Introduction To Python

AGENDA About Python How to run Python in Eclipse Basic data types, comments and assignments Sequence types: Tuples, Lists, and Strings String operations Dictionaries Operators Control of flow Functions

PYTHON Python is an interpreted, general-purpose high-level programming language The code is automatically compiled to byte code and executed, Python is suitable for use as a scripting language, Web application implementation language, etc. Its use of indentation for block delimiters is unique among popular programming languages. Python supports multiple programming paradigms, primarily but not limited to object oriented, imperative and, to a lesser extent, functional programming styles.

COMPARISONS WITH OTHER PROGRAMMING LANGUAGES

FEATURES OF PYTHON Simple Easy to learn Free and Open Source High-level Language Portable Interpreted Object Oriented Extensible Embeddable Extensive Libraries

WRITING FIRST PYTHON PROGRAM Step 1: Switch to the Python perspective Go to "Window > Open Perspective > Other..." and choose "Pydev", then click OK. If you look at the upper right corner you will see that the perspective has changed from "Java" to "Pydev".

Step 2: Create a new Python project Go to "File > New > Pydev Project" to start a wizard. In the next window that appears, enter the name of your project and select "python" and 3.0"; as the type. Make sure "create default 'src' folder and add it to the pythonpath?" is selected. Click Finish.

If you look at the upper left corner of the workspace (in the Package Explorer view), you should now see your newly created project with a "src" folder inside.

Step 3: Create a new module Select the project you just created and go to "File -> New -> Pydev Module". This will launch a new Pydev Module Wizard where you should enter a name for your module and make sure it is in the right location. Leave the Package field blank and select Finish.

Look in the Package Explorer view and you will see an icon of your new file inside the src folder, which Eclipse created when you made the new project before.

Step 4: Write and Run the Program Click on the new module created(HelloWorld.py) and type the one line code. print (‘Hello World!’)

COPY AND PASTE INTO ECLIPSE print( "hello world")

To run the python, select the module(HelloWorld.py). Right click on the module and select “Run as -> Python Run”

Look at the bottom of your screen at the Console view and you will see the message you told the computer to print. Congratulations! You have written your first program with Python.

BASIC DATA TYPES, COMMENTS AND ASSIGNMENTS

BASIC DATA TYPES Integers And Floats An integer in Python, also known as a ‘numeric literal’ Any numeric value can be assigned to it at any time, overwriting the previous value. To assign a value to an integer variable, one writes a statement after the following template: <numeric literal name> = <numeric literal value> Example: num1=5; print(num1) o/p: 5 num2=3; num3=num1/num2; print(num3) o/p: 1.66666666667 num4= 5.23/2 print(num4) o/p: 2.615

COPY AND PASTE INTO ECLIPSE num1=5; print(num1) num2=3; num3=num1/num2; print(num3) num4= 5.23/2 print(num4)

CONTI… Strings A string literal, or string , holds any combination of letters and numbers you would like it to hold. To assign a value to a string, one uses the following template: <name of string> = '<value of string>' Examples: str1= "hello" str2= 'hi'; str3= "John's" print(str1) o/p: hello print(str2) o/p: hi print(str3) o/p: john’s

COPY AND PASTE INTO ECLIPSE str1= "hello" str2= ' hi'; str3= "John's" print(str1) print(str2) print(str3)

CONTI… Some more examples on string…. Can use “ ” or ‘ ’ to specify. “abc” ‘abc’ (Same thing.) Unmatched can occur within the string. “matt’s” Use triple double-quotes for multi-line strings or strings than contain both ‘ and “ inside of them: """ string in multiple lines"""

COPY AND PASTE INTO ECLIPSE str4 = """string in multiple lines""" print(str4)

COMMENTS AND WHITESPACE Comments Start comments with # – the rest of line is ignored. Example: # this is comment in Python Whitespace Whitespace is meaningful in Python: especially indentation and placement of newlines. Use a newline to end a line of code.

CONTI… No braces { } to mark blocks of code in Python. Use consistent indentation instead. The first line with less indentation is outside of the block. The first line with more indentation starts a nested block Often a colon appears at the start of a new block.

ASSIGNMENT Binding a variable in Python means setting a name to hold a reference to some object . Assignment creates references, not copies Names in Python do not have an intrinsic type. Objects have types. Python determines the type of the reference automatically based on the data object assigned to it. You create a name the first time it appears on the left side of an assignment expression: x = 3 A reference is deleted via garbage collection after any names bound to it have passed out of scope.

ACCESSING NON-EXISTENT NAMES If you try to access a name before it’s been properly created (by placing it on the left side of an assignment), you’ll get an error. Example: var Print(var) // undefined variable error var = 3 Print(var) o/p: 3

MULTIPLE ASSIGNMENT You can also assign to multiple names at the same time. Example: x, y = 2, 3 print(x) o/p: 2 print(y) o/p: 3

COPY AND PASTE x, y = 2, 3 print(x) print(y)

NAMING RULES Names are case sensitive and cannot start with a number. They can contain letters, numbers, and underscores. Example: Valid names: var, _var, _2var, var2 Invalid names: 2var, 3var_1 There are some reserved words: and, assert, break, class, continue, def, del, elif, else, except, exec, finally, for, from, global, if, import, in, is, lambda, not, or, pass, print, raise, return, try, while

SEQUENCE TYPES: TUPLES, LISTS, AND STRINGS AND OPERATIONS ON SEQUENCE TYPES

SEQUENCE TYPES Tuple A simple immutable ordered sequence of items Items can be of mixed types, including collection types Strings Immutable Conceptually very much like a tuple. List Mutable ordered sequence of items of mixed types

SYNTAX All three sequence types (tuples, strings, and lists) share much of the same syntax and functionality. Key difference: Tuples and strings are immutable Lists are mutable The operations shown in this section can be applied to all sequence types most examples will just show the operation performed on one

DEFINING SEQUENCE TYPES Tuples are defined using parentheses ( )and commas. tu = (23, ‘abc’, 4.56, (2,3), ‘def’) Lists are defined using square brackets [ ]and commas. li = [“abc”, [34,12], 4.34, 23] Strings are defined using quotes (“, ‘, or “““). st = “Hello World” st = ‘Hello World’ st = “““This is a multi-line string that uses triple quotes.”””

COPY AND PASTE tu = (23, ' abc', 4.56, (2,3), 'def') li = [ " abc", [34,12], 4.34, 23] st5 = "Hello World" st6= 'Hello World' st7 = """This is a multi-line string that uses triple quotes.""" print(tu) print(li) print(st5) print(st6) print(st7)

ACCESSING INDIVIDUAL MEMBERS We can access individual members of a tuple, list, or string using square bracket “array” notation. Note that all are 0 based . tu = (23, ‘abc’, 4.56, (2,3), ‘def’) tu[1] # Second item in the tuple. o/p: ‘abc’ li = [“abc”, [34,12], 4.34, 23] li[1] # Second item in the list. o/p: [34,12] st = “Hello World” st[1] # Second character in string. o/p: ‘e’

COPY AND PASTE tu = (23, ' abc', 4.56, (2,3), 'def') print(tu[1]) li = [ " abc", [34,12], 4.34, 23] print(li[1]) st6 = 'Hello World' print (st6[6])

POSITIVE AND NEGATIVE INDICES tu = (23, ‘abc’, 4.56, (2,3), ‘def’) Positive index: count from the left, starting with 0. tu[1] o/p: ‘abc’ Negative lookup: count from right, starting with –1. tu[-3] o/p: 4.56

COPY AND PASTE tu = (23, ' abc', 4.56, (2,3), 'def') print(tu[1]) print(tu[-3])

SLICING tu = (23, ‘abc’, 4.56, (2,3), ‘def’) Return a copy of the container with a subset of the original members. Start copying at the first index, and stop copying before the second index. t[1:4] o/p: (‘abc’, 4.56, (2,3)) You can also use negative indices when slicing. t[1:-2] o/p: (‘abc’, 4.56, )

COPY AND PASTE tu = (23, ' abc', 4.56, (2,3), 'def') print(tu[1:4]) print(tu[1:-2])

SLICING CONTI… Omit the first index to make a copy starting from the beginning of the container. tu[:2] o/p: (23, ‘abc’) Omit the second index to make a copy starting at the first index and going to the end of the container. tu[3:] o/p: ((2,3), ‘def’)

COPY AND PASTE print(tu[:2]) print(tu[3:])

COPYING WHOLE SEQUENCE To make a copy of an entire sequence, you can use [:]. tu[:] o/p: (23, ‘abc’, 4.56, (2,3), ‘def’) Note the difference between these two lines for mutable sequences: list1=['abc', [34,12], 4.34, 23] list2 = list1 # 2 names refer to 1 ref # Changing one affects both list2 = list1[:] # Two independent copies, two refs

COPY AND PASTE list1=[ ' abc', [34,12], 4.34, 23] list2 = list1 list2[0] = " xyz" print(list1) print(list2) list2 = list1[:] list2[0] = " pqr" print(list1) print(list2)

THE ‘IN’ OPERATOR Boolean test whether a value is inside a container: >>> t = [1, 2, 4, 5] >>> 3 in t False >>> 4 in t True >>> 4 not in t False For strings, tests for substrings >>> strg= 'abcde' >>> 'c' in strg True >>> 'cd' in strg True >>> 'ac' in strg False Be careful: the in keyword is also used in the syntax of for loops and list comprehensions .

COPY AND PASTE t = [1, 2, 4, 5] if 1 in t: print( "great")

THE + OPERATOR The + operator produces a new tuple, list, or string whose value is the concatenation of its arguments. (1, 2, 3) + (4, 5, 6) o/p: (1, 2, 3, 4, 5, 6) [1, 2, 3] + [4, 5, 6] o/p: [1, 2, 3, 4, 5, 6] “ Hello” + “ ” + “World” o/p: ‘Hello World’

THE * OPERATOR The * operator produces a new tuple, list, or string that “repeats” the original content. (1, 2, 3) * 3 (1, 2, 3, 1, 2, 3, 1, 2, 3) [1, 2, 3] * 3 [1, 2, 3, 1, 2, 3, 1, 2, 3] “ Hello” * 3 ‘ HelloHelloHello’

COPY AND PASTE tu=(1, 2, 3) + (4, 5, 6) print(tu) li=(1, 2, 3) + (4, 5, 6) print(li) st=( "hello") + ("world") print(st) tu=(1, 2, 3)* 3 print(tu) li=[1, 2, 3] * 3 print(li) st=( "hello") * 3 print(st)

TUPLE VS LIST Tuples: Immutable tu = (23, ‘abc’, 4.56, (2,3), ‘def’) tu[2] = 3.14 o/p: TypeError: 'tuple' object does not support item assignment You can’t change a tuple. You can make a fresh tuple and assign its reference to a previously used name. tu = (23, ‘abc’, 3.14, (2,3), ‘def’)

CONTI … List: Mutable li = [‘abc’, 23, 4.34, 23] li[1] = 45 print(li) O/p: [‘abc’, 45, 4.34, 23] We can change lists in place. Name li still points to the same memory reference when we’re done.

COPY AND PASTE tu = (23, " abc", 4.56, (2,3), "def") tu[2] = 3.14 li = [23, " abc", 4.56, (2,3), "def"] li[2] = 3.14 print(li)

CONTI… Tuple List Immutable Mutable Fast Comparatively slow Fewer operations Lot of handy operations

OPERATIONS ON LISTS Let us consider the list shown below and perform different operations on them li = ['abc',16, [34,12], 4.34, 23] append appends the value to the end of list. li.append(‘a’) Print(li) o/p: ['abc',16, [34,12], 4.34, 23,’a’] insert Insert the value at a mentioned position in the list li.insert(2,’i’) o/p: ['abc',16,i, [34,12], 4.34, 23]

COPY AND PASTE li = [ 'abc',16, [34,12], 4.34, 23] li.append( 'a') print(li) li.insert(2, 'i') print(li)

CONTI … Index Gives the index of first occurrence of character in the list li=[a,b,c,b,d,e] li.index(‘b’) o/P: 1 Count Returns number of occurrences of the value li.count(‘b’) o/p: 2 Remove Removes first occurrence of the value li.remove(‘b’) o/p: [a,c,b,d,e]

COPY AND PASTE li = [ ' abc',16, [34,12], 4.34, 23] print(li.index( ' abc')) print(li.count( ' abc')) li.remove( ' abc') print(li)

CONTI… Reverse reverse the list li= [30,20,40,10,70,50] li.reverse() o/p: [50,70,10,40,20,30] Sort sort the list li.sort() o/p:[10,20,30,40,50,70]

COPY AND PASTE li= [30,20,40,10,70,50] li.reverse() print(li) li.sort() print(li)

UNDERSTANDING REFERENCE SEMANTICS IN PYTHON

IMMUTABLE TYPE REFERENCES In Python, the datatypes integer, float, and string (and tuple) are “immutable.” This can be understood through the following example. x=1 # value 1 is created in memory and x refers to it y=x # y now refers to value 1 y=2 # value 2 is created and y is made to refer to it print(x) # x still refers to 1, no effect on it by changing value of y print(y) o/p: 1 2

IMMUTABLE : EXAMPLE x=1 y=x X reference Type Integer Data : 1 X reference Y reference Type Integer Data: 1

CONTI… Y=2 X reference Y reference Type Integer Data: 1 Type Integer Data: 2

MUTABLE TYPE REFERENCES For other data types (lists, dictionaries, user-defined types), assignment works differently. These data types are “mutable.” When we change these data, we do it in place. We don’t copy them into a new memory address each time. If we type y=x and then modify y, both x and y are changed. Example: li=[1,2,3,4] li2=li li2.append(5) #value of li also changes print(li) o/p: [1,2,3,4,5] print(li2) o/p: [1,2,3,4,5]

MUTABLE : EXAMPLE li=[1,2,3,4] li2 = li li reference li reference li2 reference 1 2 3 4 1 2 3 4

CONTI… li2.append(5) li reference li 2reference 1 2 3 4 5

STRING OPERATIONS AND CONVERSIONS

STRING OPERATIONS Upper str=“hello” str1 = str.upper o/p: “HELLO” String format operator % The operator % allows strings to be built out of many data items in a “fill in the blanks” fashion. ex: x=34 y=“hello” z=“%sabc%d” % (y,x) o/p: helloabc34

COPY AND PASTE str= "hello" str1 = str.upper() print(str) print(str1) x=34 y= "hello" z= "% sabc%d" % (y,x) print (x) print (y) print (z)

WHY? >>> uid = "sa" >>> pwd = "secret" >>> print pwd + " is not a good password for " + uid secret is not a good password for sa >>> print "%s is not a good password for %s" % (pwd, uid) secret is not a good password for sa >>> userCount = 6 >>> print "Users connected: %d" % (userCount) Users connected: 6 >>> print "Users connected: " + userCount Traceback (innermost last): File "<interactive input>", line 1, in ? TypeError: cannot concatenate 'str' and 'int' objects

STRING CONVERSION list to String Join turns a list of strings into one string format : <separator_string>.join( <some_list> ) Example: li=[“abc”,”def”,”ghi”] str= ‘;’.join(li) print(str) o/p: abc;def;ghi

CONTI… String to list Split turns one string into a list of strings Format: <some_string> .split( <separator_string> ) Example: str = ‘abc;def;ghi’ newLi = str.split(‘;’) print(newLi) o/p: [abc, def, ghi]

COPY AND PASTE li=[ " abc","def","ghi"] str= ';'.join(li) print(str) str = ' abc;def;ghi' newLi = str.split( ';') print(newLi)

DICTIONARIES Dictionaries store a mapping between a set of keys and a set of values. Keys can be any immutable type. Values can be any type A single dictionary can store values of different types You can define, modify, view, lookup, and delete the key-value pairs in the dictionary.

CREATING AND ACCESSING DICTIONARIES Creating dict = { ‘name’:’john’ , ‘place’:’xyz’, ‘id’:1234 } Accessing dict[‘name’] o/p: ‘john’ dict o/p: { ‘name’:’john’ , ‘place’:’xyz’, ‘id’:1234 }

UPDATING Keys must be unique. Assigning to an existing key replaces its value. Dictionaries are unordered New entry might appear anywhere in the output. Example: dict[‘name’] = ‘adam’ dict[‘gender’] = ‘male’

DELETING Deleting entry from dictionary del dict[‘name’] o/p: { place: xyz, id:1234 } Deleting entire dictionary dict.clear()

OPERATIONS ON DICTIONARIES Dict = { ‘name’:’john’, ‘place’:’xyz’, ‘id’:1234 } To access keys dict.keys() #returns list of keys o/p: [name, place, id ] To access values dict.values() # return list of values o/p: [ john, xyz,1234] To access items Dict.items() # returns list of item tuples o/p: [(name,john),(place, xyz), (id, 1234)

COPY AND PASTE dict={ 'name':'john','place':' xyz','id':1234} print(dict) print(dict[ 'name']) del dict[ 'name'] print(dict) print(dict.keys()) print(dict.values()) print( dict.items())

ARITHMETIC OPERATOR Operator Description Example a=5, b=3 + Addition - Adds values on either side of the operator a+b = 5+3 = 8 - Subtraction - Subtracts right hand operand from left hand operand a-b = 5-3 = 2 * Multiplication - Multiplies values on either side of the operator a*b = 5*3 = 15 / Division - Divides left hand operand by right hand operand a/b = 5/3 = 1.66666666667 % Modulus - Divides left hand operand by right hand operand and returns remainder a%b = 5%3 = 2 ** Exponent - Performs exponential (power) calculation on operators a**b = 5**3 =125 // Floor Division - The division of operands where the result is the quotient in which the digits after the decimal point are removed. a//b = 5//3 = 1

COMPARISON OPERATORS Operator Description Example a=5, b=3 == Checks if the value of two operands are equal or not, if yes then condition becomes true. (a == b) is not true. != Checks if the value of two operands are equal or not, if values are not equal then condition becomes true. (a != b) is true. <> Checks if the value of two operands are equal or not, if values are not equal then condition becomes true. (a <> b) is true. This is similar to != operator. > Checks if the value of left operand is greater than the value of right operand, if yes then condition becomes true. (a > b) is not true. < Checks if the value of left operand is less than the value of right operand, if yes then condition becomes true. (a < b) is true.

CONTI… Operator Description Example a=5, b=3 >= Checks if the value of left operand is greater than or equal to the value of right operand, if yes then condition becomes true. (a >= b) is not true. <= Checks if the value of left operand is less than or equal to the value of right operand, if yes then condition becomes true. (a <= b) is true.

ASSIGNMENT OPERATORS Operator Description Example = Simple assignment operator, Assigns values from right side operands to left side operand c = a + b will assigne value of a + b into c += Add AND assignment operator, It adds right operand to the left operand and assign the result to left operand c += a is equivalent to c = c + a -= Subtract AND assignment operator, It subtracts right operand from the left operand and assign the result to left operand c -= a is equivalent to c = c - a *= Multiply AND assignment operator, It multiplies right operand with the left operand and assign the result to left operand c *= a is equivalent to c = c * a

CONTI…. Operator Description Example /= Divide AND assignment operator, It divides left operand with the right operand and assign the result to left operand c /= a is equivalent to c = c / a %= Modulus AND assignment operator, It takes modulus using two operands and assign the result to left operand c %= a is equivalent to c = c % a **= Exponent AND assignment operator, Performs exponential (power) calculation on operators and assign value to the left operand c **= a is equivalent to c = c ** a //= Floor Division and assigns a value, Performs floor division on operators and assign value to the left operand c //= a is equivalent to c = c // a

BITWISE OPERATORS Bitwise operator works on bits and perform bit by bit operation. Assume if a = 60; and b = 13; Now in binary format they will be as follows: a = 0011 1100 b = 0000 1101 a&b = 0000 1100 a|b = 0011 1101 a^b = 0011 0001 ~a = 1100 0011

CONTI… Operator Description Example A=60(0011 1100) B=13(0000 1101) & Binary AND Operator copies a bit to the result if it exists in both operands. (a & b) will give 12 which is 0000 1100 | Binary OR Operator copies a bit if it exists in eather operand. (a | b) will give 61 which is 0011 1101 ^ Binary XOR Operator copies the bit if it is set in one operand but not both. (a ^ b) will give 49 which is 0011 0001

CONTI… Operator Description Example A=60(0011 1100) B=13(0000 1101) ~ Binary Ones Complement Operator is unary and has the effect of 'flipping' bits. (~a ) will give -60 which is 1100 0011 << Binary Left Shift Operator. The left operands value is moved left by the number of bits specified by the right operand. a << 2 will give 240 which is 1111 0000 >> Binary Right Shift Operator. The left operands value is moved right by the number of bits specified by the right operand. a >> 2 will give 15 which is 0000 1111

LOGICAL OPERATORS Operator Description Example and Called Logical AND operator. If both the operands are true then then condition becomes true. (a and b) is true. or Called Logical OR Operator. If any of the two operands are non zero then then condition becomes true. (a or b) is true. not Called Logical NOT Operator. Use to reverses the logical state of its operand. If a condition is true then Logical NOT operator will make false. not(a and b) is false.

IDENTITY OPERATORS Identity operators compare the memory locations of two objects Operator Description Example is Evaluates to true if the variables on either side of the operator point to the same object and false otherwise. x is y, here is results in 1 if id(x) equals id(y). is not Evaluates to false if the variables on either side of the operator point to the same object and true otherwise. x is not y, here is not results in 1 if id(x) is not equal to id(y).

CONTROL OF FLOW Conditional constructs are used to incorporate decision making into programs. The result of this decision making determines the sequence in which a program will execute instructions. You can control the flow of a program by using conditional constructs.

IF STATEMENT The if statement of Python is similar to that of other languages. The if statement contains a logical expression using which data is compared, and a decision is made based on the result of the comparison. Syntax: if expression : statement(s) Here if statement, condition is evaluated first. If condition is true that is, if its value is nonzero then the statement(s) block are executed. Otherwise, the next statement following the statement(s) block is executed.

EXAMPLE: IF STATEMENT Example 1: var1 = 100 if var1: print "1 - Got a true expression value" print var1 Output: 1 - Got a true expression value 100 Example 2: var2 = 0 if var2: print “2 - Got a true expression value" print var2 print “Good bye!” Output Good bye!

ELSE STATEMENT An else statement can be combined with an if statement. An else statement contains the block of code that executes if the conditional expression in the if statement resolves to 0 or a false value. The else statement is an optional statement and there could be at most only one else statement following if . Syntax: if expression : statement(s) else: statement(s)

EXAMPLE: ELSE STATEMENT Example: 1 var1 = 100 if var1: print "1 - Got a true expression value" print var1 else: print "1 - Got a false expression value" print var1 Output: 1 - Got a true expression value 100

COPY AND PASTE n=-500 if n < 0: print ( "The absolute value of",n,"is",-n) else: print ( "The absolute value of",n,"is",n) n=500 if n < 0: print ( "The absolute value of",n,"is",-n) else: print ( "The absolute value of",n,"is",n)

ELIF STATEMENT The elif statement allows you to check multiple expressions for truth value and execute a block of code as soon as one of the conditions evaluates to true. Like the else , the elif statement is optional. However, unlike else , for which there can be at most one statement, there can be an arbitrary number of elif statements following an if . Syntax: if expression1: statement(s) elif expression2: statement(s) elif expression3: statement(s) else: statement(s)

EXAMPLE: ELIF STATEMENT Example: var = 100 if var == 200: print "1 - Got a true expression value" print var elif var == 150: print "2 - Got a true expression value" print var2 elif var == 100: print "3 - Got a true expression value" print var else: print "4 - Got a false expression value" print var print "Good bye!" Output: 3 - Got a true expression value 100 Good bye!

NESTED IF...ELIF...ELSE CONSTRUCT There may be a situation when you want to check for another condition after a condition resolves to true. In such a situation, you can use the nested if construct. In a nested if construct, you can have an if...elif...else construct inside another if...elif...else construct. Syntax: if expression1: statement(s) if expression2: statement(s) elif expression3: statement(s) else statement(s) elif expression4: statement(s) else: statement(s)

EXAMPLE: NESTED IF...ELIF...ELSE CONSTRUCT var = 100 if var < 200: print "Expression value is less than 200" if var == 150: print "Which is 150" elif var == 100: print "Which is 100" elif var == 50: print "Which is 50" elif var < 50: print "Expression value is less than 50" else: print "Could not find true expression" print "Good bye!“ Output: Expression value is less than 200 Which is 100 Good bye!

WHILE LOOP The while loop is one of the looping constructs available in Python. The while loop continues until the expression becomes false. The expression has to be a logical expression and must return either a true or a false value Syntax: while expression: statement(s) Here expression statement is evaluated first. If expression is true that is, then the statement(s) block is executed repeatedly until expression becomes false . Otherwise, the next statement following the statement(s) block is executed.

EXAMPLE:WHILE LOOP count = 0 while (count < 4): print 'The count is:', count count = count + 1 print "Good bye!" Output: The count is: 0 The count is: 1 The count is: 2 The count is: 3 The count is: 4 Good bye!

COPY AND PASTE a = 0 while a < 10: a = a + 1 if a > 5: print (a, ">",5) elif a <= 7: print (a, "<=",7) else: print ( "Neither test was true")

CONT… Infinite Loops You must use caution when using while loops because of the possibility that this condition never resolves to a false value. This results in a loop that never ends. Such a loop is called an infinite loop. An infinite loop might be useful in client/server programming where the server needs to run continuously so that client programs can communicate with it as and when required Example: var = 1 while var == 1 : # This constructs an infinite loop num = raw_input("Enter a number :") print "You entered: ", num print "Good bye!"

FOR LOOP The for loop in Python has the ability to iterate over the items of any sequence, such as a list or a string. Syntax: for iterating_var in sequence: statements(s) If a sequence contains an expression list, it is evaluated first. Then, the first item in the sequence is assigned to the iterating variable iterating_var . Next, the statements block is executed. Each item in the list is assigned to iterating_var , and the statements(s) block is executed until the entire sequence is exhausted.

EXAMPLE: FOR LOOP Example 1: for letter in ' Python': print ( 'Current Letter :', letter) Output: Current Letter : P Current Letter : y Current Letter : t Current Letter : h Current Letter : o Current Letter : n

CONTI… Example 2 fruits = [ 'banana', 'apple', ' mango'] for fruit in fruits: # Second Example print ( 'Current fruit :', fruit) O utput: Current fruit : banana Current fruit : apple Current fruit : mango

CONTI… Iterating by Sequence Index An alternative way of iterating through each item is by index offset into the sequence itself Example: fruits = ['banana', 'apple', 'mango'] for index in range(len(fruits)): print ( 'Current fruit :', fruits[index]) print ( "Good bye!") Output: Current fruit : banana Current fruit : apple Current fruit : mango Good bye!

CONTI… Since a variable often ranges over some sequence of numbers, the range() function returns a list of numbers from 0 up to but not including the number we pass to it. range(5) returns [0,1,2,3,4] So we could say: for x in range(5): print x output: 1 2 3 4 5

LOOP CONTROL You might face a situation in which you need to exit a loop completely when an external condition is triggered or there may also be a situation when you want to skip a part of the loop and start next execution. provides break and continue statements to handle such situations and to have good control on your loop.

BREAK STATEMENT The break statement in Python terminates the current loop and resumes execution at the next statement, just like the traditional break found in C. The most common use for break is when some external condition is triggered requiring a hasty exit from a loop. The break statement can be used in both while and for loops.

EXAMPLE: BREAK Example 1: for letter in 'Python' : if letter == 'h': break print ('Current Letter :', letter) Example 2: var = 10 while var > 0: print ('Current variable value :', var) var = var -1 if var == 5: break print "Good bye!"

CONTINUE STATEMENT The continue statement in Python returns the control to the beginning of the while loop. The continue statement rejects all the remaining statements in the current iteration of the loop and moves the control back to the top of the loop. The continue statement can be used in both while and for loops. Example 1: for letter in 'Python' : if letter == 'h': continue print 'Current Letter :', letter

CONTI ... Example 2 while var > 0: print ( 'Current variable value :', var) var = var -1 if var == 2: continue print ( "Good bye!")

ELSE STATEMENT USED WITH LOOPS Python supports to have an else statement associated with a loop statements. If the else statement is used with a for loop, the else statement is executed when the loop has exhausted iterating the list. If the else statement is used with a while loop, the else statement is executed when the condition becomes false. The else statement is not executed when the loop is terminated by break .

CONTI… Example for num in range(10,20): for i in range(2,num): if num%i == 0: j=num/i print ( '%d equals %d * %d' % (num,i,j)) break else: print (num, 'is a prime number')

PASS STATEMENT The pass statement in Python is used when a statement is required syntactically but you do not want any command or code to execute. The pass statement is a null operation; nothing happens when it executes. The pass is also useful in places where your code will eventually go, but has not been written yet (e.g., in stubs for example): Example: for letter in ' Python': if letter == 'h': pass print ( 'This is pass block') print ( 'Current Letter :', letter) print ( "Good bye!")

FUNCTIONS A function is a block of organized, reusable code that is used to perform a single, related action. Functions provides better modularity for your application and a high degree of code reusing. As you already know, Python gives you many built-in functions like print() etc. but you can also create your own functions. These functions are called user-defined functions.

DEFINING FUNCTIONS You can define functions to provide the required functionality. Here are simple rules to define a function in Python: Function blocks begin with the keyword def followed by the function name and parentheses ( ( ) ). Any input parameters or arguments should be placed within these parentheses. You can also define parameters inside these parentheses. The first statement of a function can be an optional statement - the documentation string of the function or docstring . The code block within every function starts with a colon (:) and is indented. The statement return [expression] exits a function, optionally passing back an expression to the caller. A return statement with no arguments is the same as return None.

CONTI… Syntax: def functionname( parameters ): "function_docstring" function_suite return [expression] Example: def printme( str ): "This prints a passed string into this function" print (str) return

CALLING A FUNCTION Defining a function only gives it a name, specifies the parameters that are to be included in the function, and structures the blocks of code. Once the basic structure of a function is finalized, you can execute it by calling it from another function or directly from the Python prompt. Example: printme("I'm first call to user defined function!"); printme("Again second call to the same function");

COPY AND PASTE def printme( str ): "This prints a passed string into this function" print (str) return printme( "I'm first call to user defined function!"); printme( "Again second call to the same function");

FUNCTION ARGUMENTS You can call a function by using the following types of formal arguments:: Required arguments Keyword arguments Default arguments

CONTI… Required arguments: Required arguments are the arguments passed to a function in correct positional order. Here the number of arguments in the function call should match exactly with the function definition. To call the function printme() you definitely need to pass one argument otherwise it would give a syntax error as follows: Example: # Now you can call printme function printme(); output: error

CONTI … Keyword arguments Keyword arguments are related to the function calls. When you use keyword arguments in a function call, the caller identifies the arguments by the parameter name. This allows you to skip arguments or place them out of order because the Python interpreter is able to use the keywords provided to match the values with parameters. You can also make keyword calls to the printme() function in the following ways: # Now you can call printme function printme( str = "My string");

CONTI… Example 2: # Function definition is here def printinfo( name, age ): "This prints a passed info into this function" print ( "Name: ", name) print ( "Age ", age) return; printinfo( age=50, name= " miki" # Now you can call printinfo function printinfo( age=50, name="miki" );

CONTI… Default arguments A default argument is an argument that assumes a default value if a value is not provided in the function call for that argument. Following example gives idea on default arguments, it would print default age if it is not passed: # Function definition is here def printinfo( name, age = 35 ): "This prints a passed info into this function" print "Name: ", name print "Age ", age return;

CONTI… # Now you can call printinfo function printinfo( age=50, name="miki" ) printinfo( name="miki" )

OTHER RESOURCES http://code.google.com/edu/languages/google-python-class/index.html

TEST 1 Write a program to check the speed. If speed is greater than 80, print 'License and registration please‘. If mood == 'terrible' or speed >= 100, print 'You have the right to remain silent.‘ Otherwise, if mood == 'bad' or speed >= 90, print "I'm going to have to write you a ticket.“, Otherwise, print "Let's try to keep it under 80 ok?"

TEST 2 Given a string s, return a string made of the first 2 and the last 2 chars of the original string, so 'spring' yields 'spng'. However, if the string length is less than 2, return instead the empty string.

Introduction To Python

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