ProgFund_Lecture_2_Data_Types_and_Selection-1.pdf

Programming Fundamentals
Lecture 2: Data Types and Selection

This Lecture
• Data types
– Common data types: integer, float, string and boolean
– Weak/Strong typing and type conversion
– Implicit and explicit conversion
– Static and dynamic typing
– Numeric, string and boolean conversions
• Selection
– If-then, if-then-else and else-if statements
– Boolean expressions
– Comparison operators and logical operators
2

Textbook
• Starting Out with Python, 4th Edition (Global)
– Tony Gaddis
• Reading the chapter(s) is required
– Read the indicated chapter(s) before class
• This week covers the following textbook chapter(s):
– Chapter 3 – Decision Structures and Boolean Logic
– Note: You may also wish to revise parts of Chapter 2 regarding data types
3

Data Types
• Last week we introduced the concept of variables
– Assigning a name to data stored in memory for later use
• We also briefly mentioned data types – the “categories” of data
– Integers, floating point numbers (floats), strings…
– The difference can be subtle; 3.14 is a float, '3.14' is a string
– All data (and hence every variable) in a program has a data type
• The data type essentially tells the language/computer how to interpret the values
that are stored in memory
– It determines what can and cannot be done with the data,
e.g. Arithmetic works on integers and floats, but not strings
– Functions often require values of a certain data type,
e.g. round() needs a float and an integer: round(2.327, 2)
5

Data Types
• The most common data types are:
• Some languages divide numeric data types further
– Data types for very large numbers, e.g. “long” in Java, or numbers with large
fractional parts, e.g. “double” in Java
• Some languages, such as C, do not support strings
– Such languages typically have a “character” or “char” data type, representing a
single character. An “array” of multiple characters can be used to represent a string
Type Description Examples
integer (int) Whole numbers 2, -500, 4294967296
float (float) Numbers with a fractional part 3.14159, -0.3, 25.0
string (str) A sequence of characters 'Hello, World!', '25', ' '
boolean (bool) A true or false value True, False
6

“Weak” and “Strong” Typing
• Different languages take different approaches to whether data types can be
mixed and converted automatically
• “Weakly typed” languages focus on being convenient
– Generally achieved by making it easier to mix different data types together and
converting between them automatically wherever appropriate
• The programming language tries to work things out for you
• “Strongly typed” languages focus on being safe
– Generally achieved by limiting the ability to mix different data types together –
conversions must be done explicitly by the programmer
• The programmer must consider and convert types manually
• Languages allow and disallow things based on their design, often ending up
somewhere between weak and strong
7

Data Type Conversion and Coercion
• Some values can be converted between different data types
– e.g. 100 (int), 100.0 (float) and '100' or '100.0' (strings)
• All languages provide ways to convert between data types
– e.g. The int(), float() and str() functions in Python
– Some languages use “(typename) value”, e.g. (int) 3.14
– Conversion will fail (or give unexpected results) if it doesn’t make sense,
e.g. trying to convert a string of 'cat' to an int
– Some conversions may involve a loss of data, e.g. converting 3.14 to an int
• Many languages do implicit (automatic) conversion between compatible data
types as needed, where possible; This is known as coercion
– Integers are commonly coerced to floats, e.g. 5 + 2.5 would become 5.0 + 2.5
– Languages like PHP can coerce numeric strings, e.g. 1 + '2' + 3.4 = 6.4
8

• “Static typing” is when a language requires you to declare (and adhere to) the
data type of every variable you create. C and Java are statically typed
– Trying to assign a value of a different type will result in an error (or coercion)
– Helps program optimisation and to prevent type-related bugs
• “Dynamic typing” is when the data type of a variable is determined by the value
it currently refers to, which can change. Python and PHP are dynamically typed
– You do not “declare” a data type for the variable, you just assign a value to it
– Convenient, particularly in languages which coerce data types automatically
Static and Dynamic Typing
String value; // declare a string
value = 1; // assign int to string
Error: Incompatible types
Java value = 1 # int
1
value = 'Potato' # string
'Potato'
value = -2.74 # float
-2.74
Python
int value; // declare an int
value = 3.14; // assign float to int
3
C
9
total
HELLO, my name is…

Type Conversion Example – Numbers
• What is happening in these examples?
– In Java (and some other “strongly typed” languages), dividing an integer by another
integer results in an integer
• The result was truncated to 0, then stored as a float of 0.0
• No rounding was performed; It simply kept the integer portion only
– In Python (and other “weakly typed” languages), the result of dividing two integers is
automatically coerced to a float
10
int value1 = 15;
int value2 = 20;
float result = value1 / value2;
System.out.println(result);
0.0
Java value1 = 15
value2 = 20
result = value1 / value2
print(result)
0.75
Python

Type Conversion Example – Numbers
• In this version, we explicitly convert value_one to a float
– We use (float) to convert value_one, an integer, to a float – 15.0
• Java will coerce value_two to match, and give the result as a float
– In Java, the result of any arithmetic is “widened” to the more encompassing data
type; int < long < float < double… e.g.
• int / int = int
• float / float = float
11
int value_one = 15;
int value_two = 20;
float result = (float) value_one / value_two; // convert value1 to float
System.out.println(result);
0.75
Java
• float / int = float
• int / float = float

Type Conversion Example – String Concatenation
• What is happening in these examples?
– Both of them are trying to combine a string and a float
• In the first one, the error message says that concatenation can only be performed
between strings, not between a string and a float
• In the second one, the error message says that you cannot add a float and a string
together – that’s not how arithmetic works!
– The goal is the same in both – concatenating them together into a single string,
i.e. 'The total is 79.4' or '79.4 is the total'
• The solution to both is to convert total to a string: str(total)
12
total = 79.4
output = 'The total is ' + total
TypeError: can only concatenate str (not "float") to str
output = total + ' is the total'
TypeError: unsupported operand type(s) for +: 'float' and 'str'
Python

• In this version, we explicitly convert total to a string
– We use the str() function to convert total to a string
• Now that there are strings on both sides of the “+”, Python can determine that you want to
concatenate them and do so without any problems
• Reminder: The total variable still refers to a float of 79.4 – the str() function simply
produces a string version of the value – it does not change the variable itself
– Concatenating numbers and strings is an area that languages handle differently
• We’ll look at some examples on the next slide!
13
total = 79.4
output = 'The total is ' + str(total)
'The total is 79.4'
output = str(total) + ' is the total'
'79.4 is the total'
Python

– PHP avoids ambiguity by using
“.” for concatenation
– It can also detect variables
inside strings due to the “$” at
the start of variable names
– JavaScript always coerces both values
to strings and concatenate them
– Even if one of the strings contains a
number…
var num = 79.4;
alert(num + ' is the total')
'79.4 is the total'
var string_num = '25';
alert(num + string_num)
'79.425'
JavaScript $total = 79.4;
echo 'The total is ' . $total;
'The total is 79.4'
echo "$total is the total";
'79.4 is the total'
PHP
total = 79.4
print(total, 'is the total. The total is', total)
'79.4 is the total. The total is 79.4'
Python
– As covered last week, Python’s print() function offers yet another way of
combining strings and numbers…
14

Type Conversion Example – Boolean Values
• True and False are the only two boolean values, but values of other data types
can always be converted to a boolean value
– The bool() function converts a value to a boolean value
• The rule that most languages follow when converting to a boolean value is:
– Values equivalent to 0, or empty/blank values are considered False
– All other values are considered True
• As always, there can be subtle differences between languages…
e.g. '0' (a string of the “zero” character) is True in Python, but False in PHP
15
bool(0) # int of 0
False
bool(0.0) # float of 0.0
False
bool('') # empty string
False
bool([]) # an empty "list"
False
Python
bool(5) # positive int
True
bool(-245.271) # negative float
True
bool('False') # string of 'False'
True
bool('0') # string of 0
True
Python

Data Types / Type Conversion Summary
• While there are general concepts such as “weak” and “strong” typing, our
examples have demonstrated that…
– Almost every language handles things slightly differently!
– Regardless of what a language allows and supports, a good programmer should
always consider data types
• Know what type your data is, and how it can be used
• Learn how your language behaves when mixing/coercing types
• Failing to do so can result in bugs and security vulnerabilities
– Always test your code thoroughly to ensure it is working
• Particularly in weakly typed languages or ones you’re new to
• You won’t always receive an error message to let you know!
16

Selection
• When a program is run, it runs the statements in the source code from start to
end (unless it encounters an error)
– Certain statements allow for a choice between different blocks of code to run based
upon the result of checking for a specific condition
– These are known as selection statements
– In this example, the beeping noise only occurs if the timer value is 0
• We will examine the most common selection statement – “if-then” (aka just “if”)
– Can be extended upon with “else” and “else-if”
– Another selection statement, “switch”, is covered in Reading 2.4
If timer value is 0
Make beeping noise
Pseudocode
18

If-Then Statements
• An if-then statement will run a block of code only if a condition
(known as a “boolean expression”) evaluates to True
– Often used run certain code in response to user input
19
condition
True
False
only run if true
// tax free threshold check
int income;
printf("Enter income: ");
scanf("%d", &income);
if(income <= 18200)
{
printf("No tax for you!");
}
C
# tax free threshold check
income = int(input('Enter income: '))
if income <= 18200:
print('No tax for you!')
Python

If-Then Statements
• The structure of an if-then statement is largely the same in most languages,
although Python strives for readability and uses indentation
– Most languages:
if (<boolean expression>)
{
<code to run if True>
}
– Python:
if <boolean expression>:
20
Boolean expression
in parentheses
No parentheses, but
a colon at the end
True-code controlled by indentation
(4 spaces is the standard/default)
True-code enclosed by braces (curly brackets)

If-Then Statements
• There are some subtle nuances that you should be aware of…
(remember, we’re learning a concept, not a language)
– In languages that use braces (curly brackets), they are usually optional if only one
statement is to be run when the expression is true:
<single statement to run if True>
<code to run regardless>
– Some languages use (or just allow) “endif” instead of braces:
endif
21

Boolean Expressions & Comparison Operators
• Selection statements choose which code to run based upon the result of a
boolean expression – i.e. An expression that results in either True or False
• You can compare values and receive a True or False result by using a
comparison operator
– Many functions will return (result in) either True or False
– A non-boolean value can be coerced to either True or False
22
Comparison Operator Description Example Example Result
< Less than 7 < 2 False
<= Less than or equal to 5 <= 5 True
> Greater than 7 > 2 True
>= Greater than or equal to 7 >= 2 True
== Equal to 7 == 2 False
!= Not equal to 7 != 2 True

If-Then / Boolean Expression Examples
• Example 1: Comparing a numeric variable to a number:
– The input was converted to a float (using the float() function), so that the
“less than” comparison with a number works as intended
• Example 2: Using a function in a comparison:
– The built-in function len() returns the “number of items in an object” (in this case,
the number of characters in the string)
– We are using a “not equal to” comparison (!=) to check if the length of the string is
not equal to 5
23
Prompt user to type exactly 5 characters
If input length != 5
Show error message
Pseudocode
value = input('Enter exactly 5 characters: ')
if len(value) != 5:
print('Error: Incorrect number of characters.')
Python
Prompt user for a score out of 100
If score < 50
Show 'You fail!'
Pseudocode
score = float(input('Enter a score out of 100: '))
if score < 50:
print('You fail!')
Python

If-Then / Boolean Expression Examples
• Example 3: Using a boolean value returned from a function:
– .isupper() is a function that can only be used on strings, hence why it is
“attached to the end” of the string variable instead of being “wrapped around it”
– .isupper() returns True if all of the letters in the string are uppercase
• Example 4: Using a coerced boolean value:
– The value variable is coerced to a boolean value (see earlier slide):
An empty string ('') is False, anything else is True
24
Prompt user to type something in upper case
If user typed something in upper case
Congratulate user
Pseudocode
value = input('Enter something in upper case: ')
if value.isupper():
print('Congratulations on typing in upper case!')
Python
Prompt user to type anything
If user typed at least one character
Congratulate user
Pseudocode
value = input('Type anything!: ')
if value:
print('Well done!')
Python

Logical Operators
• Often you will need to make multiple comparisons in a boolean expression.
For this we use logical operators
– e.g. “is A and B True?”, or “is A or B True?”
– You may also find situations where you want to know if something is not true, i.e.
turn False into True and vice-versa
– For added readability, Python uses “and”, “or” and “not” for logical operators.
Many languages use “&&”, “||” and “!”
Logical Operator
Description Example
Example
Result
Python Alternate
and && True if both comparisons are true, otherwise false 1 < 5 and 4 == 6 False
or ||
True if either (or both) comparison is true,
false if they are both false
1 < 5 or 4 == 6 True
not ! Reverse/Flip true and false not (1 < 5) False
25

Logical Operator Truth Tables & Evaluation
• These “truth tables” illustrate which situations will result in which outcome when
using logical operators:
– An “and” will only be True if both expressions are True; If either (or both) are False,
the end result is False
– An “or” will be True if either (or both) expressions are True
• Logical operators are evaluated after evaluating the expressions on either side
of them to True/False
26
and True False
True True False
False False False
or True False
True True True
False True False
5 > 2
and
6 < 4
True
and
False
False

If-Then / Logical Operator Examples
• Example 1: Using “and”:
• Example 2: Using “or”:
– count() is a function that can be used on strings. It returns the number of
occurrences of a specified string within another string
• Example 3: Using “not”:
– .isalpha() is another function that can only be used on strings.
It returns True if all of the characters in the string are letters
27
Prompt user to a number between 5 and 15
If number is between 5 and 15
Congratulate user
Pseudocode
value = float(input('Enter a number between 5 and 15: '))
if value >= 5 and value <= 15:
print('Well done!')
Python
Prompt user to type at least 5 characters, no spaces
If input length < 5 or input contains spaces
Show 'Better luck next time!'
Pseudocode
value = input('Enter at least 5 characters, with no spaces: ')
if len(value) < 5 or value.count(' ') > 0:
print('Better luck next time!')
Python
Prompt user to type something that isn't a letter
If input is not a letter
Congratulate user
Pseudocode
value = input('Type something that is not a letter: ')
if not value.isalpha():
print('Well done!')
Python

Terrible Programmer Joke Break!
Mum said, “Please go to the market and get a bottle of milk,
and if they have eggs, get six.”
I came back with six bottles of milk.
Mum asked, “Why did you get six bottles of milk?!”
I replied, “Because they had eggs!”
28

Boolean Expression & Logical Operator Notes
• Each side of an “and” or an “or” must be a full comparison
– x == 0 or 1 ✘
• Boolean expressions can have multiple logical operators
– Order of operations: “not” first, then “and”, then “or”
– e.g. True or False and not True
– Parentheses can be used to alter order of operations
– e.g. (True or False) and not True (result is False)
– Even if not altering the order of operations, parentheses are a good way to make
your code clearer and more readable
x == 0 or x == 1 ✔
True or False and False (not evaluated)
True or False (and evaluated)
True (or evaluated)
31

If-Then-Else Statements
• An if-then-else statement runs one block of code if the boolean expression is
true, and another block if it is false – it is an extension of an if-then statement
32
# test result check
result = int(input('Enter your result: '))
if result >= 50:
print('You passed!')
else:
print('You failed!')
Python
condition
True False
only run if true only run if false
// checkbox check
if (document.form.checkbox.checked)
{
alert('Checkbox checked!')
}
else
{
alert('Checkbox not checked!')
}
JavaScript

If-Then-Else Statements
• An if-then-else statement uses the word “else” to indicate the block of code to be
run if the expression is False
– Everything we have discussed regarding if-then is relevant to if-then-else
(e.g. optional braces for single statements)
– Most languages:
33
{
}
else
{
<code to run if False>
}
if <boolean expression>:
else:
<code to run if False>
– Python:

If-Then-Else Examples
– Prompt user to type their name
– If the name consists of only alphabetic
characters, print “Letters!”
– Otherwise print “Not all letters!”
– total_mark is an integer
– exam_passed is a boolean
– If both conditions are True (&& is and),
“You passed!” otherwise “You failed!”
• The else applies to all other outcomes
(one or both conditions False)
if (total_mark >= 50 && exam_passed)
{
System.out.println("You passed!");
}
else
{
System.out.println("You failed!");
}
Java
name = input('Name: ')
if name.isalpha():
print('Letters!')
else:
print('Not all letters!')
Python
34

Nested If-Then-Else Statements
• If-Then-Else statements can be “nested” within one another
– This allows you to test multiple conditions and run different code depending on which
ones are met, to implement whatever sequences of checks your program needs…
35
if <condition 1>:
<run if condition 1 True>
else:
if <condition 2>:
<run if condition 1 False
and condition 2 True>
else:
<run if both False>
Python if (<condition 1>)
{
}
else
{
if (<condition 2>)
{
<run if condition 1 False
and condition 2 True>
}
else
{
<run if both False>
}
}
Most Languages
(This is just one example of how
selection statements can be nested:
Any desired flow of logic possible!)

Else-If
• If-Then can be also expanded by using an Else-If
– This allows you to “chain” selection statements together
– Only the code following the first condition that is True is run
36
condition
1
True
False
only run if
condition 1 true
condition
2
condition
3
False
False
True only run if
condition 3 true
True only run if
condition 2 true
only run if all
conditions false
• Languages use slightly different
else-if syntax:
– “else if”
• C, C++, Java, PHP, JavaScript…
– “elseif”
• PHP, Visual Basic…
– “elsif”
• Ada, Perl, Ruby…
– “elif”
• Python, Bash…

Else-If
• While the word used for else-if may differ between languages, it works the same
in pretty much all of them
– Remember, Python uses indentation instead of braces (“{}”)!
37
if <condition 1>:
elif <condition 2>:
elif <condition 3>:
elif <condition 4>:
else:
<run if all conditions False>
Python
if (<condition 1>) {
}
else if (<condition 2>) {
}
}
}
else {
<run if all conditions False>
}
Most Languages
Placing the opening brace after the
condition/else instead of on a separate
line is a common space-saving technique

Else-If Example
• Here’s an (overused) example for grade calculation:
# calculate grade from mark
mark = float(input('Enter your mark: '))
if mark >= 80:
grade = 'HD (High Distinction)'
elif mark >= 70:
grade = 'D (Distinction)'
elif mark >= 60:
grade = 'CR (Credit)'
elif mark >= 50:
grade = 'P (Pass)'
else:
grade = 'F (Fail)'
print('Your grade is:', grade)
Python
PROMPT for mark
IF mark >= 80
SET grade to 'HD'
ELSE IF mark >= 70
SET grade to 'D'
ELSE IF mark >= 60
SET grade to 'CR'
ELSE IF mark >= 50
SET grade to 'P'
ELSE
SET grade to 'F'
DISPLAY grade
Pseudocode
38

Else-If Example
• A flowchart for this program would look like:
39
mark
>= 80
True
False
Set grade to HD
mark
>= 70
mark
>= 50
False
False
True
Set grade to P
True
Set grade to D
Set grade to F
Prompt for
mark
mark
>= 60
True
Set grade to CR
False Display
grade

Conclusion
• It is important to be aware of data types, even in languages which make it
convenient to mix them implicitly
– Know how your language coerces things, what it allows and doesn’t allow, how to
convert things explicitly, etc…
• If-Then statements allow you to make a choice in your code
– Controlled by boolean expressions, which can involve comparison operators and
logical operators
– If-Then-Else to do one thing or another
– Else-If to chain multiple comparisons together
40

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