These slides were used for the second pySIG class. It covers if-elif-else, string formatting and loops. You can find the first presentation here: http://www.slideshare.net/PranavSB/what-is-python-53192872

1. Let’s really start
Python SIG – PYA
Class 2 – 29/9/15

2. (Revision of) User input
• Complex programs = input + efficient
computation + output
• raw_input, NOT input
• raw_input – reads as string, so convert when
required (strongly typed, remember?)
• Example:
– var = raw_input(prompt)

3. ‘print’ing
• print statements, parentheses in Py3;
• But backported to 2; so print() works
• Pretty flexible: print some_string, var works
• Automatically adds newline
• Use comma to suppress

4. Useful built-ins
• len()
• range() (and xrange())
• sum(), max(), min()
• int(), float(), str()
• dir() – use it to find list and string methods
• Use help() to find out about them

5. (again) Read the documentation
• Online / Offline
• Google
• Stack Overflow
• (xkcd?)
• “Don’t have to memorise, only know how to
find what you need.”
- Me (in last class)

6. Obligatory xkcd reference[1]

7. (Revision of)Boolean
• True / False (capitalization matters!)
• >
• <
• ==
• !=
• >=
• <=
• is , is not (same as id())

8. if else elif
if SomethingThatEvaluatesToABoolean:
# code
elif SomethingElseThatEvaluatesToABoolean:
# code
else:
# code

9. Other if structures
• nested if
• elif ladders
• Basically “forking the flow of program”
• Hence called control flow

10. (Revision of)Strings
• Immutable
• Single or double, they don’t care
• Raw strings
• Escape sequences
• String slicing – string[start:stop:step]
• We count from 0 (why?)
• String methods

11. String formatting
• What is it? (Immutable)
• %something - %d, %f, %s
– ‘a = %d’ % (a)
• Prefer .format()
• {} sufficient – numbers optional
• More complex things possible

12. In-class assignment
Write a program that reads two numbers from
the user, (converting them to integers if they
are floating point numbers) and then prints
their sum along with the user input in this
format:
‘enter number 1: ’ 5.67
‘enter number 2: ’ 5.67
‘You entered 5.67 and 5.67. Adding 5 and
5, sum is 10.’

13. In-class assignment[2]
Write a program that takes input of the form
‘num1,num2,[any one of +-*/]’ as a string
and performs the given operation. In other
words, your program is a simple calculator
which can perform floating point arithmetic.
Try making improvements like asking for no.
of decimal digits to be displayed.

14. Solution required?

15. Explanation of solutions
• Essentially it is branching – splitting paths
• Be careful with ‘else’ – executes
unconditionally
• That’s why error handling – ‘try’
• Interesting fact – anything you can code, can
be coded with whatever you know now!
• (But incredibly tedious)

16. Why loops?
• Essential part of making our programs more
complex.
• When do you use for loops?
• When do you use while loops?
• If (heh.) the while loop is a ‘moving’ if,
what is the for loop?

17. Loops
• for – for when you how many iterations
• while - while you don’t know how many
iterations
• while SomethingThatEvaluatesToABoolean:
# code
• for loop syntax in Python
– for iterVar in (x)range(iterNum):
# code

18. for loops can do more!
• What is this ‘in’ anyway? (different time
complexity for different cases)
• for char in string
• for line in text
• for item in sequence
–Example: for i in [1,’a’,3]:
print i
# output: 1nan3

19. Confused / Want to know more?
Let’s do help(‘for’) and see what
we can make of it!

20. Loop shortcuts:
• break – jumps out of nearest loop
• continue – skips to top of nearest loop
• In what situation can break be used?
• In what situation can continue be used?

21. In-class assignment
• Input an angle as degrees. And find a
trigonometric ratio based on users choice[out of
sin, cos and tan]. Do this ‘n’ times, where ‘n’ was
entered by the user initially.
HINT : import math. (what is import,
from..import .. ?) [3]
• Input two strings. One takes in login_ID and
other password. It prints “login_ID is successful”
if password is password. Else should show a fail
message and keep prompting user for
password.[3]

22. In-class assignment
Find the factorial of a given number. Don’t
import math. Try to make your program break
and fix all those cases. (recursive vs.
iterative)[2]
Note that I said ‘number’. What if it user enters
a string and then it leads to a TypeError?

23. To those who have finished:
• Try to make your calculator (assignment from
earlier today) mimic a real life calculator more.
• That is, make it capable of storing the result of
a calculation and asking the user what
operation to do with that, and so on; until the
user types ‘end’ or something.

24. Thanks!
Pranav S Bijapur,
ECE, BMSCE, Bangalore
b.pranavshankar@gmail.com
Telegram - @pranavsb

25. References
• All “(Revision of)” slides were taken from class
1 presentation made by me on 11-9-15
• [1] – xkcd webcomic - https://xkcd.com/1425/
• [2] – idea by Tarun Verma
• [3] – modified form of something Tarun Verma
said