flow-control-in-python

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Flow Control

Comparison Operators

These operators evaluate to True or False depending on the values you give them.

Examples:

42 == 42

40 == 42

'hello' == 'hello'

'hello' == 'Hello'

'dog' != 'cat'

42 == 42.0

42 == '42'

Boolean evaluation

Never use == or != operator to evaluate boolean operation. Use the is or is not operators, or use implicit boolean evaluation.

NO (even if they are valid Python):

True == True

True != False

YES (even if they are valid Python):

True is True

True is not False

These statements are equivalent:

if a is True:
   pass
if a is not False:
   pass
if a:
   pass

And these as well:

if a is False:
   pass
if a is not True:
   pass
if not a:
   pass

Boolean Operators

There are three Boolean operators: and, or, and not.

The and Operator's Truth Table:

The or Operator's Truth Table:

The not Operator's Truth Table:

Mixing Boolean and Comparison Operators

(4 < 5) and (5 < 6)

(4 < 5) and (9 < 6)

(1 == 2) or (2 == 2)

You can also use multiple Boolean operators in an expression, along with the comparison operators:

2 + 2 == 4 and not 2 + 2 == 5 and 2 * 2 == 2 + 2

if Statements

if name == 'Alice':
    print('Hi, Alice.')

else Statements

name = 'Bob'

if name == 'Alice':
    print('Hi, Alice.')
else:
    print('Hello, stranger.')

elif Statements

name = 'Bob'
age = 5

if name == 'Alice':
    print('Hi, Alice.')
elif age < 12:
    print('You are not Alice, kiddo.')

name = 'Bob'
age = 30

if name == 'Alice':
    print('Hi, Alice.')
elif age < 12:
    print('You are not Alice, kiddo.')
else:
    print('You are neither Alice nor a little kid.')

while Loop Statements

spam = 0

while spam < 5:
    print('Hello, world.')
    spam = spam + 1

break Statements

If the execution reaches a break statement, it immediately exits the while loop's clause:

while True:
    print('Please type your name.')
    name = input()
    if name == 'your name':
        break

print('Thank you!')

continue Statements

When the program execution reaches a continue statement, the program execution immediately jumps back to the start of the loop.

while True:
    print('Who are you?')
    name = input()
    if name != 'Joe':
        continue
    print('Hello, Joe. What is the password? (It is a fish.)')
    password = input()
    if password == 'swordfish':
        break

print('Access granted.')

for Loops and the range() Function

print('My name is')
for i in range(5):
    print('Jimmy Five Times ({})'.format(str(i)))

The range() function can also be called with three arguments. The first two arguments will be the start and stop values, and the third will be the step argument. The step is the amount that the variable is increased by after each iteration.

for i in range(0, 10, 2):
   print(i)

You can even use a negative number for the step argument to make the for loop count down instead of up.

for i in range(5, -1, -1):
    print(i)

For else statement

This allows you to specify a statement to execute after the full loop has been executed. Only useful when a break condition can occur in the loop:

for i in [1, 2, 3, 4, 5]:
   if i == 3:
       break
else:
   print("only executed when no item of the list is equal to 3")

Importing Modules

import random

for i in range(5):
    print(random.randint(1, 10))

import random, sys, os, math

from random import *

Ending a Program with sys.exit

import sys

while True:
    print('Type exit to exit.')
    response = input()
    if response == 'exit':
        sys.exit()
    print('You typed {}.'.format(response))