---
title: Flow Control
description: The basics of python. We discuss about flow control here.
---

## Comparison Operators

| Operator | Meaning |
| -------- | ------------------------ |
| `==` | Equal to |
| `!=` | Not equal to |
| `<` | Less than |
| `>` | Greater Than |
| `<=` | Less than or Equal to |
| `>=` | Greater than or Equal to |

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

Examples:

>>> 42 == 42
True

>>> 40 == 42
False

>>> 'hello' == 'hello'
True

>>> 'hello' == 'Hello'
False

>>> 'dog' != 'cat'
True

>>> 42 == 42.0
True

>>> 42 == '42'
False

## Boolean (Logical) Operators

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

The `and` Operator’s _Truth_ Table:

| Expression | Evaluates to |
| ----------------- | ------------ |
| `True and True` | `True` |
| `True and False` | `False` |
| `False and True` | `False` |
| `False and False` | `False` |

The `or` Operator’s _Truth_ Table:

| Expression | Evaluates to |
| ---------------- | ------------ |
| `True or True` | `True` |
| `True or False` | `True` |
| `False or True` | `True` |
| `False or False` | `False` |

The `not` Operator’s _Truth_ Table:

| Expression | Evaluates to |
| ----------- | ------------ |
| `not True` | `False` |
| `not False` | `True` |

## Mixing Operators

You can mix boolean and comparison operators:

>>> (4 < 5) and (5 < 6)
True

>>> (4 < 5) and (9 < 6)
False

>>> (1 == 2) or (2 == 2)
True

Also, you can mix 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
True

## `if` Statements

The `if` statement evaluates an expression, and if that expression is `True`, it then executes the following indented code:

>>> name = 'Debora'

>>> if name == 'Debora':
...    print('Hi, Debora')
...
# Hi, Debora

>>> if name != 'George':
...    print('You are not George')
...
# You are not George

The `else` statement executes only if the evaluation of the `if` and all the `elif` expressions are `False`:

>>> name = 'Debora'

>>> if name == 'George':
...    print('Hi, George.')
... else:
...    print('You are not George')
...
# You are not George

Only after the `if` statement expression is `False`, the `elif` statement is evaluated and executed:

>>> name = 'George'

>>> if name == 'Debora':
...    print('Hi Debora!')
... elif name == 'George':
...    print('Hi George!')
...
# Hi George!

the `elif` and `else` parts are optional.

>>> name = 'Antony'

>>> if name == 'Debora':
...    print('Hi Debora!')
... elif name == 'George':
...    print('Hi George!')
... else:
...    print('Who are you?')
...
# Who are you?

## Ternary Conditional Operator

Many programming languages have a ternary operator, which define a conditional expression. The most common usage is to make a terse, simple conditional assignment statement. In other words, it offers one-line code to evaluate the first expression if the condition is true, and otherwise it evaluates the second expression.

<expression1> if <condition> else <expression2>

Example:

>>> age = 15

>>> # this if statement:
>>> if age < 18:
...    print('kid')
... else:
...    print('adult')
...
# output: kid

>>> # is equivalent to this ternary operator:
>>> print('kid' if age < 18 else 'adult')
# output: kid

Ternary operators can be chained:

>>> age = 15

>>> # this ternary operator:
>>> print('kid' if age < 13 else 'teen' if age < 18 else 'adult')

>>> # is equivalent to this if statement:
>>> if age < 18:
...     if age < 13:
...         print('kid')
...     else:
...         print('teen')
... else:
...     print('adult')
...
# output: teen

:::info
:bulb: Note that Python introduce [`match`](https://learnpython.com/blog/python-match-case-statement/) statement as another control statement in Python 3.10 to implement `switch` statement
:::

## `while` Loop Statements

The while statement is used for repeated execution as long as an expression is `True`:

>>> spam = 0
>>> while spam < 5:
...     print('Hello, world.')
...     spam = spam + 1
...
# Hello, world.
# Hello, world.
# Hello, world.
# Hello, world.
# Hello, world.

## `break` Statements

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

>>> while True:
...     name = input('Please type your name: ')
...     if name == 'your name':
...         break
...
>>> print('Thank you!')
# Please type your name: your name
# Thank you!

It also works for the `for` loop

>>> for i in range(5):
...     if i == 3:
...         break
...     print(i)
# 0
# 1
# 2

## `continue` Statements

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

>>> while True:
...     name = input('Who are you? ')
...     if name != 'Joe':
...         continue
...     password = input('Password? (It is a fish.): ')
...     if password == 'swordfish':
...         break
...
>>> print('Access granted.')
# Who are you? Charles
# Who are you? Debora
# Who are you? Joe
# Password? (It is a fish.): swordfish
# Access granted.

It also works for the `For` loop

>>> for i in range(5):
...     if i == 3:
...         continue
...     print(i)
# 0
# 1
# 2
# 4

## `For` loop

The `for` loop iterates over an iterable object like `range()`, `list`, `tuple`, `dictionary`, `set` or `string`:

>>> pets = ['Bella', 'Milo', 'Loki']
>>> for pet in pets:
...     print(pet)
...
# Bella
# Milo
# Loki

## The `range()` function

The `range()` function returns a sequence of numbers. It starts from 0, increments by 1, and stops before a specified number:

>>> for i in range(5):
...     print('Will stop at 5! or 4 ?'+str(i))
...
# Will stop at 5! or 4? 0
# Will stop at 5! or 4? 1
# Will stop at 5! or 4? 2
# Will stop at 5! or 4? 3
# Will stop at 5! or 4? 4

The `range()` function can also modify it's 3 defaults arguments. The first two 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.

# range(start, stop, step)
>>> for i in range(0, 10, 2):
...    print(i)
...
# 0
# 2
# 4
# 6
# 8

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)
...
# 5
# 4
# 3
# 2
# 1
# 0

## Ending a Program with `sys.exit()`

`exit()` function allows exiting Python.

>>> import sys

>>> while True:
...     feedback = input('Type exit to exit: ')
...     if feedback == 'exit':
...         print('You typed '+ feedback)
...         sys.exit()
...
# Type exit to exit: open
# Type exit to exit: close
# Type exit to exit: exit
# You typed exit

## The [random](https://docs.python.org/3/library/random.html) module

## `seed()`

The `seed` method is used to initialize the random number generator.

>>> random.seed(1)
>>> random.random()
# 0.13436424411240122

Setting the seed to a number will always return the same random number:

>>> random.seed(1)
>>> random.random()
# 0.13436424411240122

>>> random.seed(1)
>>> random.random()
# 0.13436424411240122

>>> random.seed(1)
>>> random.random()
# 0.13436424411240122

>>> random.seed(1)
>>> random.random()
# 0.13436424411240122

>>> random.seed(1)
>>> random.random()
# 0.13436424411240122

## `randint()`

random.randint(start: int, stop: int)

This method returns a random number between a given start and stop parameters:

>>> random.randint(1, 5)
# 3
>>> random.randint(1, 5)
# 2
>>> random.randint(1, 5)
# 5
>>> random.randint(1, 5)
# 1
>>> random.randint(1, 5)
# 3
>>> random.randint(1, 5)
# 1

## `shuffle()`

The `shuffle` method takes in an iterable and shuffle it:

>>> my_list = [1, 2, 3, 4]

>>> random.shuffle(my_list)
>>> my_list
# [1, 4, 3, 2]

>>> random.shuffle(my_list)
>>> my_list
# [2, 4, 3, 1]

>>> random.shuffle(my_list)
>>> my_list
# [4, 2, 3, 1]