control_flow.md

Control flow

When a Python program is run, the code is executed from top to bottom. The flow of the
program can be altered with various keywords, including if/else, for, while, and match.

The control flow structures can be used to executed code conditionally or multiple times.

The if statement

The if statement has the following general form:

if expression:
    statements

The if keyword is used to check if an expression is true. If it is true, a statement is
then executed. The statement can be a single statement or a compound statement. A compound
statement consists of multiple statements enclosed by the block.

#!/usr/bin/python

import random

r = random.randint(-5, 5)

print(r)

if r > 0:
    print('The r variable is positive')

A random number is generated. If the number is greater than zero, we print a message to the terminal.

r = random.randint(-5, 5)

We generate a random integer between -5 .. 5.

if r > 0:
    print('The r variable is positive')

Using the if keyword, we check if the generated number is greater than zero. The if keyword is
followed by an expression that evaluates to a boolean value. If the boolean value is True then the
block is executed. In our case, the string The r variable is positive is printed to the terminal.
If the random value is negative, nothing is done.

The else statement

We can use the else keyword to create a simple branch. If the expression following the if keyword
evaluates to False, the block following the else keyword is automatically executed.

#!/usr/bin/python

import random

r = random.randint(-5, 5)

print(r)

if r > 0:
    print('The r variable is positive')
else:
    print('The r variable is negative or zero')

Either the block following the if keyword is executed or the block following the else keyword.

The elif statement

We can create multiple branches using the elif keyword. It tests for another condition if and only if
the previous condition was not met. Note that we can use multiple elif keywords in our tests.

#!/usr/bin/python

import random

r = random.randint(-5, 5)

print(r)

if r > 0:
    print('The r variable is positive')
elif r == 0:
    print('The r variable is zero')
else:
    print('The r variable is negative')

If the first condition evaluates to True, e.g. the entered value is less than zero, the first block
is executed and the remaining two blocks are skipped. If the first condition is not met, then the
second condition following the if else keywords is checked. If the second condition evaluates to True,
the second block is executed. If not, the third block following the else keyword is executed.
The else block is always executed if the previous conditions were not met.

The while statement

The while keyword is used to create a cycle. The statements inside the while loop are executed until
the expression evaluates to False.

#!/usr/bin/python

# while_kwd.py

numbers = [22, 34, 12, 32, 4]
mysum = 0

i = len(numbers)

while i != 0:

   i -= 1
   mysum = mysum + numbers[i]

print("The sum is:", mysum)

We want to calculate the sum of all values in the numbers list. We utilize the while loop.
We determine the length of the list. The while loop is executed over and over again, until the i is equal
to zero. In the body of the loop, we decrement the counter and calculate the sum of values.

The break statement

The break keyword is used to interrupt the cycle if needed.

#!/usr/bin/python

# break_kwd.py

import random

while True:

    val = random.randint(1, 30)
    print(val, end=" ")

    if val == 22:
        break

print()

In our example, we print random integer numbers. If the number equals to 22, the cycle is interrupted
with the break keyword. The while True creates an endless cycle. The break is used to jump out of
this endless cycle.

The continue statement

The continue statement is used to interrupt the current cycle without jumping out of the whole cycle.
It initiates a new cycle.

#!/usr/bin/python

# continue_kwd.py

num = 0

while num < 1000:

      num = num + 1

      if num % 2 == 0:
         continue

      print(num, end=" ")

print()

In the example we print all numbers smaller than 1000 that cannot be divided by number 2 without a remainder.

The for statement

The for/in keywords are used to iterate over items of a collection in order that they appear in the container.

#!/usr/bin/python

# for_kwd.py

lyrics = """
Are you really here or am I dreaming
I can't tell dreams from truth
for it's been so long since I have seen you
I can hardly remember your face anymore
"""


for i in lyrics:

    print(i, end=" ")

In the example, we have a lyrics variable having a strophe of a song. We iterate over the text and print
the text character by character. The comma in the print statement prevents from printing each character
on a new line.

The for statement has a rich syntax and it is covered in a separate section in a more detail.

Pattern match

Pattern matching is a powerful control flow construct that allows us to compare a value against
a series of patterns and executing code based on which pattern matches. It is a much more
advanced construct than the if/else statements.

#!/usr/bin/python

grades = ['A', 'B', 'C', 'D', 'E', 'F', 'FX']

for grade in grades:

    match grade:
        case 'A' | 'B' | 'C' | 'D' | 'E' | 'F':
            print('passed')
        case 'FX':
            print('failed')

We have a list of grades. For A throug F grades, we pass the example.
For the FX grade, we fail the exam.

grades = ['A', 'B', 'C', 'D', 'E', 'F', 'FX']

We define a list of grades.

for grade in grades:

First, we go over the list with a for loop.

match grade:
    case 'A' | 'B' | 'C' | 'D' | 'E' | 'F':
        print('passed')
    case 'FX':
        print('failed')

In each for cycle, we match a value agains the given patterns. The first case branch
matches against several values separated with |. In the second branch, we match
agains single value 'FX'.

$ ./main.py  
passed
passed
passed
passed
passed
passed
failed