Master Python Control Flow: If, Loops, and List Comprehensions Explained

1 Conditional Statements (if)

1.1 Simple if

An if statement evaluates a condition; if the condition is true, the following block runs, otherwise it is skipped.

>> n = 10
>>> if n > 5:  # because 10 > 5, condition true
...     print('n>5')
... 
>>> if n > 50:  # because 10 < 50, condition false
...     print('n>5')

1.2 if … else

>> n = 10
>>> if n > 50:  # false, go to else
...     print('n>50')
... else:
...     print('n<=50')
>>> n<=50

1.3 if … elif … else

Combining if, elif, and else allows multiple mutually‑exclusive branches.

>> n = 10
>>> if n > 50:
...     print('n>50')
>>> elif n > 20:
...     print('20<n<=50')
>>> elif n > 10:
...     print('10<n<=20')
>>> else:
...     print('n<=10')
>>> n<=10

2 Loop Statements

2.1 for Loop

A for loop iterates over each element of a sequence.

>> for i in [1,3,5,7]:
...     print(i)
1
3
5
7

>> for i in [1,3,5,7]:
...     print(i**2)  # square of i
1
9
25
49

>> s = 0
>>> for i in [1,3,5,7]:
...     s = s + i
...     print(i, s)
(1, 1)
(3, 4)
(5, 9)
(7, 16)

for

can be combined with if to filter items.

>> for i in [1,3,5,7]:
...     if i > 4:
...         print(i)
5
7

2.2 while Loop

A while loop repeats as long as its condition remains true.

>> n = 10
>>> while n > 5:
...     print('n>5')

The above would loop forever, so we decrement n inside the loop.

>> n = 10
>>> while n > 5:
...     print('n>5')
...     n = n - 1  # decrease each iteration

2.3 continue

continue

skips the rest of the current iteration and proceeds to the next.

>> for i in [1,2,3,4]:
...     if i % 2 == 0:
...         continue
...     print(i, 'cannot be divided by 2')
1 cannot be divided by 2
3 cannot be divided by 2

2.4 break

break

terminates the loop immediately.

>> for i in [1,2,3,4]:
...     if i % 2 == 0:
...         break
...     print(i, 'cannot be divided by 2')
1 cannot be divided by 2

2.5 range

The range function generates a sequence of integers.

>> list(range(5))
[0, 1, 2, 3, 4]
>>> list(range(1,5))
[1, 2, 3, 4]

Using range to sum numbers from 1 to 100:

>> s = 0
>>> for i in range(101):
...     s = s + i
>>> s
5050

3 List Comprehensions

List comprehensions combine loops and optional conditions to build lists concisely.

>> [i**3 for i in range(1,5)]
[1, 8, 27, 64]

Generating cubes of odd numbers:

>> [i**3 for i in range(1,8) if i % 2 == 1]
[1, 27, 125, 343]