Python Loops Tutorial

Loops are used to repeatedly execute a block of code. In Python, there are mainly two types of loops:

For Loops
While Loops

For Loops

The for loop in Python is used to iterate over a sequence (list, tuple, dictionary, set, or string) or other iterable objects.

for i in range(5):
    print(i)

Question: What will be the output if we change range(5) to range(2, 5)?

Answer: The output will start from 2 and end at 4 (not including 5).

for i in range(2, 5):
    print(i)

2
3
4

While Loops

The while loop in Python is used to execute a block of code as long as the condition is true.

i = 0
while i < 5:
    print(i)
    i += 1

Question: What will happen if we forget to increment the variable i inside the while loop?

Answer: If we forget to increment the variable i, the condition i < 5 will always be true, resulting in an infinite loop.

Nested Loops

Loops can be nested inside each other. This means you can have a loop inside another loop.

for i in range(3):
    for j in range(2):
        print(f"i={i}, j={j}")

i=0, j=0
i=0, j=1
i=1, j=0
i=1, j=1
i=2, j=0
i=2, j=1

Question: How many times will the inner loop execute?

Answer: The inner loop will execute 2 times for each iteration of the outer loop. Since the outer loop runs 3 times, the inner loop will run a total of 6 times.

Loop Control Statements

Python provides various loop control statements to change the loop's execution flow:

break: Terminates the loop statement and transfers execution to the statement immediately following the loop.
continue: Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating.
pass: A null statement, used as a placeholder in loops.

Break Example

for i in range(5):
    if i == 3:
        break
    print(i)

0
1
2

Continue Example

for i in range(5):
    if i == 3:
        continue
    print(i)

Pass Example

for i in range(5):
    if i == 3:
        pass
    print(i)

Questions and Exercises

Question: Write a Python program to print the first 10 Fibonacci numbers using a while loop.

Answer:

a, b = 0, 1
count = 0
while count < 10:
    print(a)
    a, b = b, a + b
    count += 1

Question: How can you use a for loop to iterate over a dictionary's keys and values?

Answer: You can use the items() method to get key-value pairs.

my_dict = {'a': 1, 'b': 2, 'c': 3}
for key, value in my_dict.items():
    print(f"{key}: {value}")

a: 1
b: 2
c: 3