For and While Loops in Python

Loops repeat a block of code. Python has two loops: for goes through every item of a sequence (string, list, dictionary, range), and while repeats as long as a condition is True.

for Loop over a String and a List

x = "Sudheer"
for letter in x:
    print(letter)

numbers = [10, 20, 62, 45]
for item in numbers:
    print(item)

Nested loop over a list of strings

fruits = ["Banana", "Orange", "Apple", "Mango", "Papaya"]
for fruit in fruits:
    print(fruit)
    for letter in fruit:      # inner loop goes through each letter
        print(letter)

break: Stop the Loop

break exits the loop as soon as a condition is met. Where you put print decides whether the matching item is printed.

fruits = ["Banana", "Orange", "Apple", "Mango", "Papaya"]

# print first, then check -> Apple IS printed
for fruit in fruits:
    print(fruit)
    if fruit == "Apple":
        break

# check first, then print -> Apple is NOT printed
for fruit in fruits:
    if fruit == "Apple":
        break
    print(fruit)

continue: Skip One Item

continue skips the rest of the current iteration and moves to the next item.

for fruit in fruits:
    if fruit == "Orange":
        continue          # Orange is skipped
    print(fruit)

Looping with range()

for x in range(6):           # 0 to 5 (6 is not included)
    print(x)

for x in range(2, 18):       # 2 to 17
    print(x)

for x in range(2, 20, 2):    # 2, 4, 6 ... 18 (step of 2)
    print(x)

for x in range(10, 0, -1):   # reverse: 10 down to 1
    print(x)

for Loop with else

The else block runs only when the loop finishes normally, that is, without break.

for x in range(50):
    if x == 32:
        break
    print(x)
else:
    print("Loop finished")    # not printed, because the loop was broken at 32

Looping Through a Dictionary

employee = {"name": "Rahul", "age": 30, "salary": 75000}

for key in employee:                  # keys
    print(key)

for value in employee.values():       # values
    print(value)

for key, value in employee.items():   # keys and values
    print(key, ":", value)

Nested Loops

for i in range(1, 4):
    for j in range(1, 4):
        print(i, j)            # 9 combinations

# multiplication tables of 1, 2 and 3
for i in range(1, 4):
    for j in range(1, 11):
        print(i, "x", j, "=", i * j)
    print()

break and continue in nested loops

break and continue affect only the loop they are written in (the inner loop here).

for i in range(1, 4):
    for j in range(1, 4):
        if j == 2:
            break              # leaves the inner loop only
        print(i, j)

for i in range(1, 4):
    for j in range(1, 4):
        if j == 2:
            continue           # skips j == 2 only
        print(i, j)

while Loop

A while loop runs as long as its condition is True. Always change the variable inside the loop, or it never ends.

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

while with break

i = 1
while i <= 5:
    print(i)
    if i == 3:
        break
    i += 1

Nested while loop

i = 1
while i <= 3:
    j = 1
    while j <= 3:
        print(i, j)
        j += 1
    i += 1

while with continue: reading user input

user_input = input("Enter a number: ")
while user_input != "exit":
    if user_input == "skip":
        print("Skipping this iteration")
        user_input = input("Enter a number: ")
        continue
    print("You entered:", user_input)
    user_input = input("Enter a number: ")

Real Example: Login Attempts with while-else

The user gets 3 attempts. The else block runs only if the loop ends without a successful login (no break).

correct_password = "admin123"
attempts = 3

while attempts > 0:
    password = input("Enter password: ")
    if password == correct_password:
        print("Login successful")
        break
    attempts -= 1
    print("Invalid password")
else:
    print("Account locked")