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Control Flow

Loops: for, while & range

Iterate with for over any sequence, count with range(), repeat with while, control loops with break, continue and the loop else clause, and write the classic loop programs.

4 min read7 runnable examples4-question quiz

for loops

Python's for loops over the items of any iterable — a list, string, range, dictionary, file …

Python
for subject in ["C", "DSA", "Python"]:
    print("Studying", subject)

for ch in "abc":
    print(ch.upper(), end=" ")
print()
Output
Studying C
Studying DSA
Studying Python
A B C
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range()

range(start, stop, step) produces numbers from start up to but not including stop:

Python
print(list(range(5)))           # 0..4
print(list(range(2, 10, 3)))    # 2, 5, 8
print(list(range(10, 0, -2)))   # counting down
for i in range(1, 6):
    print(f"7 x {i} = {7 * i}")
Output
[0, 1, 2, 3, 4]
[2, 5, 8]
[10, 8, 6, 4, 2]
7 x 1 = 7
7 x 2 = 14
7 x 3 = 21
7 x 4 = 28
7 x 5 = 35
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enumerate and zip

Python
names = ["Asha", "Ravi", "Meera"]
marks = [91, 78, 85]
for i, name in enumerate(names, start=1):
    print(i, name)
for name, m in zip(names, marks):
    print(f"{name:6} {m}")
Output
1 Asha
2 Ravi
3 Meera
Asha   91
Ravi   78
Meera  85
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while loops

Use while when you do not know in advance how many times to repeat:

Python
total = 0
count = 0
n = int(input("Number (0 to stop): "))
while n != 0:
    total += n
    count += 1
    n = int(input("Number (0 to stop): "))
print(f"{count} numbers, sum {total}")
Output
Number (0 to stop): 5
Number (0 to stop): 12
Number (0 to stop): -3
Number (0 to stop): 0
3 numbers, sum 14
Sample input is includedTry it Yourself

break, continue and loop-else

  • break leaves the loop immediately.
  • continue skips to the next iteration.
  • A loop's else block runs only if the loop finished without break — perfect for searches.
Python
def is_prime(n):
    if n < 2:
        return False
    for d in range(2, int(n ** 0.5) + 1):
        if n % d == 0:
            break                  # found a divisor
    else:
        return True                # no break: no divisor found
    return False

print([n for n in range(1, 40) if is_prime(n)])

for n in range(1, 11):
    if n % 3 == 0:
        continue
    print(n, end=" ")
print()
Output
[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37]
1 2 4 5 7 8 10
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Classic loop programs

Python
# Fibonacci
a, b = 0, 1
fib = []
for _ in range(12):
    fib.append(a)
    a, b = b, a + b
print("Fibonacci:", fib)

# Sum of digits and reverse
n = 40213
digits = 0
rev = 0
while n > 0:
    digits += n % 10
    rev = rev * 10 + n % 10
    n //= 10
print("digit sum:", digits, "reversed:", rev)

# Armstrong numbers
arm = [n for n in range(1, 1000) if n == sum(int(d) ** len(str(n)) for d in str(n))]
print("Armstrong:", arm)

# GCD with Euclid's algorithm
x, y = 84, 36
while y:
    x, y = y, x % y
print("gcd(84, 36) =", x)
Output
Fibonacci: [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]
digit sum: 10 reversed: 31204
Armstrong: [1, 2, 3, 4, 5, 6, 7, 8, 9, 153, 370, 371, 407]
gcd(84, 36) = 12
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Pattern printing

String multiplication makes patterns much shorter than in C:

Python
n = 5
for i in range(1, n + 1):
    print("* " * i)
print()
for i in range(1, n + 1):
    print(" " * (n - i) + "*" * (2 * i - 1))
print()
for i in range(1, n + 1):
    print(" ".join(str(j) for j in range(1, i + 1)))
Output
* 
* * 
* * * 
* * * * 
* * * * * 

    *
   ***
  *****
 *******
*********

1
1 2
1 2 3
1 2 3 4
1 2 3 4 5
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Test yourself

4 questions · pick an answer to check it instantly

  1. 1

    What does list(range(3, 12, 4)) produce?

  2. 2

    When does a for loop's else block run?

  3. 3

    How many times does this print?

    Python
    i = 10
while i > 0:
    print(i)
    i -= 3
  4. 4

    What does print("ab" * 3) output?

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