Python For Loops

1. What is a For Loop?

A loop is a block of code that executes multiple times. A for loop repeats for a defined number of iterations.

for i in range(10):
    print(i)   # prints 0 through 9

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2. The range() Function

Controls how many times and in what way the loop runs.

Single argument — stop only

for i in range(10):   # 0, 1, 2 ... 9

Two arguments — start and stop

for i in range(5, 20):   # 5, 6, 7 ... 19

Three arguments — start, stop, step

for i in range(5, 20, 2):   # 5, 7, 9 ... 19

Important: The stop value is never included — the loop ends as soon as i reaches it.

Negative step (counting down)

for i in range(10, -20, -5):   # 10, 5, 0, -5, -10, -15

Summary

Argument	Role	Default
start	First value of i	0
stop	Loop ends when i reaches this	required
step	Amount added to i each iteration	1

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3. Iterating Through Collections

Lists, tuples, and strings are iterable — you can loop directly through them.

Method 1 — By index (using range + len)

lst = [1, 2, 3, 4, 5]
for i in range(len(lst)):
    print(lst[i])

✅ Have access to the index. ⚠️ More verbose.

Method 2 — By item (direct iteration)

for element in lst:
    print(element)

✅ Clean and simple. ❌ No access to the index.

Method 3 — By index AND item (enumerate)

for i, element in enumerate(lst):
    print(i, element)

✅ Access to both index and element. ✅ Preferred when you need both.

Works the same way for tuples and strings.

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4. Practical Example — Summing Numbers

result = 0
for i in range(1, 11):   # 1 through 10 (stop at 11 to include 10)
    result += i           # same as: result = result + i
print(result)             # → 55

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5. break and continue

break — stop the loop immediately

lst = [1, 2, 3, 3, 4, 4, 1, 2]
for num in lst:
    if num == 4:
        break        # exits the loop as soon as 4 is seen
    print(num)
# Output: 1 2 3 3

continue — skip the current iteration

for num in lst:
    if num == 4:
        continue     # skips printing 4, keeps looping
    print(num)
# Output: 1 2 3 3 1 2

Keyword	Effect
break	Exits the loop entirely
continue	Skips to the next iteration

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6. Nested For Loops

A for loop inside another for loop. The inner loop runs completely for each iteration of the outer loop.

for i in range(10):
    for j in range(10):
        print(j)
# print runs 10 × 10 = 100 times

⚠️ Name iterator variables differently (e.g. i, j, w) to avoid confusion.

Iterating through a nested list

lst = [[1, 2], [3, 4], [5, 6], [7, 8]]

for i in range(len(lst)):
    interior = lst[i]
    for j in range(len(interior)):
        print(interior[j])
# Output: 1 2 3 4 5 6 7 8

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7. Common Mistake — Declaring Variables Inside the Loop

# ❌ WRONG — list gets reset on every iteration
for i in range(3):
    numbers = []          # resets to empty each time!
    numbers.append(int(input("Enter a number: ")))

# ✅ CORRECT — declare the list BEFORE the loop
numbers = []
for i in range(3):
    numbers.append(int(input("Enter a number: ")))
print(numbers)

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8. The pass Keyword

A placeholder that does nothing — prevents errors when a loop body is empty.

for i in range(10):
    pass    # valid, does nothing

Without pass, an empty loop body causes an IndentationError.

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9. For-Else Statement

The else block runs only if the loop was NOT exited with break.

words = ("hello", "name", "world")
target = "name"

for word in words:
    if word == target:
        print("Found the word!")
        break
else:
    print("Didn't find the word.")

• break encountered → else block is skipped
• Loop finishes normally → else block runs

This removes the need for a separate found = False boolean flag.

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Cheat Sheet

# Basic loop
for i in range(10):          # 0–9
for i in range(2, 10):       # 2–9
for i in range(0, 10, 2):    # 0, 2, 4, 6, 8

# Iterating collections
for element in lst:                    # by item
for i in range(len(lst)):              # by index
for i, element in enumerate(lst):      # both

# Loop control
break       # exit loop immediately
continue    # skip to next iteration
pass        # do nothing (placeholder)

# For-else
for x in collection:
    if condition:
        break
else:
    # runs only if break was never hit