For Loops with range() and Iterating Over Collections

A for loop in Python is used to iterate over the elements of a sequence or other iterable (like lists, strings, tuples, dictionaries, sets, etc.), executing a block of code for each element.

Python’s for loop works more like the “for-each” loop in other languages rather than the traditional C-style loop with an index.

This means:

• No need to manually manage an index or increment a counter.

• Python handles stepping through the sequence automatically.

Iterating Over a Sequence

Syntax:

for <variable> in <sequence>:
    <block of code>

• On each iteration:

  • <variable> is assigned the next element from <sequence>.

  • The code block executes using that value.

• Loop continues until all elements are processed.

Example – looping through a list:

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print("I have a", fruit)

Output:

I have a apple
I have a banana
I have a cherry

💡 The loop runs 3 times (once for each fruit). No index like i++ is needed.

Example – looping through a string:

for ch in "PYTHON":
    print(ch, end=" ")

Output:

P Y T H O N

Example – looping through a dictionary:

mydict = {"name": "Alice", "age": 25}
for key, val in mydict.items():
    print(key, ":", val)

Output:

name : Alice
age : 25

💡 Tip:
If you need both the index and value, use enumerate():

for idx, val in enumerate(["a", "b", "c"]):
    print(idx, val)

The range() Function for Numeric Loops

range() generates a sequence of numbers, which you can loop over.

Basic example:

for i in range(5):  # 0 to 4
    print("Iteration", i)

Output:

Iteration 0
Iteration 1
Iteration 2
Iteration 3
Iteration 4

Custom start and step:

print(list(range(2, 6)))      # [2, 3, 4, 5]
print(list(range(0, 10, 2)))  # [0, 2, 4, 6, 8]
print(list(range(10, 0, -1))) # [10, 9, 8, ..., 1]

Example – summing numbers:

total = 0
for n in range(1, 6):  # 1 to 5
    total += n
print("Sum of 1 to 5 is", total)

Output:

Sum of 1 to 5 is 15

Example – ignoring the loop variable:

for _ in range(3):
    print("Hello")

Output:

Hello
Hello
Hello

Break and Continue

• break → Stop the loop immediately.

• continue → Skip current iteration, go to next.

Example – using break & continue:

numbers = [1, 3, 7, 10, 15]
for num in numbers:
    if num == 10:
        print("Found 10! Stopping the search.")
        break
    if num % 2 == 0:
        continue
    print("Odd number:", num)

Output:

Odd number: 1
Odd number: 3
Odd number: 7
Found 10! Stopping the search.

💡 Key points:

• break → jumps out of the loop entirely.

• continue → skips to the next iteration.

• Works in both for and while loops.

Recap:

• Use for item in collection when iterating over items.

• Use for i in range(n) when looping a fixed number of times.

• Use enumerate() if you need an index.

• Use break to stop early, continue to skip iterations.

Mini-Exercise:

1. Loop over the string "LOOP" and print each character on its own line.

2. Use range(1, 11) to print all even numbers from 1 to 10.

3. Write a loop that goes through a list and stops if it finds "STOP".