for Loops and range()

When the number of repetitions is clear, a for loop is often easier to read than a while loop. It expresses “take values from a sequence one by one” in a single line.

range()

range() generates integer sequences and is commonly used with for.

for i in range(5):
    print(i, end=' ')

print()

for i in range(10, 15):
    print(i, end=' ')

print()

for i in range(1, 10, 2):
    print(i, end=' ')

Output:

0 1 2 3 4
10 11 12 13 14
1 3 5 7 9

range(start, stop, step) starts at start, increases by step, and stops before stop.

Fixed-count accumulation

To sum integers from 1 to 100, use range(1, 101). Since range excludes the stop value, the stop value must be 101.

total = 0

for i in range(1, 101):
    total += i

print("Sum =", total)

Output:

Sum = 5050

This version makes the value range of i easier to see than an equivalent while loop.

01total = 002for i in range(1, 101):03    total += i04print(total)

>>>

Fibonacci sequence

The first two Fibonacci numbers are 1, 1. Starting from the third term, each term equals the sum of the previous two terms.

n = int(input("Enter the number of terms: "))

if n <= 0:
    print("Please enter a positive integer.")
elif n == 1:
    print(1)
else:
    first = 1
    second = 1
    print(first, second, end=' ')

    for i in range(3, n + 1):
        current = first + second
        print(current, end=' ')
        first = second
        second = current

    print()

One possible run:

Enter the number of terms: 10
1 1 2 3 5 8 13 21 34 55

The most important updates are:

first = second
second = current

They slide the two-number window forward, so the next loop can keep using the previous two terms.

01first = 1; second = 102for i in range(3, 11):03    current = first + second04    first = second05    second = current