Python 6a – For Loops

Overview

Iteration—repeating a set of instructions—is fundamental in programming. In Python, a for loop runs a block of code a specific number of times, usually via range() which generates the sequence of integers for your loop variable.

Simple for loops (single value)

A basic for loop repeats the indented code a fixed number of times:

for i in range(5):
    print("This is a loop!")

Counting with the loop variable

range(5) starts at 0 and ends at 4:

for i in range(5):
    print("Loop number", i)

Start counting at 1 by specifying the start value:

for i in range(1, 6):
    print("Loop number", i)

Using a step value

Control the increment (or decrement) with the third value in range():

# Even numbers from 2 to 10
for i in range(2, 11, 2):
    print(i)

Count backwards with a negative step:

for i in range(10, 0, -1):
    print(i)

User-defined loops

Let the user decide how many times to loop and what to print:

loopnum = int(input("Times to repeat: "))
word = input("Word to repeat: ")

for i in range(loopnum):
    print(word)

Practice tasks

Task 1 – Repeat your name

Create a loop that prints your name 20 times.

Task 2 – Are we there yet?

Print “Are we there yet?” 150 times.

Task 3 – 100 to 150

Print every number from 100 to 150 (inclusive):

for i in range(100, 151):
    print(i)

Task 4 – Even numbers 10 to 30

Use an appropriate step to print even numbers from 10 to 30:

for i in range(10, 31, 2):
    print(i)

Task 5 – Countdown

Use a negative step to print a countdown from 10 to 1:

for i in range(10, 0, -1):
    print(i)

Task 6 – Many Happy Birthdays

Ask the user for their age and then print “Happy Birthday!” that many times.

Task 7 – House number and name

Ask for a house number and a name, then print the name as many times as the number.