Introduction to Programming

Loops and conditional statements can be nested

for-loops can be nested within conditional statements, and conditional statements can be nested within loops.

for-loops within conditional statements

Here is the structure of a for-loop nested within an if-statement:

if <condition>:
    for i in range(...):
        # indented code block

In this case, the loop executes if the condition evaluates to True, otherwise it is skipped.

Another possibility is for-loops nested within an if-else statement:

if <condition>:
    for i in range(...):
        # indented code block
else <condition>:
    for i in range(...):
        # indented code block

Examples: Trace the following code examples, confirming the output.

a = 5
b = 1

if a > b:
    for i in range(5):
        print("a")
else:
    for i in range(5):
        print("b")

a
a
a
a
a

a = 5
b = 10

if a > b:
    for i in range(5):
        print("a")
else:
    for i in range(5):
        print("b")

b
b
b
b
b

Conditional statements within for-loops

Here is the structure of an if statement nested within a for-loop:

for i in range(...):
    if <condition>:
        # indented code block

In this case, the condition is checked newly on each iteration of the loop. In some of the iterations it may evaluate True, and then in others it may be False. The check is done independently each time. The indented code block only executes on the iterations where the condition comes out to be True.

Of course, inside the for-loop we may instead have an if-else statement or an if-else ladder.

Examples: trace the following code examples, confirming the output.

for i in range(8):
    if i % 2 == 0:
            print(i)

0
2
4
6

Putting it all together

Here's a complete program that uses an if-else statement nested within a for-loop. Carefully trace the program and predict the image that results. You can run the code to test if you were correct.

"""
Produce a drawing of randomly-placed circles in two colors
Mystery output: Trace the code to find out. Run it to check.
Author: COMP 1351 Instructor
Date:
File: two_colors.py
Course: COMP 1351
Assignment: Preview assignment for for-loops
Collaborators: 1351 Instructors
Internet Sources: None
"""

import dudraw
from random import random

dudraw.set_canvas_size(500, 500)
dudraw.clear(dudraw.LIGHT_GRAY)

for i in range(10000):
    # generate random x and y locations:
    x = random()
    y = random()
    # set color based on position
    if y > 0.5:
        # Magenta is a hot-pink color
        dudraw.set_pen_color(dudraw.MAGENTA)
    else:
        # Cyan is a turquoise color
        dudraw.set_pen_color(dudraw.CYAN)

    # draw the circle at the randomly-chosen location:
    dudraw.filled_circle(x, y, 0.01)
    # outline the circle with a black edge:
    dudraw.set_pen_color(dudraw.BLACK)
    dudraw.circle(x, y, 0.01)

# display the final image until the window is closed
dudraw.show(float('inf'))