while Loops with if Statements

Instructions

Quick review

n = 1
while n <= 4:
    if n % 2 == 0:     # 4 spaces
        print(n, "even")   # 8 spaces
    else:
        print(n, "odd")
    n += 1             # back to 4 spaces

count = 0
n = 1
while n <= 20:
    if n % 5 == 0:
        count += 1     # only when True
    n += 1             # every time
print(count)           # 4

age = int(input("Age: "))
while age < 1 or age > 120:
    print("Not a real age.")
    age = int(input("Age: "))
print("Thanks!")

• The while body is indented 4 spaces. The body of an if inside the loop is indented 8 spaces. The indentation tells Python which lines belong to which block.
• The update (like n += 1) must run every time through the loop, so it lines up with the if (4 spaces), not inside it. If it only runs when the if is True, the loop can get stuck forever.
• % gives the remainder. n % 2 == 0 means n is even. n % 3 == 0 means n is a multiple of 3.
• and is True only when both sides are True. or is True when at least one side is True. not flips True and False.
• Each side of and / or must be a full comparison: write x == "yes" or x == "y", not x == "yes" or "y".
• Python checks if / elif from the top and runs only the first True branch. else never has a condition.

Messages you may see in the terminal

• Stopped: this loop ran 1000 times... means an infinite loop: the condition never becomes False. Check that the update is indented to line up with the if (not inside it), and that it moves the right way. (Real Python keeps going until you press Ctrl + C, which shows KeyboardInterrupt.)
• Stopped: the program asked for more input than the user typed means a loop that asks with input() kept going after it should have stopped. Check the loop condition.
• SyntaxError: invalid syntax. Maybe you meant '==' ... means a single = was used in a condition. Use == to compare.
• SyntaxError: expected ':' means a while, if, elif, or else line is missing its colon, or an else has a condition after it.
• No error at all is possible too! A wrong operator or a wrong and / or just prints the wrong thing. Compare your output with the expected output.

How this works

1. Enter your first name, last name initial, and class period, then click Start Assignment. Your start time is recorded.
2. The assignment opens in a Python editor with four files (tabs). Each file holds several small programs. Each orange numbered box or drop down is one item. There are 20 items.
3. Click ▶ Run to run every program in that file. Programs that use input() get their answers typed for you (the comment says what the user types). Run does not grade you, so compare with the expected output.
4. When you finish all four tabs, click Submit for Score. Your end time is recorded and your score appears. Answers cannot be changed after submitting.

Student Information