Teacher note: “Waiting” while

Researchers have found that some students hold the following misconception: if statements “wait” for their conditional statement to become true. When that happens (at whichever point in the program), the corresponding block of code is executed.

This misconception might also appear in students’ understanding of the while loop: the loop body is executed whenever its condition becomes true.

Activity: CodeLens 1 (WaitingWhile1)

In the above example, the student might predict that the while loop will execute as soon as size is set to 0 at the end of the program, displaying the following output:

1
2
3
4

On the contrary, the while loop is never executed in the example, since size is not less than 5 at the point of evaluation of its conditional statement. The loop is never returned to even after size is set to 0.

To find out if students hold these misconceptions, show these program examples and ask them to predict the output. Alternatively, observe them as they trace through these examples by hand line-by-line. Then, have them compare their predictions to results from tracing the code using CodeLens, and, if different, ask them to try and explain why. Emphasize the sequential flow of execution through the program.

Let’s say that you showed Sam this program that simulated a thermostat of an air conditioner:

targetTemperature = 75
roomTemperature = 90
while roomTemperature > targetTemperature:
    roomTemperature = roomTemperature - 2

targetTemperature = 64
print(roomTemperature)

csp-8-5-2: If Sam had the “waiting” while misconception, what would he predict the output would be?

• 74
• This is the actual value to be printed. If Sam predicted 74, then he has a correct understanding of the while loop.
• 64
• If Sam held this misconception, he would predict that the while loop would run twice: the second time when targetTemperature is set to 64.
• 75
• If Sam chose this option, he might have a correct understanding of the while loop, but his arithmetic will be slightly off: since the program reduces roomTemperature by 2 each time, and since roomTemperature starts at 90, an even number, we expect the final roomTemperature value to be an even number as well, not 75.

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