teacher note Teacher note: “Waiting” if

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.

For instance, consider this program:

Activity: CodeLens 1 (WaitingIf1)

If a student understands this correctly, she would say that since size is not equal to 5 when the if statement is evaluated, “Hello” is never printed. The program proceeds to execute the for loop and never returns to the if statement to evaluate that again. Step through the above code using CodeLens to see that the if statement is never executed.

A student who holds the above misconception, however, would say that after size becomes 5 at the end of the loop, the if statement (which had been “waiting” all this time) would see that its condition would now evaluate to true. “Hello” would then be printed.

In this case, the student fails to see that each line of the program is executed in sequence, one line at a time. Instead, she views all lines of the program as being active all at once, monitoring the status of the variables constantly.

Compare the previous program to the following example, in which the intended behavior is to display “Hello” when size becomes 5:

Activity: CodeLens 2 (WaitingIf2)

To find out if students hold this misconception, 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.

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