misconception-if-while.md

Misconception: "Waiting" if and 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.

For instance, consider this program:

size = 0
if size == 5:
    print("Hello")

for i in [1, 2, 3, 4, 5]:
    size = size + 1
    print(size)

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.

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.

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

size = 10
while size < 5:
    size = size + 1

size = 0

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.

A final variant of this misconception is believing that the while loop exits immediately after its condition becomes false. For instance:

size = 0
while size < 5:
    size = size + 1
    print(size)

size becomes 5 at the last iteration of the loop, but before its value is printed. A student who held this misconception would predict that the output of this program would be as follows, leaving out the 5:

1
2
3
4

The correct behavior of while loops is that its condition is only checked every after the loop body is executed completely.

To prevent this misconception, encourage students to trace through the examples line-by-line using CodeLens. This way, the students see the sequential flow of execution through the program.

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