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back to the lessonWhich values does the while loop show?
importance: 4
For every loop iteration, write down which value it outputs and then compare it with the solution.
Both loops alert the same values, or not?
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The prefix form ++i:
let i = 0;
while (++i < 5) alert( i );
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The postfix form i++
let i = 0;
while (i++ < 5) alert( i );
The task demonstrates how postfix/prefix forms can lead to different results when used in comparisons.
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From 1 to 4
let i = 0;
while (++i < 5) alert( i );
The first value is i = 1, because ++i first increments i and then returns the new value. So the first comparison is 1 < 5 and the alert shows 1.
Then follow 2, 3, 4… – the values show up one after another. The comparison always uses the incremented value, because ++ is before the variable.
Finally, i = 4 is incremented to 5, the comparison while(5 < 5) fails, and the loop stops. So 5 is not shown.
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From 1 to 5
let i = 0;
while (i++ < 5) alert( i );
The first value is again i = 1. The postfix form of i++ increments i and then returns the old value, so the comparison i++ < 5 will use i = 0 (contrary to ++i < 5).
But the alert call is separate. It’s another statement which executes after the increment and the comparison. So it gets the current i = 1.
Then follow 2, 3, 4…
Let’s stop on i = 4. The prefix form ++i would increment it and use 5 in the comparison. But here we have the postfix form i++. So it increments i to 5, but returns the old value. Hence the comparison is actually while(4 < 5) – true, and the control goes on to alert.
The value i = 5 is the last one, because on the next step while(5 < 5) is false.