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Which values shows the while?

importance: 4

For every loop, write down which values it shows, in your opinion. And then compare with the answer.

Both loops alert same values or not?

  1. The prefix form ++i:

    let i = 0;
    while (++i < 5) alert( i );
  2. 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.

  1. 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.

  2. 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.