GOTOphobia Considered Harmful (In C)

Re:GOTOphobia Considered Harmful (In C) by thaumasiotes on Sunday February 26, 2023 @08:02PM (#34951703)

> The first program listing is on page 24 of the PDF. Try to follow the logic of the program. Why does line 400 go to 280? What paths can lead to line 400? Who knows!

Without looking at the post-program material, this isn't exactly a difficult question to answer.

Line 400 is preceded by some print statements:

    370    PRINT "PRESS 'E' TO END, SPACE TO CONTINUE"
    380    GET R$:IF R$="" THEN [goto] 380
    390    IF R$="E" THEN [goto] 120
    400    L=0:GOTO 280

So we have a prompt that says "press E to end, space to continue", and then branches one of three ways: if you provide no input, the prompt is shown again; if you provide an E, the entire program restarts from scratch, and if you do anything other than that, the count of lines drawn on screen is reset to 0 and the next 18 lines of the chart are drawn.

We can assume that line 400 will be hit whenever a piece of chart is drawn to the screen.

The program's structure here is a nested loop: there is a loop between lines 280 and 400 (displaying the chart indefinitely, 18 lines at a time) containing another loop between lines 300 and 360 (displaying 18 lines of a chart, one line at a time).

Why is this supposed to be an example of spaghetti code?

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Re:GOTOphobia Considered Harmful (In C) by j16sdiz on Sunday February 26, 2023 @11:00PM (#34952804)

Now imagine you have got a typo:

    400  L=0:GOTO 290

This would be almost impossible to debug.

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Re:GOTOphobia Considered Harmful (In C) by thaumasiotes on Monday February 27, 2023 @04:06AM (#34954454)

That would correspond to the following C:

    /* 280 */
    do_something();
    for (;;) {
      /* 290 */
      /* display chart in blocks of 18 lines */
      /* 400 */
      L = 0;
    }

when the correct code is this:

    for (;;) {
      /* 280 */
      do_something();
      /* 290 */
      /* display chart in blocks of 18 lines */
      /* 400 */
      L = 0;
    }

The bug is that the call to do_something() precedes the outer loop when it should be inside the loop.

Is that easier to debug in C than it is in the BASIC program? What's the difference?

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