Labels and goto — Maple v0.0.10beta Documentation

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# Labels and `goto`

A label gives a name to a line of code within a function. You can label a line by writing a name for it, then a colon (`:`), before the line starts. The `goto` keyword allows program flow to transfer to a labeled line from anywhere within the same function.

Warning

The use of `goto` is discouraged in C and C++ programming. It is never necessary to use `goto` to write a program.

Unless you know what you’re doing, using `goto` tends to encourage code which is harder to debug and understand than programs without `goto` that do the same thing. That said, however, it’s sometimes useful; see below for a concrete example.

## Using Labels and goto

Labels and `goto` are probably best explained through example. Let’s start with an example of how to label lines. The first line (`int x = analogRead(some_pin);`) in the loop function below has label `readpin`. The third line (`delay(x);`) has label `startdelay`. The second line (`SerialUSB.println(x);`) does not have a label:

    void loop() {
    readpin:
           int x = analogRead(some_pin);
           SerialUSB.println(x); // for debugging
    startdelay:
           delay(x);
           // ... more code ...
    }

Anything which can be a variable name can be a label.

Let’s say that we wanted to print `x` only if it was very large, say at least 2000. We might want to do this just so anybody watching on a serial monitor would know they were in for a longer wait than usual. We can accomplish this through the use of a `goto` statement that skips the printing if `x` is less than 2000:

    void loop() {
    readpin:
           int x = analogRead(some_pin);
           if (x < 2000) {
               goto startdelay;
           }
           SerialUSB.println(x); // for debugging
    startdelay:
           delay(x);
           // ... more code ...
    }

In this modified program, whenever `x` is less than 2000, the body of the if statement in the second line is executed. The `goto` statement inside the `if` body skips straight to the line labeled `startdelay`, passing over the line doing the printing.

A `goto` does not have to “move forwards”; it can go “backwards”, too. For example, the following program prints “5” forever (why?):

    void loop() {
    printfive:
        SerialUSB.println(5);
        goto printfive;
        SerialUSB.println(6);
    }

## When to Use goto

As mentioned above, use of `goto` is generally discouraged. However, when used with care, `goto` can simplify certain programs. One important use case for `goto` is breaking out of deeply nested for loops or if logic blocks. Here’s an example:

    for(int r = 0; r < 255; r++) {
        for(int g = 255; g > -1; g--) {
            for(int b = 0; b < 255; b++) {
                if (analogRead(0) > 250) {
                    goto bailout;
                }
                // more statements ...
            }
            // innermost loop ends here
        }
    }
    bailout:
    // more code here

In the above example, whenever the analog reading on pin 0 was greater than 250, the program would jump to the line labeled `bailout`, exiting all three loops at once.

While there is already a break keyword for breaking out of a loop, it will only break out of the innermost loop. So, if instead of saying “`goto bailout;`”, there was a “`break;`” instead, the program would only exit from the loop with header “`for(int b = 0; b < 255; b++)`”. The program would continue at the line which reads “`// innermost loop ends here`”, which is clearly undesirable if you wanted to leave all three loops at once.

More examples of when `goto` is a good choice are given in Donald Knuth’s paper, “Structured Programming with go to Statements”; see below for a link.

## See Also

- Dijkstra, Edsger W. Go To Statement Considered Harmful (PDF)
- Knuth, Donald. Structured Programming with go to Statements (PDF)

License and Attribution

Portions of this page were adapted from the Arduino Reference Documentation, which is released under a Creative Commons Attribution-ShareAlike 3.0 License.

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