The GOTO Keyword in LOGO Turtle Programming

ACMer

7 years ago

LogoTurtle is the first and currently only one Chrome Extension for Logo Programming. I personally use this tool to teach my sons the turtle graphics.

This is probably the first version of LOGO that supports GOTO keyword.

How to Install?

Chrome Webstore: https://chrome.google.com/webstore/detail/logo-turtle/dcoeaobaokbccdcnadncifmconllpihp. For Opera browsers, the workaround is to first install Chrome Extension Gadget. And similarly for Firefox, you can install Chrome Store Foxified before you install LogoTurtle.

The GOTO in Logo

Let’s see a example in using GOTO:

rt 10
goto @start
repeat [5 fd 100 rt 144]

@start
make "i 0

@hello
fd 100
rt 90
make "i :i+1
if :i<4  [goto @hello] 

fd 100

You can define a label using @ symbol, the labels are case-sensitive. Once defined, you can use goto keyword to jump to that location whenever needed.

Scan the Source for Labels

The following Javascript code scans the LOGO source and put labels into a hash table – as long as it is not preceded with a goto keyword – see github src.

scanForLabels(s, i, U) {
    let prev = "";
    while (i < U) {
        // jump Comments and white spaces
        i = this.skipTo(s, i, U);
        if (i >= U) {
            break;
        }
        // get next word and next index
        let x = getNextWord(s, i, U);
        i = x.next;
        let word = x.word;
        if (word == '') {
            continue;
        }
        let lword = word.toLowerCase();
        let y = getNextWord(s, i, U);
        let word_next = y.word;		
        if ((lword[0] === '@') && (prev !== 'goto')) { // label
            if (lword.length == 1) {
                this.pushErr(word, LOGO_ERR_INVALID_LABEL);
                return false;
            }
            // save the (label, index) into the hash table
            const label = lword.substring(1);
            this.labels[label] = i;
            continue;
        }		
        i = y.next;
        prev = word.toLowerCase();
    }
    console.log("labels: ", this.labels);
}

Then, when we meet the goto keyword, we can quickly look up the index in the hash table – see github

case "goto":
    if (!word_next.startsWith("@")) { 
        // variable must be noted with double quotes
        this.pushErr(word, LOGO_ERR_INVALID_LABEL);
        return false;
    }
    // get the label name
    let label_1 = word_next.substring(1); 
    // not a valid variable name
    if (!this.labels[label_1]) {
        this.pushErr(word, LOGO_ERR_INVALID_LABEL);
        return false;
    }
    // goto that index
    i = this.labels[label_1];
    break;