How to format float to omit zeros at the end of the fraction

What I would do:
Build a function doing a loop and check (and cut) last zero of %.3f formatted string.

Until some better idea comes up, these work:

function formatShortFloat(value as Float) as String { // ciq_1_3_0
    var str = value.format(FLOAT_FORMAT_STRING_3);
    var chars = str.toCharArray(); // ciq_1_3_0
    var end;
    for (end = str.length() - 1; chars[end] == '0' && end > 0; --end) {
    }
    if (chars[end] == '.') {
        --end;
    }
    return str.substring(0, end + 1) as String;
}

or if need to work on very old devices < CIQ 1.3.0:

function formatShortFloat(value as Float) as String { // code:-3,optimized code:+5,data:+5
    var str = value.format("%.3f");
    var end = str.length();
    var lastChar;
    do {
        lastChar = str.substring(end - 1, end);
        --end;
    } while ("0".equals(lastChar));
    if (".".equals(lastChar)) {
        --end;
    }
    return str.substring(0, end + 1) as String;
}

Solution adapted from [https://stackoverflow.com/a/22885197]

Probably more code than you'd like.

    // Count the number of decimal digits, disregarding trailing 0s
    // This won't work for if num.toString() happens to be converted
    // to exponential notation (e.g. "1.23e-50") but doesn't seem to happen
    // in Monkey C and it isn't a practical scenario anyway
    //
    // adapted from https://stackoverflow.com/a/22885197
    function numSignificantDecimalDigits(num as Numeric) as Number {
        var str = num.toString();
        var decimalIndex = str.find(".");
        if (decimalIndex == null) {
            return 0;
        }

        // obtain fractional part of the number
        var fractionalStr = str.substring(decimalIndex + 1, str.length());
        if (fractionalStr.toNumber() == 0) {
            return 0;
        }

        // prefix with "1" to handle leading zero
        var n = ("1" + fractionalStr).toNumber();

        while (n != 0 && n % 10 == 0) {
            n /= 10; // kill the 0s at the end of n;
        }
        return Math.floor(Math.log(n, 10)).toNumber(); // get the number of digits
    }

    function formatRemoveTrailingDecimalZeroes(num as Numeric, numDecimals as Numeric) as String {
        var sigDecimals = numSignificantDecimalDigits(num);
        var actualDecimals = sigDecimals < numDecimals ? sigDecimals : numDecimals;
        return num.format("%." + actualDecimals  + "f");
    }

    function test_numSignificantDecimalDigits() {
        System.println("numSignificantDecimalDigits(1): " + numSignificantDecimalDigits(1));
        System.println("numSignificantDecimalDigits(1.0): " + numSignificantDecimalDigits(1.0));
        System.println("numSignificantDecimalDigits(1.001): " + numSignificantDecimalDigits(1.001));
        System.println("numSignificantDecimalDigits(1.23450): " + numSignificantDecimalDigits(1.23450));
        System.println("numSignificantDecimalDigits(1.1234): " + numSignificantDecimalDigits(1.1234));
        System.println("numSignificantDecimalDigits(1.123): " + numSignificantDecimalDigits(1.123));
        System.println("numSignificantDecimalDigits(1.12): " + numSignificantDecimalDigits(1.12));
        System.println("numSignificantDecimalDigits(1.1): " + numSignificantDecimalDigits(1.1));
    }

    function test_formatRemoveTrailingDecimalZeroes() {
        System.println("formatRemoveTrailingDecimalZeroes(1, 3): " + formatRemoveTrailingDecimalZeroes(1, 3));
        System.println("formatRemoveTrailingDecimalZeroes(1.0, 3): " + formatRemoveTrailingDecimalZeroes(1.0, 3));
        System.println("formatRemoveTrailingDecimalZeroes(1.001, 3): " + formatRemoveTrailingDecimalZeroes(1.001, 3));
        System.println("formatRemoveTrailingDecimalZeroes(1.23450, 3): " + formatRemoveTrailingDecimalZeroes(1.23450, 3));
        System.println("formatRemoveTrailingDecimalZeroes(1.1234, 3): " + formatRemoveTrailingDecimalZeroes(1.1234, 3));
        System.println("formatRemoveTrailingDecimalZeroes(1.123, 3): " + formatRemoveTrailingDecimalZeroes(1.123, 3));
        System.println("formatRemoveTrailingDecimalZeroes(1.12, 3): " + formatRemoveTrailingDecimalZeroes(1.12, 3));
        System.println("formatRemoveTrailingDecimalZeroes(1.1, 3): " + formatRemoveTrailingDecimalZeroes(1.1, 3));
    }

Output:

numSignificantDecimalDigits(1): 0
numSignificantDecimalDigits(1.0): 0
numSignificantDecimalDigits(1.001): 3
numSignificantDecimalDigits(1.23450): 4
numSignificantDecimalDigits(1.1234): 4
numSignificantDecimalDigits(1.123): 3
numSignificantDecimalDigits(1.12): 2
numSignificantDecimalDigits(1.1): 1
formatRemoveTrailingDecimalZeroes(1, 3): 1
formatRemoveTrailingDecimalZeroes(1.0, 3): 1
formatRemoveTrailingDecimalZeroes(1.001, 3): 1.001
formatRemoveTrailingDecimalZeroes(1.23450, 3): 1.235
formatRemoveTrailingDecimalZeroes(1.1234, 3): 1.123
formatRemoveTrailingDecimalZeroes(1.123, 3): 1.123
formatRemoveTrailingDecimalZeroes(1.12, 3): 1.12
formatRemoveTrailingDecimalZeroes(1.1, 3): 1.1

...works for me...

        // TEST for shortening a FloatString without zeros at the end:
        var testFloat = 120.00000f ;
        System.println( cutFloatZeros(testFloat.format("%.3f")));

    function cutFloatZeros(floatFigure) as String {
        var floatStr = floatFigure;
         while ( (floatStr.substring(floatStr.length()-1, null )).equals("0") ) {
            floatStr = floatStr.substring(0, floatStr.length()-1);
        }
        if ( (floatStr.substring(floatStr.length()-1, null )).equals(".") ) {
            floatStr = floatStr.substring(0, floatStr.length()-1);
        }
         return floatStr;
    }

Yeah, this is the same algorithm as my second, except for 2 things:

1. it only works on CIQ 3.3.2+ because you pass null to substring

2. I think it's less efficient because it does lot more substrings

Yes, you're right!