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I have a scenario where I have three numbers:

  1. 17
  2. 10
  3. 90

I need to convert those into whole percentage values (so that when added, total 100% as you'd expect). I have this function:

function roundPercentageTotals(num1, num2, num3) {
    var total = num1 + num2 + num3;  // 117

    var num1Total = (num1 / total) * 100;  // 14.529914529914531
    var num2Total = (num2 / total) * 100;  //  8.547008547008546
    var num3Total = (num3 / total) * 100;  // 76.92307692307693

    var num1ToDecimal = num1Total.toFixed(1); // 14.5
    var num2ToDecimal = num2Total.toFixed(1); //  8.5
    var num3ToDecimal = num3Total.toFixed(1); // 76.9

    var totalPercentage = parseInt(num1ToDecimal) + parseInt(num2ToDecimal) + parseInt(num3ToDecimal); // 98

    return { percentage1: Math.round(num1ToDecimal, percentage2: Math.round(num2ToDecimal), percentage3: Math.round(num3ToDecimal) };
}

In my example, the total percentage calculated is 98%. Followed by:

  1. Percentage1 = 15
  2. Percentage2 = 9
  3. Percentage3 = 77

Which adds up to 101%, where am I going wrong?

Thanks for any help in advance!

matisetorm
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Rob
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4 Answers4

6

You're getting 98% in the first calculation because you're rounding the values down, and then getting 101% in your second because you're rounding them up.

Change your parseInt() to parseFloat() to get your totals to be closer to 100% instead of 98%. parseInt() floors decimals, it does not round them.

In regards to your second calculation totaling 101%: By rounding up 14.5 to 15, and 8.5 to 9, you've just added a full 1%. This leaves you with 101% instead of 100%.

The bottom line is that you cannot consistently achieve an even 100% if you're going to round the exact values, unless you fudge your percentages to fit somewhere along the way.

Tyler Roper
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6

Ok, so it looks like mathematically, I cannot achieve exactly what I was looking for. However, I needed to round figures up so it equalled 100% in the end (all be in that some of the figures where rounded, so not totally accurate).

Here's my solution, just in case this is useful to someone else:

function roundPercentageTotals(numArr) {

    // Total of all numbers passed.
    var total = numArr[0] + numArr[1] + numArr[2];

    // Percentage representations of each number (out of 100).
    var num1Percent = Math.round((numArr[0] / total) * 100);
    var num2Percent = Math.round((numArr[1] / total) * 100);
    var num3Percent = Math.round((numArr[2] / total) * 100);

    // Total percent of the 3 numbers combined (doesnt always equal 100%).
    var totalPercentage = num1Percent + num2Percent + num3Percent;

    // If not 100%, then we need to work around it by subtracting from the largest number (not as accurate but works out).
    if (totalPercentage != 100) {
        // Get the index of the largest number in the array.
        var index = getLargestNumInArrayIndex(numArr);

        // Take the difference away from the largest number.
        numArr[index] = numArr[index] - (totalPercentage - 100);

        // Re-run this method recursively, until we get a total percentage of 100%.
        return roundPercentageTotals(numArr);
    }

    // Return the percentage version of the array passed in.
    return [num1Percent, num2Percent, num3Percent];
}

function getLargestNumInArrayIndex(array) {
    return array.indexOf(Math.max.apply(Math, array));
}

Pass an array of the numbers into roundPercentageTotals, such as roundPercentageTotals([13,54,38]) and it will return the whole percentage (or nearest percentage I should say) figures in an array.

Rob
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    It worked for me, but when taking the difference from the largest number I've done `numArr[index] = numArr[index] - Math.abs(totalPercentage - 100);` because when totalPercentage was less than 100 entered in an infinite loop – Angel F. Sep 24 '19 at 10:49
4

percent-round is doing just that. did the job for me. just pass values and get percentages back that always add up to 100%.

const percentRound = require ('percent-round');
percentRound([16, 56, 18]); // [18, 62, 20]
Arootin Aghazaryan
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  • If only you'd seen this post three years ago! lol - thanks for the update - hope this helps others too! – Rob Sep 08 '19 at 13:07
1

You cannot convert those numbers in percentage without decimals. It will work only if the numbers are divided by 100. So the answere here must be (1. 14.5 , 2. 8.5 , 3. 76.9). And as you can see there is a "0.1" percent missing for the same reason of the decimals you threw (i.e by converting 14.529914529914531 to 14.5).

Dion Zac
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