How do I round a number (any number) down by 100s?

Question

I'm trying to round numbers down to the lowest 100. Basically 199 -> 100, 2021 -> 2000, etc. How can I make this in C# program?

Possible duplicate of [Built in .Net algorithm to round value to the nearest 10 interval](http://stackoverflow.com/questions/274439/built-in-net-algorithm-to-round-value-to-the-nearest-10-interval) — Meirion Hughes, Jan 29 '17 at 07:42
That's not rounding, that is truncation. Math.Truncate() for floating point numbers. Don't forget about negative values. — Hans Passant, Jan 29 '17 at 08:22

Kesty · Accepted Answer · 2017-01-29T07:31:18.763

5

This will do it:

private static int RoundDown(int input)
{
    return input / 100 * 100;
}

The reason this works is as follows:

Dividing an integer value by 100 will give the integer portion of the result. This means, if we take the integer 199 and divide by 100, the mathematical result is 1.99 but because we're working with integers, only the 1 is retained. Multiplying this value again by 100 will give you 100 which is the desired result.

edited Jan 29 '17 at 07:31

answered Jan 29 '17 at 06:57

Kesty

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RoundDown 99 gives 100? – kouty Jan 29 '17 at 07:03
It's not supposed to. RoundDown 199 is supposed to give 100. – Kesty Jan 29 '17 at 07:03
1

sorry! ;( @Kesty – kouty Jan 29 '17 at 07:05
1

@Kesty - A little explanation as to why this works would be nice. If a novice saw this they would expect `/ 100 * 100` would return the original value. – Enigmativity Jan 29 '17 at 07:25

How do I round a number (any number) down by 100s?

1 Answers1