1

i have some understanding problems, with the percent calculation in c#

Why is this working.

100 * usedDiskSpace/maxDiskSpace

and this not...

((usedDiskSpace/maxDiskSpace)*100)

I didn't get it.

Why ?

for better explanation. I try to calculate the percentage of a harddisk volume. I programmed in other languages like Actionscript or php, or JAVA. I do not understand, why the result of the lower calculation will result a 0 value.

Glowhollow
  • 132
  • 3
  • 13
  • 2
    Possible duplicate of [Why does integer division in C# return an integer and not a float?](https://stackoverflow.com/questions/10851273/why-does-integer-division-in-c-sharp-return-an-integer-and-not-a-float) – mjwills Jun 07 '18 at 00:47
  • It would be awesome if you could provide a [mcve] (that we can copy and paste into a console app and run). The answer depends on the variable types involved. – mjwills Jun 07 '18 at 00:49

5 Answers5

5

You have a differing order of operations.

Your first example, 100 * usedDiskSpace/maxDiskSpace, is the same as (100*usedDiskSpace)/maxDiskSpace.

Your second example, (usedDiskSpace/maxDiskSpace)*100 is doing the division before the multiplication. In integer division, anything after a whole number is truncated. Since (presumably) maxDiskSpace is always larger than usedDiskSpace, the result will be 0 * 100, or 0.

We can work through this with some toy numbers for the sake of an example.

int a = 10;
int b = 20;

Console.WriteLine(100*a/b); // Output: 50
// 100*10/20
// (100*10)/20
// 1000/20
// 50
Console.WriteLine((a/b)*100); // Output: 0
// (10/20)*100
// (0)*100 <- 0, rather than 0.5 due to how integer division works
// 0
Jonathon Chase
  • 9,396
  • 21
  • 39
3

Integer Division and Order of Operations

When you divide the two integers, you get an integer result. Say you have values 40 and 50. When all you have are integers, 40/50 is just 0. There's no such thing as .8 in integer land. Even if you also multiply by 100, the division has already happened and 100*0 is still 0.

But if you multiply the numerator by 100 first, you get 4000/50. That still leaves room for an integer result: 80.

That's what happened here. The first example executes the multiplication first. The second example executes the division first.

Joel Coehoorn
  • 399,467
  • 113
  • 570
  • 794
2

The issue is likely caused by integer division.

The other answers are great if you are looking for a result that is a whole number (e.g. 10%). If you are looking for a result with multiple decimal points (e.g. 10.57%) then consider using:

var bob = ((usedDiskSpace * 1.0 / maxDiskSpace) * 100);

or even simpler:

var bob = usedDiskSpace * 100.0 / maxDiskSpace;

Multiplying by 1.0 (or 100.0) will ensure that integer division is not used (since 1.0 and 100.0 are double.

mjwills
  • 23,389
  • 6
  • 40
  • 63
1

This is because you made integer division. Just cast as float or double to achieve desired result.

int usedDiskSpace = 230, maxDiskSpace = 980;

// integer division
int res = 100 * usedDiskSpace / maxDiskSpace;
// 23
res = ((usedDiskSpace / maxDiskSpace) * 100);
// 0

// floating point division
float resF =100f * usedDiskSpace / maxDiskSpace;
// 23.4693871
resF = (((float)usedDiskSpace / maxDiskSpace) * 100);
// 23.4693871
Hossein Golshani
  • 1,847
  • 5
  • 16
  • 27
0

In mathematical calculus, arrangement of parentheses are determinative. In the following expression:

100 * usedDiskSpace/maxDiskSpace

first 100 multiplied in usedDiskSpace then divided to maxDiskSpace. For example:

100 * 50 / 20 = 5000 / 20 = 250

But in the second one the calculation is as follows:

((usedDiskSpace/maxDiskSpace)*100)

first usedDiskSpace divided to maxDiskSpace then multiplied in 100. The previous example in this scenario acts as follows:

((50 / 20) * 100) = 2 * 100 = 200

Please note that if usedDiskSpace and maxDiskSpace variables are an integer type, in that case, division result will be an integer one:

50 / 20 = 2 (not 2.5)

To obtain an equal result in your mentioned expressions, you should define usedDiskSpace and maxDiskSpace variables as float or double types.

Mohammad Afrashteh
  • 348
  • 4
  • 18