C# Bug or Something Wrong

Question

I have an issue with C# not calculating correctly for me to draw my progress bar.

int width = 130;
int maxValue = 20;
int value = 20;

int percent = (width / maxValue) * value

Now it should return 130 so it mean my progress bar is full but it returns 120 so I don't know what is happening.

here is and image of progress bar http://imgur.com/sUbshxk

I also tested the formula with VB.NET and it worked perfectly.

I am using VS2013 in Windows 7 x86.

C# bug? For future reference...it is __never__ a bug in the C# compiler. — Steve Wellens, Nov 30 '14 at 18:24
If you don't want to start casting, you can change your formula: percent = (width * value) / maxValue — the_lotus, Dec 01 '14 at 14:51

Soner Gönül · Accepted Answer · 2014-11-30T18:27:50.427

3

130 / 20 performs integer divison

From / Operator (C# Reference)

When you divide two integers, the result is always an integer. For example, the result of 7 / 3 is 2

That's why it always discards the fractional part and it returns 6. That's why your result will be 6 * 20 which is equal to 120.

As a solution, you can change your integer division to floating-point division.

For example;

var percent = (130.0 / 20) * 20;
var percent = (130 / 20.0) * 20;

That means, you need to define one of your variable as a double, or cast one of them to double in your calculation.

edited Nov 30 '14 at 18:27

answered Nov 30 '14 at 18:21

Soner Gönül

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@Downvoter care to comment at least so I can see where I might be wrong? – Soner Gönül Nov 30 '14 at 20:05
1

It looks like every answer in this question was downvoted. Perhaps a serial downvoter? – Chris Dunaway Dec 01 '14 at 16:34
@ChrisDunaway Yeah, it smells like.. – Soner Gönül Dec 02 '14 at 13:15

score 1 · Answer 2 · answered Nov 30 '14 at 18:21

1

You need to cast the values to double:

int percent = (int)(((double)width / (double)maxValue) * (double)value);

130 / 20 = 6.5 and it will be implicitely converted to integer which makes 6 of it. So the wrong value is used for the following multiplication

answered Nov 30 '14 at 18:21

Flat Eric

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Or use doubles in the first place. – Robert Harvey Nov 30 '14 at 18:22

score 1 · Answer 3 · answered Nov 30 '14 at 18:23

Since you're dividing two int variables, you're performing integer division. 130/20 is 6.5, which, in integer context is truncated to 6. It is then multiplied by 20, to generate the result of 120.

You could avoid this issue completely by defining your variables as doubles:

double width = 130;
double maxValue = 20;
double value = 20;

double percent = (width / maxValue) * value;

score 0 · Answer 4 · answered Nov 30 '14 at 18:22

In most programming languages, dividing an integer by an integer always results in an integer.

If you don't want an integer result, make sure at least one of the operands is a float or double:

int width = 130;
float maxValue = 20.0;
int value = 20;

int percent = (width / maxValue) * value

C# Bug or Something Wrong

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