Dividing by two integers does not return expected result

Question

I'm currently writing a program that requires a preview of a live display, but the preview, of course, is scaled down. However, when I scale the PictureBox down, the size is incorrect. For the scale to be correct the width and height need to be at a 4:3 ratio. Here's the code:

private void FindOptimalRes(PictureBox picBox)
{
    double h = Height / 4;
    double ratio = 4 / 3;
    picBox.Size = new Size((int)(h * ratio), (int)h);
}

In testing, Height (the height of the form) is 400, so, the width of the new size should be 133. But it always gets resized to 100×100! Why?

You should make sure whether YOU have the wrong before say that C# has a math wrong — , Jul 14 '12 at 21:44
Why was this question downvoted? Just because someone is ignorant about something is not a reason for downvoting his question. He even showed his code and everything. — Darin Dimitrov, Jul 14 '12 at 21:44
I think, perhaps: "Try to choose a more accurate/issue-reflecting and less combative title .." — , Jul 14 '12 at 21:54
This is a good read: http://stackoverflow.com/questions/9841332/why-is-the-division-result-between-two-integers-truncated Also, I've changed your title to make it easier for future visitors with a similar problem to find this post. I see minitech removed your last line - thanks @minitech! — dash, Jul 14 '12 at 22:07
Okay.. Thank you, Dash. I apologize for my.. noobiness? Whatever. :P Thanks, again. Lol.. — Steven, Jul 14 '12 at 22:38
So, I know this is an old post, but I just wanted to point out that, at the time of making this question, I was fairly new to C#. I know Java pretty well, but I was not aware that you had to specify that the variables were doubles for them to work in division properly. Just wanted to clear that up.. — Steven, Nov 28 '12 at 01:48

score 4 · Accepted Answer · answered Jul 14 '12 at 21:43

4 and 3 are both ints, so it gets turned to 1. Make them something floating-point:

double ratio = 4.0 / 3.0;

Note that you're also making the same mistake with Height (it doesn't matter right now, but it will - change it to 4.0). And if this is the actual code, why divide by four to multiply by four again?

private void FindOptimalRes(PictureBox picBox)
{
    picBox.Size = new Size(Height / 3, Height / 4);
}

score 3 · Answer 2 · 2012-07-15T23:07:18.457

C#'s math is "correct". The understanding of what is being done is .. missing :-)

The expression 4 / 3 (of type int / int) will evaluate to the integer value 1 as it is using integer division (both operands are integers). The resulting 1 is then implicitly coerced to a double value on assignment.

On the other hand 4d / 3 will "work" (and results in a double 1.333_) because now it is double / int -> double / double (by promotion) -> double using the appropriate floating point division.

Similarly, for Height / 4 (assuming Height is an integer), these would work:

(double)Height / 4          // double / int -> double
Height / 4d                 // int / double -> double
(double)Height / (double)4  // double / double -> double

Happy coding!

For the record, I do know that C#'s math is correct. It's been quite a while since I've programmed in general and nearly a year since in C#, so I simply forgot that little rule. But thank you for the correct answer and for reminding me of this. :P — Steven, Jul 14 '12 at 22:10

Marlon · Answer 3 · 2012-07-14T21:48:34.733

3

You are doing integer division:

double ratio = 4 / 3; // evaluates to 1

This won't give you the value you are looking for because the decimal point is being truncated, thus evaluating to 1 instead of 1.333. At least one of the operands needs to be a double:

double ratio = 4.0 / 3.0; // evaluates to 1.333

Same goes for Height. Change the 4 to 4.0.

edited Jul 14 '12 at 21:48

answered Jul 14 '12 at 21:43

Marlon

19,924
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Thank you. You realized I was kidding about C#'s math being off and you gave me a straight (and correct) answer. :) – Steven Jul 14 '12 at 21:49

Ria · Answer 4 · 2012-07-14T22:00:11.230

Make sure division result is double

double ratio = (double) 4 / 3; // double division

and no need to set your input values to double.

var num1 = // an integer number
var num2 = // an integer number

//result is integer, because of integer/integer uses 'integer division'
double result = num1 / num2; 

//result is double , because of you forced to 'double division'
double result = (double) num1 / num2;

score 0 · Answer 5 · answered Jul 14 '12 at 21:43

0

Maybe you should do a decimal division and not integer division:

double h = Height / 4.0;
double ratio = 4 / 3.0;

If C# Math was off many things around the world would be off as well.

answered Jul 14 '12 at 21:43

Darin Dimitrov

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score 0 · Answer 6 · answered Jul 14 '12 at 21:59

You are doing integer division.

what you need to do is this :

private void FindOptimalRes(PictureBox picBox)
{
    double h = Height / 4D; // or Height / 4.0
    double ratio = 4D / 3D; // or 4.0 / 3.0
    picBox.Size = new Size((int)(h * ratio), (int)h); // Size is now correct [133,100]
}

when you do a mathematical operation with an integer literal (no decimal places) it is implicitly typed as an int.

Simply appending a capital D at the end of your literals (4D, 3D) will type them as doubles, and your math will be correct. Alternatively you can write 4.0, 3.0

Dividing by two integers does not return expected result

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