incorrect double to integer convertion

Question

I am realy sorry to ask so simple question, but I realy can't figure out what is the problem. I want to convert any mark to 12 grade scale. I am trying to this as following:

double coef=(4/20)*12;
int mark=(int)coef;

But when i execute this code via debugger I see that coef is equal to zero. Why if this is double variable?

What is the value of `coef`? What is value of `(4/20)*12`? What is the value of `(4/20)`? (Hint: its a hint) — Miserable Variable, Nov 19 '12 at 19:32
Possible duplicate of [How can I divide two integers to get a double?](http://stackoverflow.com/q/661028/299327) — Ryan Gates, Nov 19 '12 at 19:43

score 12 · Accepted Answer · answered Nov 19 '12 at 19:31

12

Because even though you're assigning the result to a double value the calculation is performed using integers. And 4 / 20 in integer arithmetic in C# is 0. So you need something like

double coef = 4.0 / 20 * 12;

The 4.0 ensures that the rest of the calculation uses doubles. But often it's actually a good habit to make all your numbers explicit so that readers don't have to know about type conversion rules:

double coef = 4.0 / 20.0 * 12.0;

answered Nov 19 '12 at 19:31

Joey

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@seeker If this solved your problem, then you should accept it as an answer. – Ryan Gates Nov 19 '12 at 19:39

score 2 · Answer 2 · answered Nov 19 '12 at 19:31

2

It's integer division until it needs to be converted...

4/20 = 0
0 * 12 = 0

answered Nov 19 '12 at 19:31

Austin Salonen

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score 2 · Answer 3 · answered Nov 19 '12 at 19:35

Another alternative would be to do:

4 * 12 / 20;

But be aware that the result will truncate to 2 (because this is still integer division), which is okay, because that's what your cast to int would have done anyway.

Also, be aware that casting to int will truncate, not round. Use Math.Round if you want to actually round the result. It makes no difference with the numbers you gave, but I assume you'll not actually going to be only working with these constants!

score 1 · Answer 4 · answered Nov 19 '12 at 19:31

1

You're doing integer division first, then multiplying by 12.

When you do:

(4/20)

That's evaluating to 0, because they're both integers.

Modify it so explicitly state doubles by:

double coef = (4.0 / 20.0) * 12;

answered Nov 19 '12 at 19:31

Dave Zych

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score 1 · Answer 5 · answered Nov 19 '12 at 19:32

1

try

double coef = (4.0/20.0)*12;

then

int mark = (int)coef;

answered Nov 19 '12 at 19:32

Mihai

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razeh · Answer 6 · 2012-11-19T20:05:11.913

1

All of the constants in the code are integers, and the result of 4/20, as an integer, is 0.

You'll want to rewrite it as:

(4.0/20.0)*12.0;

edited Nov 19 '12 at 20:05

answered Nov 19 '12 at 19:32

razeh

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incorrect double to integer convertion

6 Answers6