I tried my first program in mingw. And i got the following output. What is wrong in my program?

Question

Program:

#include<stdio.h>
main()
 {
  int fahr;
   float cel;
   for(fahr=0;fahr<300;fahr=fahr+20)
   {
   cel=(5/9)*(fahr-32);
     printf("\n %d \t %f",fahr,cel);
    }
}

Output which i get:

   0       0.000000
   20      0.000000
   40      0.000000
   60      0.000000
   80      0.000000
   100     0.000000
   120     0.000000
   140     0.000000
   160     0.000000
   180     0.000000
   200     0.000000
   220     0.000000
   240     0.000000
   260     0.000000
   280     0.000000

While it's fairly obvious what's wrong here, a general tip is to give just a couple of test cases that show your error (rather than every 20 degrees from 0 to 280), and state explicitly what result you expected to get. — me_and, Sep 15 '12 at 21:57

score 2 · Answer 1 · answered Sep 14 '12 at 17:50

2

5/9 is zero in whole numbers, try cel=(fahr-32)*5/9 or float the whole boat.

answered Sep 14 '12 at 17:50

Michael Krelin - hacker

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`fahr` is an `int` too, so the reordering doesn't change anything, you need at least one `x.0` or cast in that. – Daniel Fischer Sep 14 '12 at 18:07
@DanielFischer, reordering does a great deal, because after multiplying `(fahr-32)` by 5 division by nine is much more meaningful. And will actually yield sane values. To have all the precision float is helpful, tho. – Michael Krelin - hacker Sep 14 '12 at 18:11
1

Right, overlooked that, you'll get 0 only for values between 31 and 33, which is not obscenely far off. – Daniel Fischer Sep 14 '12 at 18:14
Yes, the only problem is that `float` for `cel` makes no sense if calculation is done in integers. Loss of precision may be justified if there's no float involved. – Michael Krelin - hacker Sep 14 '12 at 18:17

score 2 · Accepted Answer · answered Sep 14 '12 at 17:54

Division between two ints always results in another int. If one of the two terms is a float or a double the other one gets automatically promoted to that type, thus yelding the correct result.

So typeof(5/9) = int while typeof(5.0/9) = typeof(5/9.0) = typeof(5.0/9.0) = double.

Therefore the correct version is:

cel=(5./9.)*(fahr-32);

Note: This happens everytime there's a mathematical expression with types of different 'rank', types of lower ranks are promoted to match the highest.

score 0 · Answer 3 · answered Sep 14 '12 at 17:55

Give this a try:

main()  
{   
  int fahr;  
  float cel;  
  for(fahr=0;fahr<300;fahr=fahr+20) 
  {  
    cel=(5./9.)*(fahr-32);  
    printf("\n %d \t %f",fahr,cel);   
  } 
}

5/9 is (as a type) int/int which will give an int. You can convert the math by changing them to 5.0/9.0, or you can just 5./9.

I tried my first program in mingw. And i got the following output. What is wrong in my program?

3 Answers3