C giving wrong answer to calculation

Question

I am coding a very simple program in C but I keep getting wrong answers to calculations that I am doing. The final output that I want needs to have no decimal places so I am using int as the data type even though the answer will not be an integer. Here is the code:

int numberOfInches = (100/254)*101;

I either get the answer 0 if I use int as the data type or crazy long numbers if I try using float or double. Any ideas on what I am doing wrong?

The `/` operator with two integers performs an integer division. So `100 / 254` will always produce `0`. What you want is `float numberOfInches = (100/254.0)*101;` — Thomas W., Oct 07 '16 at 13:25
Possible duplicate of [What is the behavior of integer division in C?](http://stackoverflow.com/questions/3602827/what-is-the-behavior-of-integer-division-in-c) — High Performance Mark, Oct 08 '16 at 13:04
What `printf()` code did you use that resulted in "crazy long numbers". I suspect that is the key to code's problem. — chux - Reinstate Monica, Oct 19 '16 at 22:16

score 5 · Answer 1 · edited Oct 07 '16 at 14:12

5

100 / 254

This is integer division, which will get 0. You then multiply by 101.

To do floating point division, at least one of the operands of / must be floating point:

int n = (int)((100. / 254.) * 101.);

edited Oct 07 '16 at 14:12

Bo Persson

90,663
31
146
203

answered Oct 07 '16 at 13:26

bolov

72,283
15
145
224

Pirate · Answer 2 · 2016-10-07T13:31:04.567

1

Obviously it will give 0.

int numberOfInches = (100/254)*101;

When it will calculated first it will evaluate inner bracket. so (100/254) will be 0 And when you multiply 0 to 101, i.e 0*101 = 0.

It will be 0.

To get the correct output, use the following.

int numberOfInches =(int)((100.0/254)*101);

edited Oct 07 '16 at 13:31

answered Oct 07 '16 at 13:26

Pirate

2,886
4
24
42

score 1 · Answer 3 · answered Oct 07 '16 at 13:27

When both operands of the / operator are integer, it performs integer division, i.e. the result is the quotient with the fraction part trucated. The result of 100/254 is less that 1, so it rounds down to 0.

You can either make one of the constants floating point:

(100.0/254)*101

Or you can do the division last:

(100*101)/254

score 1 · Answer 4 · answered Oct 07 '16 at 13:29

You use an int. The results are rounded to the nearest int value:

100/254 = 0 THEN 0 * 101 = 0 SO, final result is 0.

I think you can do something like :

int numberOfInches = 100 * 101 / 254;

result: 100 * 101 = 10100 THEN 10100 / 254 = 39.7xxxxxx SO, final result is 40.

score 1 · Answer 5 · answered Oct 07 '16 at 13:31

1

Division in parentheses is integer-type, so 100/254=0. If you want to calculate value with a fractional part try:

int numberOfInches = (100.0 / 254.0) * 101.0;

answered Oct 07 '16 at 13:31

Tomek Smolarek

11
1

score 0 · Answer 6 · answered Oct 07 '16 at 13:26

0

Why not using float ? try 100.0f/254.0f

answered Oct 07 '16 at 13:26

Isukthar

185
8

C giving wrong answer to calculation

6 Answers6