round() function produces a strange number

Question

I declare double variables t and tau, and assign them values 1 and 0.00001 This line of code produces 536870912 in the console. What can be the reason?

printf("%i\n",round(t/(tau*double(2))));

By the way, I write code in a C++ compiler, but practically it's C.

"*but practically it's C*" - No it isn´t. You use `double(2)`, which isn´t possible in C. — RobertS supports Monica Cellio, Apr 12 '20 at 12:57
Code compiled with a C++ compiler is *by definition* C++ code. You *might* be able to compile it with a C compiler but also might not. Even if it compiles, the behavior might differ. As such, I'll remove the C tag, as you are using C++. — ljrk, Apr 12 '20 at 22:01
@Kalana This is *is* a C++ queston as the code above is shown to be C++, thus the C tag *is* wrong. Some people watch the C tag and are greatly annoted by C++ questions that are wrongly tagged. My edit was approved by others, please stop reverting these. Additionally, in C++ the header `stdio.h` is not necessarily available or has the intended behavior. One should use the `cstdio` and `cmath` headers instead. There are many minor differences with C and C++, eg. your main function does specify it doesn't take arguments in C++ -- but the same code doesn't do that in C. Stop mingling C and C++. — ljrk, Apr 13 '20 at 08:13
He trying to run `C` code. There for my answer was based on `C`. If he asked answer for `c++` my answer never be accepted. There for your edit suggestion on my answer was rejected by community not by me. — Kalana, Apr 13 '20 at 10:54
@Kalana No, `double(2)` is not C code, so he's obviously running C++. I've thus removed the C tag from the question but he's reverted the edit with no reasoning. Also, please tag me when answering, otherwise I won't get notified. — ljrk, Apr 18 '20 at 18:24
@larkey his first and only tag was `C`. There for my answer was base on `C`. That was actually happened. — Kalana, Apr 18 '20 at 18:36
@Kalana if you compile code with a C++ compiler, it's C++. And his code was evidently not C, despite his claims. — ljrk, Apr 18 '20 at 20:54

score 3 · Answer 1 · answered Apr 12 '20 at 12:43

3

round returns a double. Printing it with %i is undefined behaviour.

Use %f for printing double:

printf("%f\n",round(t/(tau*double(2))));

answered Apr 12 '20 at 12:43

P.P

117,907
20
175
238

this returns me "nan" :( – Nick The Dick Apr 12 '20 at 12:44
2

`round` can return `NaN` too! – P.P Apr 12 '20 at 12:45
how should I modify this line so that it return ```50000``` ? Or it should as it is? – Nick The Dick Apr 12 '20 at 12:49
@RobertSsupportsMonicaCellio `%f` can print double too. See https://port70.net/~nsz/c/c11/n1570.html#7.21.6.1 – P.P Apr 12 '20 at 12:53
@NickTheDick I am not sure how you defined `t` and `tau`. See https://ideone.com/muZtuy which prints 5000. – P.P Apr 12 '20 at 12:59
@usr yes i realise now it's an issue with my code or compiler - ill try to look into it. Thank you! – Nick The Dick Apr 12 '20 at 13:00
Interesting. Your are right. Here is a according question just linked through: https://stackoverflow.com/questions/19952200/scanf-printf-double-variable-c – RobertS supports Monica Cellio Apr 12 '20 at 13:04

score 2 · Accepted Answer · answered Apr 12 '20 at 12:57

Use %lf instead of %i and remove double line it is unnecessary because you are already defined variables as double.

#include <stdio.h>
#include <math.h>

int main()
{
    double t = 1, tau = 0.00001;

    printf("%lf\n", round(t/(tau*2)));

    return 0;
}

Output -: 50000.000000

If you want 50000 only you can edit your code into like this

double t = 1, tau = 0.00001;
int answer;

answer = round(t/(tau*2));
printf("%i\n", answer);

Output -: 50000

round() function produces a strange number

2 Answers2