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cceil
AGeek
AGeek

Reputation: 5225

Implementation of ceil function in C

I have two questions regarding ceil() function..

  1. The ceil() function is implemented in C. If I use ceil(3/2), it works fine. But when I use ceil(count/2), if value of count is 3, then it gives compile time error.

    /tmp/ccA4Yj7p.o(.text+0x364): In function FrontBackSplit': : undefined reference toceil' collect2: ld returned 1 exit status

    How to use the ceil function in second case? Please suggest.

  2. How can I implement my own ceil function in C. Please give some basic guidelines.

Thanks.

Upvotes: 4

Views: 31499

Answers (5)

msg
msg

Reputation: 166

You can use this

#define (x < 0 ? (int)x : (int)x + 1)

Upvotes: 0

nintendo
nintendo

Reputation: 69

Try this out:

#define CEILING_POS(X) ((X-(int)(X)) > 0 ? (int)(X+1) : (int)(X))
#define CEILING_NEG(X) (int)(X)
#define CEILING(X) ( ((X) > 0) ? CEILING_POS(X) : CEILING_NEG(X) )

Check out the link for comments, proof and discussion: http://www.linuxquestions.org/questions/programming-9/ceiling-function-c-programming-637404/

Thanks to Vilhelm Gray and carveone for pointing out that the linked definition of CEILING_NEG(X) is incorrect.

Upvotes: 6

Anonymous
Anonymous

Reputation: 1

double ceil (double x) {
    if (x > LONG_MAX) return x; // big floats are all ints
    return ((long)(x+(0.99999999999999997)));
}

Upvotes: -1

Adam Rosenfield
Adam Rosenfield

Reputation: 400692

The ceil() function is implemented in the math library, libm.so. By default, the linker does not link against this library when invoked via the gcc frontend. To link against that library, pass -lm on the command line to gcc:

gcc main.c -lm

Upvotes: 3

Michael Aaron Safyan
Michael Aaron Safyan

Reputation: 95639

The prototype of the ceil function is:

double ceil(double)

My guess is that the type of your variable count is not of type double. To use ceil in C, you would write:

#include <math.h>
// ...
double count = 3.0;
double result = ceil(count/2.0);

In C++, you can use std::ceil from <cmath>; std::ceil is overloaded to support multiple types:

#include <cmath>
// ...
double count = 3.0;
double result = std::ceil(count/2.0);

Upvotes: 3

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