C - random number generation

Question

newbie in C here. I'm trying to have a different value for a variable everytime I run a program. I have this code:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define ADD 1
#define SUB 2
#define MUL 3
#define DIV 4

int main(void)
{
    double out;
    int choice;
    float x, y;
    printf("Welcome to our first calculator program!\n");
    printf("===========================\n");
    printf("Please insert 2 numbers:\n");
    scanf("%f%f", &x, &y);
    choice = 1 + srand(time(NULL)) % 4;
    if(choice == ADD)
            out = x + y;
    else if(choice == SUB)
            out = x - y;
    else if(choice == MUL)
            out = x * y;
    else
    {
            if(y != 0)
                    out = x / y;
            else
            {
                    out = 0;
                    fprintf(stderr, "error");
                    return 1;
            }
    }
    printf("The outcome is: %0.2lf\n", out);
    return 0;
}

but it always gives me 4 in choice. I don't get why...

Can you help me? Thanks.

Are you sure that code compiles? srand (or "seed random") should have a void return type, and rand should be used to get the actual random number. — user2864740
– user2864740, Commented Feb 26, 2014 at 22:19
@user2864740 gcc version 4.7.2 (Debian 4.7.2-5) complains error: void value not ignored as it ought to be. — Morten Jensen
– Morten Jensen, Commented Feb 26, 2014 at 22:35
@user2864740 Visual Studio 2010 complains also: error C2296: '%' : illegal, left operand has type 'void' .. Wonder which compiler allows this :) ? — Morten Jensen
– Morten Jensen, Commented Feb 27, 2014 at 9:21

Morten Jensen · Accepted Answer · 2014-02-27 11:38:02Z

3

Right now you are calling srand and expecting a random number. You should seed the entropy pool by calling srand and THEN calling rand to get the random number :)

An example of usage taken from: http://www.cplusplus.com/reference/cstdlib/srand/

int main ()
{
  printf ("First number: %d\n", rand()%100);
  srand (time(NULL));
  printf ("Random number: %d\n", rand()%100);
  srand (1);
  printf ("Again the first number: %d\n", rand()%100);

  return 0;
}

See how you seed by calling srand and then retrieve the random integer from rand

You need to do this:

srand(time(NULL));
choice = 1 + rand() % 4;

instead of this:

choice = 1 + srand(time(NULL)) % 4;

edited Feb 27, 2014 at 11:38

answered Feb 26, 2014 at 22:22

Morten Jensen

5,9573 gold badges46 silver badges57 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Ed Heal · Accepted Answer · 2014-02-26 22:19:13Z

1

You need to seed the random number generator - see srand - http://linux.die.net/man/3/srand.

Use something like the current time.

Then use rand to get a random number

answered Feb 26, 2014 at 22:19

Ed Heal

60.3k18 gold badges91 silver badges137 bronze badges

Collectives™ on Stack Overflow

C - random number generation

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related