How to use random() in C

Question

So I am currently learning C, and I have some confusion about how the random() function works. I know I have to provide a seed, but I don't know how to actually generate the random number.

srandom(seed);
long int value= random(40);

When I try this it gives me a compiler error:

too many arguments to function ‘long int random()

The end goal is to get a random number between 1 and 40. I should add that the goal is to not use the rand() function.

Please do not place `srand()` before each call to `rand()`. Call it once at the start of the program. I do not know why you renamed those library functions. What are they? How can we know what they do? Are they non-standard functions or your own? — Weather Vane, Feb 25 '17 at 18:22
Read the POSIX specification for [`random()`](http://pubs.opengroup.org/onlinepubs/9699919799/functions/random.html) — it tells you a lot of what to do. For the rest, read [Is this C implementation of Fisher-Yates shuffle correct?](http://stackoverflow.com/a/3348142) to see how to generate an unbiassed random number in the range 0..N (for 1..40, you'll use a random number in the range 0..39 and add 1). — Jonathan Leffler, Feb 25 '17 at 21:34

score 6 · Answer 1 · edited May 23 '17 at 11:54

6

The answer to your question is precisely answered by the compiler. You are passing '40' to the function random() which it is not supposed to receive.

The signature for random() is:

long int random (void)

void keyword here is indicator that the function in hand is not supposed to receive anything.

Here's an example program to generate 6 random numbers.

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int num;
    int max = 5; 
    for (int c = 0; c <= max; c++){
      num = random();
      printf("%d\n",num);         
   }
   return 0;
}

Output:

fire-trail@Sahai-Yantra:~$ gcc try.c 
fire-trail@Sahai-Yantra:~$ ./a.out
1804289383
846930886
1681692777
1714636915
1957747793
424238335

To reach your end goal of generating random numbers within a given range using random(), see How to generate a random number from within a range.

edited May 23 '17 at 11:54

Community

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answered Feb 25 '17 at 18:31

Mehar Charan Sahai

183
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So far, so good. Now he wants to generate a number in the range 1..40; how do you go about doing that? – Jonathan Leffler Feb 25 '17 at 21:36
1

Already answered here, http://stackoverflow.com/questions/2509679/how-to-generate-a-random-number-from-within-a-range – Mehar Charan Sahai Feb 25 '17 at 21:43
OK; so update your answer to reference the question, or include an outline of the information and the cross-reference to its source. – Jonathan Leffler Feb 25 '17 at 21:45
@JonathanLeffler is this okay? Or should I add up anything? newbie here. – Mehar Charan Sahai Feb 25 '17 at 21:53
1

I recommend using the link's title rather than an uninformative 'follow this link'. The latter looks like spam, even though it isn't. I've updated as I suggest. I might avoid using the bold for 'end goal' and I'd probably use back-ticks around `random()` so it appears in constant-width font, but those are second-order refinements. – Jonathan Leffler Feb 25 '17 at 21:57
I guess I'm not supposed to comment this but noted and thanks! – Mehar Charan Sahai Feb 25 '17 at 21:59
`random()` is not a standard function. it is a posix extension. – chqrlie Feb 25 '17 at 22:03

score 3 · Answer 2 · answered Feb 25 '17 at 18:19

3

The answer is right there in the error thrown by the compiler

too many arguments to function ‘long int random()

Change random(40) to random()

answered Feb 25 '17 at 18:19

Aniruddh Dikhit

662
4
10

msc · Answer 3 · 2017-02-25T18:26:59.043

2

There is no argument in random() function.

See man page of random(). Prototype of random()

long int random(void);

edited Feb 25 '17 at 18:26

answered Feb 25 '17 at 18:20

msc

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How to use random() in C

3 Answers3