Random not declared in scope

Question

I am getting the error:

tester.cpp|20|error: 'rand' was not declared in this scope|

Did I miss to include something here?

void tester::volumeset(bool A_B, int i, int v)
{
    if (A_B == true)
    {
        A_volumn[i] = rand(v+1);
    }else{
        B_volumn[i] = rand(v+1);
    }
}

Have you added the correct `#include` file or even declared the function? — Tony The Lion, Nov 26 '12 at 15:58
Yea i have this #include #include #include #include #include — soniccool, Nov 26 '12 at 15:58
I saw this on c++ forums unless its not an actualfunction in c++? — soniccool, Nov 26 '12 at 15:59
I've never seen that C++ function. C has a `rand()` function — Tony The Lion, Nov 26 '12 at 15:59
Just so you know, `#include`ing `` and `` are the same thing. For system headers (wrapped in `<>`) that came from C, you can either use their original name with a `.h`, as in `` or use the newer C++ name which drops the `.h` and adds a `c` at the beginning: ``. — Ari, Nov 26 '12 at 16:04
Indirectly related, not a dupe: http://stackoverflow.com/questions/686353/c-random-float/686373 — John Dibling, Nov 26 '12 at 16:05
@Ari: they're not the same. `` puts standard C functions in the `std` namespace and may add a bunch of overloads (http://stackoverflow.com/a/8734292/166749). — Fred Foo, Nov 26 '12 at 16:07

Fred Foo · Accepted Answer · 2012-11-26T16:07:46.763

9

random is not a standard C++ function; it's a POSIX function, so it's not available on Windows. Use rand instead, or better, the new C++11 randomness library.

edited Nov 26 '12 at 16:07

answered Nov 26 '12 at 15:59

Fred Foo

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How do you access the POSIX `random` function? I've never heard of it before. – Xymostech Nov 26 '12 at 16:02
1

@soniccool: `rand()` returns a random number in the range `[0, RAND_MAX)`, so `rand() % k` will give (non-uniformly distributed) random number in the range `[0, k)`. – Fred Foo Nov 26 '12 at 16:03
How would i rewrite this `random(v+1);`` in c++? – soniccool Nov 26 '12 at 16:04

score 8 · Answer 2 · answered Jul 29 '17 at 20:52

8

rand is part of cstdlib, try including cstdlib in your code.

#include <cstdlib>

or

#include <stdlib.h>

answered Jul 29 '17 at 20:52

Rishabh Agrahari

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score 1 · Answer 3 · answered Nov 26 '12 at 16:00

1

You want to use rand(), not random(). You can see some examples of how to use it here: http://www.cplusplus.com/reference/cstdlib/rand/

answered Nov 26 '12 at 16:00

Ari

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score 0 · Answer 4 · edited Jul 11 '19 at 14:12

0

when i compile my code with Code::Blocks i don't need to #include <cstdlib>, with DevC++ i simply added #include <cstdlib> and it works for me.

edited Jul 11 '19 at 14:12

Josh Lee

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answered Jul 11 '19 at 13:57

AJcleverprogrammer

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What's the difference from [Rishabh Agrahari's answer](https://stackoverflow.com/a/45393947/5376789)? – xskxzr Jul 11 '19 at 14:19

Tesfahunegn Minwuyelet · Answer 5 · 2018-01-09T17:38:36.997

enter code hereonly including #include or #include may not work it is better to use for instance I get a challenge when I use rand(100) but instead of this when I use rand()%100 it works well. try and enjoy it.

for(i=0;i<cust;i++)
{

  rsr[i]=rand(100);
 }

it works when I change it to 
 for(i=0;i<cust;i++)
 {

  rsr[i]=rand()%100;
  }

if you want to make it 1-100 use below but 0-99 use the above

for(i=0;i<cust;i++)
 {

  rsr[i]=rand()%(100+1);
  }

for(i=0;i<cust;i++)
 {

  rsr[i]=rand()%100+1;
  }

Random not declared in scope

5 Answers5