Perform a random permutation on an array in C++

Question

I have one int* array and I want to perform a random permutation to this array. The function is similar with randperm in MATLAB. For example, I have an array int* A=[0 1 1 0] and if I call randperm(A,sizeof A) then one possible output would be A=[1 0 0 1]

This is my function but there seems to be an error. Can anyone determine what this error is?

void randperm(int* permMatrix,int n)
{
    int i, j, t;
    for(i=0; i<n; i++) {
        j = rand()%(n-i)+i;
        t = permMatrix[j];
        permMatrix[j] = permMatrix[i];
        permMatrix[i] = t;
    }
    for(i=0; i<n; i++) {
        printf("%d ",permMatrix[i]);
    }
}

Error output shows such as A=[1 0 0 0] or A=[0 1 0 0]. I lost one bit equal 1 — Jame, Nov 06 '13 at 06:38
A variable of type `int *` is not a matrix, it can be an array though. — Some programmer dude, Nov 06 '13 at 06:39
If this is not homework and you really need to implement it yourself, I'd suggest you use [`std::random_shuffle`](http://en.cppreference.com/w/cpp/algorithm/random_shuffle) instead. — Daniel Frey, Nov 06 '13 at 06:41
@DanielFrey: It is not homework. I know random_shuffle function. But I want to edit my function. THanks — Jame, Nov 06 '13 at 06:44
Dear all, Thank you for support. I editted it. Problem it is j = rand()%(n-i)+i;-error — Jame, Nov 06 '13 at 06:48

score 6 · Answer 1 · answered Nov 06 '13 at 06:49

6

It's always best to use standard library facilities where they exist. In this case, the <algorithm> header has a function std::random_shuffle() that does exactly what you want:

#include <algorithm>

void randperm(int* matrix, int size)
{
    std::random_shuffle(matrix, matrix + size);
}

answered Nov 06 '13 at 06:49

Tristan Brindle

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+1. I am still trying to figure out what the reason for the down-vote could be. – juanchopanza Nov 06 '13 at 06:56
1

Probably the typo in the first word of the title – kfsone Nov 06 '13 at 07:12

score 0 · Answer 2 · answered Nov 06 '13 at 07:12

0

Implementation of 'randperm' is correct but, may be sizeof(A) is not evaluated as 4. Please Check out value of 'n'.

answered Nov 06 '13 at 07:12

Taemin Hong

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Perform a random permutation on an array in C++

2 Answers2