Shuffling function not working properly

Question

I have created a shuffling program that uses a 5X5 array and a vector. The vector should store the values 1 through 25 inclusive and the array should just have 0 for each element. Once the shuffle() function is passed it should randomly position the 1 through 25 values of the vector in the array.

void Match::shuffle() {

    std::vector<int> vec(25);

    int randNum = rand() % (vec.size());

    for (int i = 1; i < 26; i ++) {
            vec.push_back(i);
    }
    for (int i = 0; i < 5; i++) {
            for (int j = 0; j < 5; j++) {
            backArr[i][j] = vec.at(randNum);
            vec.erase(vec.begin() + randNum);

            randNum = rand()%vec.size();
            }
    }

}

It is difficult to offer solutions when the problem statement is simply, "it doesn't work". Please [edit] your question to give a more complete description of what you expected to happen and how that differs from the actual results. See [ask] for hints on what makes a good explanation. — Toby Speight, Sep 27 '16 at 16:45
http://stackoverflow.com/questions/6127503/shuffle-array-in-c. This might help — Parnab Sanyal, Sep 27 '16 at 16:51
http://www.cplusplus.com/reference/algorithm/shuffle/ see this also — Parnab Sanyal, Sep 27 '16 at 16:53
I thought creating a std::vector of 25 entries, followed by 25 `push_backs` ends up with a vector of size 50 — infixed, Sep 27 '16 at 17:13

chtz · Answer 1 · 2016-09-28T09:16:12.053

In the last run (i.e., after backArr[i][j] has been set) you remove the last element from the vector and calculate randNum = rand()%vec.size(); which calculates rand() % 0 which will fail.

Just calculate randNum before backArr[i][j] = vec.at(randNum); (also remove the randNum in front of the loops).

Addendum: As pointed out by @infixed in the comments, you also create a vector of 25 (default-initialized) elements and afterwards push 25 additional elements (due to this, the first part of my answer is actually not true). To solve that, you either need to remove the (25) when declaring vec or replace the push_back instructions by corresponding vector accesses (mind the off-by-1).

Thank you so much this helps a lot. – Drew Sep 27 '16 at 19:31 — Drew, Sep 27 '16 at 19:31

score 0 · Answer 2 · answered Sep 27 '16 at 17:29

0

why not use std::random_shuffle ?

answered Sep 27 '16 at 17:29

Renaud

209
2
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I would like to avoid using std:random_shuffle – Drew Sep 27 '16 at 19:26

Shuffling function not working properly

2 Answers2