Passing a 2 dimensional array to a function using a pointer in C++

Question

I've been trying to pass this array to the function but i keep on getting, error C2664: 'correctans' : cannot convert parameter 2 from 'std::string [3][3]' to 'std::string **' , don;t mind the silly questions in the code its just random for testing.

code:

    #include <iostream>
    #include <string>
    using namespace std;

int correctans(string *arr1, string **arr2, int *arr3, int questions, int choices)
{
int count=0;
int ans;

    for(int i=0; i<questions; i++)
    {
        cout << "Question #" << i+1;
      cout << arr1[i] << endl;
      for(int j=0; j<choices; j++)
          cout << j+1 << arr2[i][j] << " ";
      cout << "your answer:";
      cin >> ans;
      if(ans==arr3[i])
          count++;
    }

    return count;
}

int main()
{
    int correct;
    string Questions[3]={"HowAreYou", "HowManyHandsDoYouHave", "AreYouCrazyOrCrazy"};
    string Choices[3][3]={{"Banana", "Peanut", "Fine"},{"Five", "Two", "One"},{"I'mCrazy", "I'mCrazyBanana", "I'mDoubleCrazy"}};
    int Answers[3]={3, 2, 3};

    correct=correctans(Questions, Choices, Answers, 3, 3);
    cout << "You have " << correct << " correct answers" <<endl;

    return 0;
}

virusrocks · Accepted Answer · 2013-06-10 15:19:29Z

1

Here you go

int correctans(string *arr1, string (&arr2)[3][3], int *arr3, int questions, int choices)`

answered Jun 10, 2013 at 15:19

virusrocks

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2 Comments

WildBeginner Over a year ago

but I have a question what if i didn't know what is the size of the 2d array what can i use instead of [3][3] cause i tried changing the number just from the function but it didn't work i want to be able to set it up to any size is that possible?

fogbit Over a year ago

@WildBegginner: you could make correctans template function. template<typename T, int SIZE> correctans( ..., string(&arr)[SIZE][SIZE], ... ). But if you don't know the size at the compile time, then it doesn't work.

IanPudney · Accepted Answer · 2013-06-10 15:23:00Z

Passing multidimensional arrays can get very confusing. I recommend creating a singlepointer to the start of your array, and passing that pointer:

    #include <iostream>
    #include <string>
    using namespace std;

int correctans(string *arr1, string *arr2, int *arr3, int questions, int choices)
{
int count=0;
int ans;

    for(int i=0; i<questions; i++)
    {
        cout << "Question #" << i+1;
      cout << arr1[i] << endl;
      for(int j=0; j<choices; j++)
          cout << j+1 << arr2[i][j] << " ";
      cout << "your answer:";
      cin >> ans;
      if(ans==arr3[i])
          count++;
    }

    return count;
}

int main()
{
    int correct;
    string Questions[3]={"HowAreYou", "HowManyHandsDoYouHave", "AreYouCrazyOrCrazy"};
    string Choices[3][3]={{"Banana", "Peanut", "Fine"},{"Five", "Two", "One"},{"I'mCrazy", "I'mCrazyBanana", "I'mDoubleCrazy"}};
    int Answers[3]={3, 2, 3};

    string* choicesPtr=&Choices[0][0];
    correct=correctans(Questions, choicesPtr, Answers, 3, 3);
    cout << "You have " << correct << " correct answers" <<endl;

    return 0;
}

This code compiles and executes.

tlvs · Accepted Answer · 2013-06-10 15:21:04Z

0

If I remember correctly you can use something like 'string [][] & rArray'
(or exactly: string [3][3] & rArray), of course you should pass the actual dimensions as parameters as well.

The compiler accepted this:
string arr2[3][3]
as parameter. But i would try to improve passing a pointer instead of copying an array by value. Since you are using stl::string you could also try vector< vector< string > >.

edited Jun 10, 2013 at 15:21

answered Jun 10, 2013 at 15:14

tlvs

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Comments

kotlomoy · Accepted Answer · 2013-06-10 15:23:37Z

0

Well, as compiler said std::string [3][3] can not be converted to std::string **.

You can try this

int correctans(string *arr1, string (* arr2)[ 3 ], int *arr3, int questions, int choices)

or this

int correctans(string *arr1, string arr2[][ 3 ], int *arr3, int questions, int choices)

But better solution is to use std::vector.

edited Jun 10, 2013 at 15:23

answered Jun 10, 2013 at 15:14

kotlomoy

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2 Comments

WildBeginner Over a year ago

the second one worked but it is not passed using a pointer. thanks though @kotlomoy

kotlomoy Over a year ago

@WildBegginner Actually string arr2[][3] implicitly converted to string (* arr2)[3]. So both are identical for compiler. BTW the first one works now (I missed parentheses in first edition of my post)

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