Passing dynamically allocated 2D char array causes segmentation error?

Question

I am having trouble using a function.

I have two functions.

createTwoDArray: prompts user for row and column sizes, creates a new 2D array and returns it while also modifying the row and column variables passed to it.

printTwoDArray: should take in a 2d array and print everything. However, when calling this function, segmentation fault occurs immediately. Not one line of code inside the function is called even.

Thank you :)

int column, row;
char** createTwoDArray(int& column, int& row) {
   int min, max, i, j;
   cout << "\nPlease enter row size:";
   cin >> i;
   row = i;
   cout << "\nPlease enter column size:";
   cin >> j;
   column = j;
   char** dynamicArray2 = new char*[column];
   for(i = 0; i < row; i++) {
     dynamicArray2[i] = new char[column];
     for(j = 0; j < column; j++) {
       dynamicArray2[i][j] = '\0';
    }
   }
   return dynamicArray2;
}
void printTwoDArray(char** array, int row, int column) {
//
}

//
char** array2 = new createTwoDArray(column, row)
printTwoDArray(array2, column, row); //this causes the     segmentation error
//

You really should be validating that your input succeeds. If by accident a non-digit character is typed (such as catching ' while pressing Enter) things break quickly in your code. — David C. Rankin
– David C. Rankin, Commented Jun 17, 2019 at 1:55

R.J. Dunnill · Accepted Answer · 2019-06-17 02:12:56Z

2

There are two errors: 'column' was used to allocate rows, and row and column were mixed up when calling printTwoDArray().

Here is the fixed code. It runs fine in Visual C++.

#include "pch.h"
#include <iostream>

int column, row;
char** createTwoDArray(int& column, int& row) {
    int min, max, i, j;
    std::cout << "\nPlease enter row size:";
    std::cin >> i;
    row = i;
    std::cout << "\nPlease enter column size:";
    std::cin >> j;
    column = j;

    // *** Use row, not column to allocate the number of rows.
    char** dynamicArray2 = new char*[row]; 
    for (i = 0; i < row; i++) {
        dynamicArray2[i] = new char[column];
        for (j = 0; j < column; j++) {
            dynamicArray2[i][j] = '\0';
        }
    }
    return dynamicArray2;
}

void printTwoDArray(char** array, int row, int column) {
    printf("\nPrinting %d rows:\n\n", row);

    for (int i = 0; i < row; i++)
    {
        for (int j = 0; j < column; j++)
        {
            printf(" %2d", array[i][j]);
        }

        printf("\n");
    }
}

int main()
{
    //
    char** array2 = createTwoDArray(column, row);

    // Pass row and column in the right order!
    printTwoDArray(array2, row, column); 
    //

    return 0;
}

answered Jun 17, 2019 at 2:12

R.J. Dunnill

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2 Comments

rustytoaster21 Over a year ago

Thank you this fixed it! Is there any reason why the program would crash instantly if I pass it the wrong order of row/column? Shouldn't at least some lines of the function execute before it crashes?

R.J. Dunnill Over a year ago

You're welcome! FYI for me, it didn't crash instantly when I tried, just when it attempted to access out-of-bounds memory.

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