CODEWITHSUNDEEP
c++functionpointers
Beach Williams
Beach Williams

Reputation: 153

Pointer to pointer function parameter

The function should simply read a matrix. Why does it freeze after I enter the first character?

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

using namespace std;

void as(char **p,int n,int m)
{
    int i, j;
    for (i = 0; i < n; i++)
        for (j = 0; j < m; j++)
        {
            cout << "p[" << i << "][" << j << "]=";
            cin >> p[i][j];
        }
}

int main()
{
    char *a[100];
    as(a, 3, 3);
    return 0;
}

Upvotes: 3

Views: 404

Answers (3)

Raindrop7
Raindrop7

Reputation: 3911

You have a great problem in your code. You are facing a UB in:

char *a[100]; // an array of 100 pointer to a character
// it is not initialized yet
  • Above no element is initialized (even not allocated).

To correct your code:

char *a[100];
// Allocating the array of 100 elements on the heap:
for(int i(0); i < 100; i++){
    a[i] = new char[100]; // let's say your array is n = m
}

as(a, 3, 3);

for(int i = 0; i < 3; i++){
    for(int j(0); j < 3; j++)
        cout << a[i][j] << ", ";
    cout << endl;
}

Last but not least Don't forget to free up memory when you are done with the dynamic array:

for(int i = 0; i < 100; i++)
    delete[] a[i];

Upvotes: 0

0___________
0___________

Reputation: 67476

I have for you simple pseudo array on mallocs reallocs and pointers. Maybe it will be interesting for you:

typedef struct arr_str_t{
    size_t rows, columns;
    char **table;
}dynamicStringTable_t;

int CreateStringTable(dynamicStringTable_t **ptr, int rows, int columns)
{
    int result = 0;
    *ptr = (dynamicStringTable_t *)malloc(sizeof(dynamicStringTable_t));
    if (ptr == NULL) return - 1;
    (*ptr)->rows = rows;
    (*ptr)->columns = columns;
    (*ptr) -> table = (char *)malloc(rows * columns * sizeof(char *));
    if (*ptr == NULL)
    {
        free(*ptr);
        return -1;
    }
    for (int i = 0; i < rows * columns; i++) (*ptr)->table[i] = NULL;
    return 0;
}

char *getString(dynamicStringTable_t *ptr, int x, int y)
{
    char *result = (ptr == NULL || x >= ptr->columns || y >= ptr->rows || !x || !y) ? NULL : "";
    if (result != NULL)
    {
        result = ptr->table[x + y * ptr->rows];
    }
    return result;
}

int putString(dynamicStringTable_t *ptr, int x, int y, const char *str)
{
    int result = (ptr == NULL || x >= ptr->columns || y >= ptr->rows || str == NULL || !x || !y) * -1;
    if (!result)
    {
        char *tmp = (char *)realloc(ptr->table[x + y * ptr->rows], (strlen(str) + 1) * sizeof(char));
        if (tmp == NULL) result = -2;
        else
        {
            ptr->table[x + y * ptr->rows] = tmp;
            strcpy(tmp, str);
        }
    }
    return result;
}

int removeString(dynamicStringTable_t *ptr, int x, int y)
{
    int result = (ptr == NULL || x >= ptr->columns || y >= ptr->rows || !x || !y) * -1;

    if (!result)
    {
        free(ptr->table[x + y * ptr->rows]);
        ptr->table[x + y * ptr->rows] = NULL;
    }

        return result;
}

int destroyStringTable(dynamicStringTable_t *ptr, int x, int y)
{
    int result = (ptr == NULL || x >= ptr->columns || y >= ptr->rows || !x || !y) * -1;

    if (!result)
    {
        if (ptr->table != NULL)
        {
            for (int i = ptr->rows * ptr->columns - 1; i >= 0; i--)
                free(ptr->table[i]);
            free(ptr->table);
        }
        free(ptr);
    }
    return result;
}

Upvotes: 0

Christophe
Christophe

Reputation: 73376

This is undefined behavior: your array is an array of 100 pointers to char. But you've never initialized them. So when you address p[i] it gets an uninitialized pointer that could point anywhere, and when you dereference it with p[i][j] you might then freeze or suffer of anyother symptom of undefined behavior.

If you want to learn to use pointers and arrays:

Solution 1: define your array as char a[100][100];

Solution 2: in the outer loop of as(), start to allocate the chars with p[i] = new char[m];

If you want to learn modern C++:

Solution 3: Forget about memory allocation and deallocation and use vectors instead. The vectors are totally dynamic, so no maximum of 100 rows anymore:

void as(vector<vector<char>> &p, int n, int m)
{
    p.resize(n); 
    int i, j;
    for (i = 0; i < n; i++) {
        p[i].resize(m); 
        for (j = 0; j < m; j++)
        {
            cout << "p[" << i << "][" << j << "]=";
            cin >> p[i][j];
        }
    }
}

int main()
{
    vector<vector<char>>a;
    as(a, 3, 3);
    return 0;
}

If you want to try online...

Solution 4: you want modern C++, but you'd like to use your the elements in a[] as they were a string, for easy output and manipulation, just use the same code as above but replace vector<vector<char>> with vector<string>

And here you can look online the slightly simplified code.

Upvotes: 3

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