动态分配的二维数组问题

Question

I am trying to understand what I am doing wrong when attempting to dynamically allocate the 2D array "allWordMultiArray" and assign values to it. I've been reading a lot of articles online (and in Stackoverflow in particular) and tried to implement it in a lot of ways but unsuccessfully. using C++ 14.

So, the following code creates a warning of "Using uninitialized memory '*allWordMultiArray[i]'". Obviously, when trying to cout the values in the 2D array it prints garbage. Can someone point to me what the problem is?

Here's a summary of the code:

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <string>
using namespace std;

const int letters = 26;

int main() {
....

int numWords, pos;
string input;
int** allWordMultiArray;
cin >> numWords;

allWordMultiArray = new int* [numWords];

    for (int i = 0; i < numWords; i++) {
        cin >> input;

        allWordMultiArray[i] = new int[letters];
        for (unsigned long int j = 0; j < input.size(); j++)
        {
            pos = (input[j] - 'a');

            //Here is the problem. Garbage values
            allWordMultiArray[i][pos]++;
        }

    }
return 0;
}

Your help is very much appreciated.

Answer 1

This line:

allWordMultiArray[i] = new int[letters];

does allocate memory for an array, but the values in the array are indeterminate. Reading from these values, like here:

allWordMultiArray[i][pos]++;

will invoke undefined behavior.

Just initialize the array when you allocate it:

allWordMultiArray[i] = new int[letters] {};
                                    //  ^^