如何在C ++中从动态数组保存值？

Question

EDIT: The problem has been solved, I was accessing data that had not been initialized because of the incorrect while() condition. I have changed it from OR to AND. It now works as intended. Thanks!

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I am trying to find the intersection between two arrays in C++. I have written code that does what I want, but when I delete[] the arrays it breaks resulting in a floating point exception. (Division by zero?) How can I save the value I want, without causing a memory leak in my program?

This code works exactly how I intend it to work if I omit the delete[] statements, but I believe this causes a memory leak. It will return 1 if I omit the statement biggestIntersection = *(factorsa + i); What can I do to save the value at factorsa + i and subsequently delete the array to avoid a memory leak?

const int Fraction::findgcf(int a, int b) const{
    a = std::abs(a); //absoute value
    b = std::abs(b);

    int* factorsa = new int[a]; //dynamic array of ints to store factors of a
    int* factorsb = new int[b]; //dynamic array of ints to store factors of b


    int numFactorsa = 0;
    for(int i = 1; i <= a; i++){//iterate over the ints from 1 to a
        if(a % i == 0) {//if we find a factor of a
            *(factorsa + numFactorsa) = i;// and append that to the array
            numFactorsa++;
        }

    }

    int numFactorsb = 0;
    for(int i = 1; i <= b; i++){
        if(b % i == 0){
            *(factorsb + numFactorsb) = i;
            numFactorsb++;
        }
    }

    int biggestIntersection = 1;

     int i = 0, j = 0;
    while(i < numFactorsa || j < numFactorsb){//while we are in the bounds of the two arrays
        if(*(factorsa + i) < *(factorsb + j)){ //if the factor of a is less than the factor of b
            i++;                               //move the index of a up one
        } else if (*(factorsa + i) > *(factorsb + j)){ //if the factor of b is less than the factor of a
            j++;                                       //move the index of b up one
        } else {                                    //otherwise they must be equal
            biggestIntersection = *(factorsa + i); //so that is the new biggest intersection between the sets
            i++; j++;
        }
    }

    delete [] factorsa;
    delete [] factorsb;
    return biggestIntersection;

Answer 1

You really should be using std::vector. Then you don't have to worry about cleaning up.

const int Fraction::findgcf(int a, int b) const{
    a = std::abs(a); //absoute value
    b = std::abs(b);

    std::vector<int> factorsa(a);
    std::vector<int> factorsb(b);

    int numFactorsa = 0;
    for(int i = 1; i <= a; i++){//iterate over the ints from 1 to a
        if(a % i == 0) {//if we find a factor of a
            factorsa[numFactorsa] = i;// and append that to the array
            numFactorsa++;
        }

    }

    int numFactorsb = 0;
    for(int i = 1; i <= b; i++){
        if(b % i == 0){
            factorsb[numFactorsb] = i;
            numFactorsb++;
        }
    }

    int biggestIntersection = 1;

    int i = 0, j = 0;
    while(i < numFactorsa || j < numFactorsb){//while we are in the bounds of the two arrays
        if(factorsa[i] < factorsb[j]){ //if the factor of a is less than the factor of b
            i++;                               //move the index of a up one
        }
        else if (factorsa[i] > factorsb[j])
        {                                              //if the factor of b is less than the factor of a
            j++;                                       //move the index of b up one
        } else {                                    //otherwise they must be equal
            biggestIntersection = factorsa[i];      //so that is the new biggest intersection between the sets
            i++; j++;
        }
    }

    return biggestIntersection;
}

Answer 2

The biggest problem - and likely what is causing your error, though a minimal example would make it clearer - is that you're accessing memory you haven't initialized. This can produce unpredictable behavior.

int* factorsa = new int[a]; doesn't set each int in that array to zero - the contents of the array could be literally anything. Later, in your first for loop, you do set the values for some array locations, but not all of them. And so in your final for loop, you have no way of knowing what you're going to output. It will depend on the more-or-less random contents of the memory location you asked new to provide.

(Also, as a comment notes, your while loop condition is wrong.)