Question

I’m writing a program in C to do a simple dynamic programming algorithm where you return the minimum number of coins needed to add up to a certain amount. Here’s my code:

#include <limits.h>
#include <stdlib.h>
#include <stdio.h>

/*
This function returns the minimum number of stamps required for a given value.
It assumes that the given array contains the available stamp sizes, and that it
always contains 1, so a solution is always possible
*/
int min_number_of_stamps(const int* array, size_t array_size, int request) {

    /* Construct a table with dimensions (array_size+1)*(request+1) */ 
    int numRows = array_size + 1;
    int numCols = request + 1;
    int **DPtable; 
    DPtable = malloc(numRows*sizeof(int));
    int i;
    for (i = 0; i < numRows; i++) {
        DPtable[i] = malloc(numCols*sizeof(int));
    }
    printf("%d",DPtable[4][0]);
    int r, c, useIt, loseIt;
    for (r = 0; r < numRows; r++) {
        for (c = 0; c < numCols; c++) {
            printf("%d,%d\n", r, c);
            if (c==0) {
                printf("1\n");
                //if the amount of change is 0, 0 coins are needed 
                DPtable[r][c] = 0;
            } 
            else if ((r==0) || c < array[r-1]) {
                printf("2\n");
                //if there are no coins or if the change needed is less than 
                //the smallest coin available, then 'infinity' coins are needed
                DPtable[r][c] = INT_MAX;
            } 
            else { 
                printf("3\n");
                useIt = DPtable[r][c-array[r-1]] + 1;
                loseIt = DPtable[r-1][c];
                if (useIt <= loseIt) { 
                    //if 'use it' requires fewer coins than 'lose it,' then 
                    //'use it' coins are needed. 
                    DPtable[r][c] = useIt;
                } 
                else { 
                    //if 'lose it' requires fewer coins, 'lose it' coins are needed 
                    DPtable[r][c] = loseIt; 
                }
            }
        }
    }

    return DPtable[numRows][numCols];

}

int main() {
    const int array[] = {1,5,10,25};
    const int* stamps = &array[0];
    printf("%d", min_number_of_stamps(stamps, 4, 44));
}

I’m getting a segfault when my inner for loop gets to the case where r=4 and c=0. I left my debugging print statements in because I’m lazy, but you can see where I got stuck. If I access the same place in the array outside of my for loops, there’s no problem. But in the for loop, I get a `Segmentation fault: 11′ message after it outputs “4,0” for the array element and “1” for the if case it’s in. Can anyone see what I’m missing?

Answer 1

You’re allocating dpTable incorrectly. It should be

DPtable = malloc(numRows*sizeof(int*));

See if that fixes the problem.