rewritten

/* Program to compute thermal equilibrium map of a heatbath bath has walls at T=0,
two heat sources (T=10(50,50), T=7.2(40,60)) and a sink at T=-1.2(70,25). Bath can
be approximated by 100x100 grid where each node's temperature evolves iteratively
as the average of its 4 nearest neighbours and itself. Assume all undefined grid
points to be T=0 on start. */

#include <stdio.h> // printf
#include <stdlib.h> // calloc, free
#include <math.h> // fabs

#define ARRAY_SIZE 100
#define MAX_RELATIVE_ERROR 0.001 // 0.1%

/* Function prototypes */
void setConstantCells(float **array, int size, int x, int y);
void printArray(float **array, int size, int x, int y);
void calcNewCellTemp(float **array, float **arrayTemp, int size, int x, int y);
int equilibriumCheck(float **array, float **arrayTemp, int size, int x, int y);
void updateTempArray(float **array, float **arrayTemp, int size, int x, int y);
void printResult(float **array, int size, int i);
void deallocateArrays(float **array, float **arrayTemp, int size, int i);

int main(int argc, char *argv[]){
    /* Wide scope variables */
    int x, y, i;
    int success = 0;
    /* Pointer setup and array allocation */
    float **array, **arrayTemp; // pointer to pointer to ints
    array = (float **)malloc(sizeof(float *) * (ARRAY_SIZE + 1)); // column 0
    arrayTemp = (float **)malloc(sizeof(float *) * (ARRAY_SIZE + 1));
    for(i = 0; i < ARRAY_SIZE + 1; i++){
        array[i] = (float *)malloc(sizeof(float) * (ARRAY_SIZE + 1)); // generate rows
        arrayTemp[i] = (float *)malloc(sizeof(float) * (ARRAY_SIZE + 1));
    }

    /* Body of program */
    setConstantCells(array, ARRAY_SIZE, x, y);
    while(success != 1){
        i++;
        calcNewCellTemp(array, arrayTemp, ARRAY_SIZE, x, y);
        setConstantCells(arrayTemp, ARRAY_SIZE, x, y);
        success = equilibriumCheck(array, arrayTemp, ARRAY_SIZE, x, y);
        updateTempArray(array, arrayTemp, ARRAY_SIZE, x, y);
    }
    printResult(array, ARRAY_SIZE, i);
    deallocateArrays(array, arrayTemp, ARRAY_SIZE, i);
    return 0;
}

/* Resets the values which remain constant throughout */
void setConstantCells(float **array, int size, int x, int y){
    /* Set edges to 0 */
    for(y = ARRAY_SIZE; y >= 0; y--){
        array[0][y] = 0;
        array[ARRAY_SIZE][y] = 0;
    }
    for(x = 0; x <= ARRAY_SIZE; x++){
        array[x][0] = 0;
        array[x][ARRAY_SIZE] = 0;
    }

    /* Set heat sources / sinks */
    array[40][60] = 7.2;
    array[50][50] = 10;
    array[70][25] = -1.2;
}

/* Debugging print function */
void printArray(float **array, int size, int x, int y){
    for(y = ARRAY_SIZE; y >= 0; y--){
        for(x = 0; x <= ARRAY_SIZE; x++){
            //printf("(%d,%d) = %.2f\t", x, y, array[x][y]);
            printf("%.2f\t", array[x][y]);
        }
        printf("\n");
    }
    printf("\n");
}

/* Calculate temperature changes and assign to arrayTemp */
void calcNewCellTemp(float **array, float **arrayTemp, int size, int x, int y){
    int up, down, left, right;
    for(y = ARRAY_SIZE; y >= 0; y--){
        for(x = 0; x <= ARRAY_SIZE; x++){
            up = y + 1;
            down = y - 1;
            left = x - 1;
            right = x + 1;
            if(up > ARRAY_SIZE)
                up = ARRAY_SIZE;
            if(down < 0)
                down = 0;
            if(left < 0)
                left = 0;
            if(right > ARRAY_SIZE)
                right = ARRAY_SIZE;
            arrayTemp[x][y] = (array[x][y] + array[left][y] + array[right][y] + array[x][down] + array[x][up]) / 5;
        }
    }
}

/* Check for thermal equilibrium */
int equilibriumCheck(float **array, float **arrayTemp, int size, int x, int y){
    float relativeError;
    int totalThermalEquilibrium = 1;
    for(y = ARRAY_SIZE; y >= 0; y--){
        for(x = 0; x <= ARRAY_SIZE; x++){
            relativeError = fabs(array[x][y] - arrayTemp[x][y]) / arrayTemp[x][y];
            if(relativeError > MAX_RELATIVE_ERROR)
                totalThermalEquilibrium = 0;
        }
    }
    return totalThermalEquilibrium;
}

/* Set array to updated values */
void updateTempArray(float **array, float **arrayTemp, int size, int x, int y){
    for(y = ARRAY_SIZE; y >= 0; y--){
        for(x = 0; x <= ARRAY_SIZE; x++){
            array[x][y] = arrayTemp[x][y];
        }
    }
}

void printResult(float **array, int size, int i){
    float percentageAccuracy = MAX_RELATIVE_ERROR * 100;
    printf("Finished at iteration #: %d\n", i);
    printf("Thermal equilibrium attainted, with maximum %g%% change between iterations for any points\n", percentageAccuracy);
    printf("Temperature @ (55,55) = %.2f\n", array[55][55]);
}

void deallocateArrays(float **array, float **arrayTemp, int size, int i){
    for(i = 0; i < ARRAY_SIZE + 1; i++){
        free(array[i]);
        free(arrayTemp[i]);
    }
    free(array);
    free(arrayTemp);
}