A better approach to add big numbers, Project Euler #13 [closed] - c

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Project euler problem 13
For C program I tried the problem with practical approach, i.e. not defining the data in the code but rather using scanf for taking input.
But I cannot understand why the output is wrong! I get 1373762303 while it should be 5537376230.
It seems ok for 2-3 numbers.
#include <stdio.h>
#include <ctype.h>
#include <strings.h>
#define NUM 50
#define STRINGS 100
#define OUTPUT 10
int main(void) {
char str[STRINGS][NUM+1];
int answer[NUM+1] = {0};
int carry = 0, out_digits = OUTPUT;
for(int i = 0; i < STRINGS; i++){
scanf("%s", str[i]);
for(int j = NUM; j >=0; j--){
answer[j] += (str[i][j] - 48) + carry;
if(answer[j] > 9){
carry = answer[j] / 10;
answer[j] %= 10;
}else{
carry = 0;
}
}
}
printf("--------------------------------------------------\r\n");
printf("%d",carry);
for(int j = 0; j < OUTPUT-1; j++){
printf("%d",answer[j]);
}
printf("\r\n--------------------------------------------------");
return 0;
}

Answer 1

scanffills in the array you give it with a string constructed from the input including a null terminator byte. You allocate 51 bytes correctly but when you start adding the digits, you start at index 50 which is the index of the nul byte. The actual digits are from indexes 0 to 49.
This means that you will get a carry into the units digit of your answer at some point because the answer for that digit is calculated as
answer[50] += (str[i][50] - 48) + carry;
// ^^^^ correction applied for ASCII
Another issue is that you forget to reset carry to zero at the beginning of adding each new number.
This might work (not tested) but it still doesn't really handle overflows in the top digits
for(int i = 0; i < STRINGS; i++){
scanf("%s", str[i]);
carry = 0; // Reset the carry
for(int j = NUM - 1; j >=0; j--){
answer[j] += (str[i][j] - 48) + carry;
if(answer[j] > 9){
carry = answer[j] / 10;
answer[j] %= 10;
}else{
carry = 0;
}
}
}

Answer 2

I got the answer, it was the missing carry over when adding the two numbers. Following is the solution:
#include <stdio.h>
#include <ctype.h>
#include <strings.h>
#define NUM 50
#define STRINGS 100
#define OUTPUT 10
int main(void) {
char str[STRINGS][NUM+1];
int answer[NUM+1] = {0};
int carry = 0, out_digits = OUTPUT, intm_carry = 0;
for(int i = 0; i < STRINGS; i++){
scanf("%s", str[i]);
for(int j = NUM; j >=0; j--){
answer[j] += (str[i][j] - 48) + carry;
if(answer[j] > 9){
carry = answer[j] / 10;
answer[j] %= 10;
}else{
carry = 0;
}
}
intm_carry += carry;
carry = 0;
}
printf("--------------------------------------------------\r\n");
printf("%d", intm_carry);
for(int j = 0; j < OUTPUT-1; j++){
printf("%d",answer[j]);
}
printf("\r\n--------------------------------------------------");
return 0;
}