Manipulating Big Numbers as strings - c

Question

I have a problem that goes:
Create a C program that inputs large integers as strings.
Then every character is converted into the corresponding digit.
After that I have to create a function addBigNumbers() that has 3 matrices.
addBigNumbers(char *a1, char *a2, char *res)
a1 and a2 will contain the 2 large numbers that I want to add,res will contain the sum of those as a number sequence. We want the function that we created to check if the strings contains numbers only.
If it contains only numbers then res equals 1 and it prints the sum of those numbers else res is equal to 0 (max number length is 1000)
After that first function we want to create a function for subtraction.
So far I haven't gotten to subtraction since I stuck in the first one and I need your help.
This is the code that I have so far:
#include <stdio.h>
#include <stdlib.h>
#define N 1000
/* run this program using the console pauser or add your own getch, system("pause") or input loop */
int addHugeNumbers(char *a1, char *a2, char *res){
int y=0, u=0, h=0;
res=strcat(a1,a2);
if(strlen(a1)>strlen(a2)){
y=atoi(a1);
u=atoi(a2);
h=y+u;
}
else{
y=atoi(a1);
u=atoi(a2);
h=u+y;
}
printf("%d", h);
}
int main(int argc, char *argv[]) {
char res[N];
char a1[N/2];
char a2[N/2];
scanf("%s", &a1);
scanf("%s", &a2);
addHugeNumbers(a1, a2, res);
return 0;
}
The problem I have is that if I input ex. 23 23 it outputs 2346 which is obviously wrong but it got 46 correct, when I input 1234 123 it outputs 1234246 which is all wrong.
Where it gets weird is if i input something like 1234r 123 or anything else that has a character in it, it outputs the exact sum.

Answer 1

The problem is res=strcat(a1,a2), which does something very different than that what you think: it appends a2 to a1, and it does not "create" a new string. See, for example strcat-definition at cppreference.com:
char *strcat( char *dest, const char *src )
Appends a copy of the null-terminated byte string pointed to by src
to the end of the null-terminated byte string pointed to by dest. The
character src[0] replaces the null terminator at the end of dest. The
resulting byte string is null-terminated.
So you are manipulating your input before calculating something, and that's what you will observe then when using a debugger.
Further, scanf("%s", &a1) looks suspicious; it should be scanf("%s", a1);. Your compiler should have warned you.
You'd probably rethink addBigNumbers, probably adding the digits in a loop rather than converting them to (somehow always) limited integral data types in between. This task is actually nothing for beginners in C; take the following fragment to study:
#define N 1000
int addHugeNumbers(char *a1, char *a2, char *res){
char resultBuffer[N];
int i1 = (int)strlen(a1);
int i2 = (int)strlen(a2);
int carryOver = 0;
int ri = 0;
while (i1 > 0 || i2 > 0) { // until both inputs have been read to their beginning
i1--;
i2--;
// read single digits and consider that a string might have already
// been read to its beginning
int d1 = i1 >= 0 ? a1[i1] - '0' : 0;
int d2 = i2 >= 0 ? a2[i2] - '0' : 0;
// check for invalid input
if (d1 < 0 || d1 > 9 || d2 < 0 || d2 > 9) {
return 0;
}
// calculate result digit, taking previous carryOver into account
int digitSum = d1 + d2 + carryOver;
carryOver = digitSum / 10;
digitSum %= 10;
resultBuffer[ri++] = digitSum + '0';
}
// write the last carryOver, if any
if (carryOver > 0) {
resultBuffer[ri++] = carryOver + '0';
}
// copy resultBuffer into res in reverse order:
while(ri--) {
*res++ = resultBuffer[ri];
}
// terminate res-string
*res = '\0';
return 1;
}
int main(int argc, char *argv[]) {
char res[N];
char a1[N/2] = "123412341234";
char a2[N/2] = "1231";
if (addHugeNumbers(a1, a2, res)) {
printf("result: %s\n", res);
} else {
printf("invalid number.\n");
}
return 0;
}