Converting String Related Function from C++ to C - c++

Question

there is a function which I have written it myself for large numbers' Add (+) operator in C++, now I'm trying to convert it to C language but there is some problems with strings :( I tried so many things but it seems it is not working, Here is a piece of my code in C++:
#include <iostream>
#include <stdlib.h>
using namespace std;
string num1,num2,Result;
string AddF (string num1, string num2)
{
string Result;
char d;
int sum,f=0,Len1=num1.length(),Len2=num2.length();
while(Len1>=0 || Len2>=0)
{
sum=0;Len1--;Len2--;
if (Len1>=0)
sum+=num1[Len1]-'0';
if (Len2>=0)
sum+=num2[Len2]-'0';
d=((sum+f)%10)+'0';
Result=d+string(Result);
f=(sum+f)/10;
}
d=f+'0';
if (f!=0)
Result=d+string(Result);
if(Result[0]=='0')
Result.erase(0,1);
return Result;
}
int main()
{
cout << "Please Enter 1th Number: "; cin >> num1;
cout << "Please Enter 2th Number: "; cin >> num2;
Result=AddF(num1,num2);
}
Now My Converted Code in C:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char *num1,*num2,*Result;
char* AddF (char *num1, char *num2)
{
char *Result,*d;
int sum,f=0,Len1=strlen(num1),Len2=strlen(num2),Temp;
while(Len1>=0 || Len2>=0)
{
sum=0;Len1--;Len2--;
if (Len1>=0)
sum+=num1[Len1]-'0';
if (Len2>=0)
sum+=num2[Len2]-'0';
Temp=((sum+f)%10);
itoa(Temp,d,10);
strcat(d,Result);
Result=d;
f=(sum+f)/10;
}
itoa(f,d,10);
if (f!=0)
{
strcat(d,Result);
Result=d;
}
if(Result[0]=='0')
memmove(&Result[0], &Result[0 + 1], strlen(Result) - 0);
return Result;
}
int main()
{
printf ("Please Enter 1th Number: "); scanf ("%s",&num1);
printf ("Please Enter 2th Number: "); scanf ("%s",&num2);
Result=AddF(num1,num2);
}
It will Crash when It reach last line.

Answer 1

Your code doesn't work because of several reasongs
You have to allocate d. I would recomend you declare d as an array and use the snptrinf or _snprintf functions something like
char d[32];
int x = 3;
snprintf(d, sizeof(d), "%d", x);
You have to pass an allocated memory buffer to scanf too, you can pass an array, something like
char num1[32];
char num2[32];
scanf("%31s", num1);
scanf("%31s", num2);
and the same goes for Result
You are returning an array instead of a pointer from the AddF() function, you can use a temporary array and then copy the string at the end, and return the copy, don't solve the problem using global variables.
You are assigning to Result unecessarily, if you use strcat() the destination string will be always appendend to the end of the source string.
And this will in fact be a mistake, since you declared Result as a pointer, and you are making point somewhere else, thus loosing the reference to the original pointer.
This might be what you want
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char *AddF (char *number0, char *number1)
{
char *result;
int sum;
int carry;
int lengths[2];
int count;
if ((number0 == NULL) || (number1 == NULL))
return NULL;
lengths[0] = strlen(number0);
lengths[1] = strlen(number1);
result = malloc(lengths[0] + lengths[1] + 1);
if (result == NULL)
return NULL;
carry = 0;
count = 0;
while (lengths[0] >= 0 || lengths[1] >= 0)
{
lengths[0]--;
lengths[1]--;
sum = 0;
if (lengths[0] >= 0)
sum += number0[lengths[0]] - '0';
if (lengths[1] >= 0)
sum += number1[lengths[1]] - '0';
memmove(result + 1, result, ++count);
result[0] = ((sum + carry) % 10) + '0';
carry = (sum + carry) / 10;
}
result[count] = '\0';
if (result[0] == '0')
memmove(result, result + 1, count);
if (carry == 0)
return result;
memmove(result + 1, result, ++count);
result[0] = carry + '0';
result[count] = '\0';
return result;
}
int main()
{
char *numbers[2] = {NULL, NULL};
char *result;
size_t i;
for (i = 0 ; i < sizeof(numbers) / sizeof(*numbers) ; i++)
{
char chr;
size_t length;
length = 0;
printf ("Please Enter %ldth Number: ", 1 + i);
while ((chr = getchar()) != '\n')
{
char *number;
number = realloc(numbers[i], (2 + length));
if (number == NULL)
{
if (numbers[0] != NULL)
free(numbers[0]);
if (numbers[1] != NULL)
free(numbers[1]);
return -1;
}
number[length] = chr;
numbers[i] = number;
length += 1;
}
numbers[i][length] = '\0';
}
result = AddF(numbers[0], numbers[1]);
if (result != NULL)
{
printf("%s\n", result);
free(result);
}
if (numbers[0] != NULL)
free(numbers[0]);
if (numbers[1] != NULL)
free(numbers[1]);
return 0;
}