Question

I’m trying to create a function (in C) to solve an addition that is placed in a buffer variable. So far it works for equations that result in single-digit numbers, however if the result is more than one digit, it causes problems.. For example, if I place 2+3+5+2 in the buffer, outputs 102, not 12. Can anyone point out where my errors are and how I can fix them?

Here’s my code:

char buffer[50];

void *adder(void)
{
    int bufferlen;
    int value1, value2;
    int startOffset, remainderOffset;
    int i;
    char result[10];
    while (1) 
    {
        startOffset = remainderOffset = -1;
        value1 = value2 = -1;

        bufferlen = strlen(buffer);
        for (i = 0; i < bufferlen; i++) {
            if(value1 == -1)
            {
                if(isNumeric(buffer[i]))
                {
                    value1 = string2int(&buffer[i]);
                    startOffset = i;
                }
            }
            else
            {
                if(buffer[i] == 43)
                {
                    if(isNumeric(buffer[i+1]))
                    {
                        value2 = string2int(&buffer[i+1]);
                        remainderOffset = i+1;
                    }
                }
            }

            if(value1 != -1 && value2 != -1)
            {
                int k=0;
                int j=0;
                int2string((value1 + value2),result);
                /* print out the number we've found */
                fprintf(stdout, "Re:%s\n", result);
                int resultlen = strlen(result);
                for(k=startOffset; k < bufferlen; k++)
                {
                    if(j < resultlen)
                    {
                        buffer[k] = result[j];
                        j++;
                    }
                    else
                    {
                        /* shift the remainder of the string to the left */
                        printf("A1-Buffer:%s\nk=%i\n", buffer, k);
                        if(j > 0)
                        buffer[k] = buffer[k+2];
                        else
                        buffer[k] = buffer[k+2];
                        printf("A2-Buffer:%s\n", buffer);
                        i = i - remainderOffset;
                        startOffset = remainderOffset = -1;
                        value1 = value2 = -1;   
                    }
                }
            }
        }
    break;
    }

}

Edit: Ah, sorry, forgot about the extra functions. This function is for a school assignment and the isNumeric() and String2int() are exactly the same as isdigit() and atoi() and were given to us to use in the function.

Edit 2: After the numbers are added, the result has to be added back into the buffer in the expressions place like so: (2+3) -> (5) the reason is that eventually more functions will be written to handle multiplication, division, etc. This is where my main frustration lies – putting the result in place of the expression and shifting the buffer left the proper amount.

Answer 1

char buffer[50];

Why 50 bytes? Why not 100? You should accept a pointer as an argument instead of relying on a global. Also, buffer is not a descriptive name.

void *adder(void)

The function does not return a void *; it has no return value. You might consider

/* return value of expression */
int evaluate_add_expression( char const *in_string )

or

/* return status code */
int evaluate_add_expression( char const *in_string, int *out_value ) 

Next, you declare a number of variables without initializing them. Move the declarations into the loop and include initializations with the declarations, i.e. int value1 = -1, value2 = -1; Of course, those names are also not descriptive at all.

            if(buffer[i] == 43)

The value '+' is a better way to represent the plus character than the magic number 43.

---

If I were you, I’d avail myself of the sscanf function.

int evaluate_add_expression( char const *in_string, int *sum ) {
    int addend;
    size_t offset, offset_delta;

    if ( sscanf( in_string, " %d %zn", sum, &offset ) != 1 ) return 1;

    while ( sscanf( in_string + offset, "+ %d %zn", &addend, &offset_delta ) == 1 ) {
        * sum += addend;
        offset += offset_delta;
    }

    return in_string[ offset ] != '\0'; /* check that all input was consumed */
}

This works fine and is about 15% as long.