How to dynamically expand a string in C

Question

I have a function that does some calculations on a set of numbers recursively. I also want to print calculations in each recursion call, passing the string from the previous calculation and concatenating it with the current operation. An example output might look like this:

3
(3) + 2
((3) + 2) / 4
(((3) + 2) / 4) x 5
((((3) + 2) / 4) x 5) + 14
... and so on

So the second call gets 3 and adds + 2 to it, the third call is passed (3) + 2, etc. My prototype of a recursive function is as follows:

void calc_rec(int input[], int length, char * previous_string);

I wrote two helper functions that will help me with this operation, but they explode when I test them:

/**********************************************************************
 * dynamically allocate and append new string to old string and return a pointer to it
 **********************************************************************/
 char * strapp(char * old, char * new)
 {
     // find the size of the string to allocate
     int len = sizeof(char) * (strlen(old) + strlen(new));

     // allocate a pointer to the new string
     char * out = (char*)malloc(len);

     // concat both strings and return
     sprintf(out, "%s%s", old, new);

     return out;
 }

/**********************************************************************
 * returns a pretty math representation of the calculation op
 **********************************************************************/
 char * mathop(char * old, char operand, int num)
 {
     char * output, *newout;
     char fstr[50]; // random guess.. couldn't think of a better way.
     sprintf(fstr, " %c %d", operand, num);
     output = strapp(old, fstr);
     newout = (char*)malloc( 2*sizeof(char)+sizeof(output) );
     sprintf(newout, "(%s)", output);
     free(output);
     return newout;  
 }


void test_mathop()
{
    int i, total = 10;
    char * first = "3";
    printf("in test_mathop\n");
    while (i < total)
    {
        first = mathop(first, "+", i);
        printf("%s\n", first);
        ++i;
    }
}

strapp() (), mathop() ( "(3) +2" ), char ('+', ' - ' ..) int , "((3) +2)/3". , ? .

Answer 1

0.

strlen sizeof :

newout = (char*)malloc( 2*sizeof(char)+sizeof(output) );

Sizeof char* (- 4 ) , output.

, @Agnel, i , , .

, strstr strcat .

Answer 2

, , - :

int len = sizeof(char) * (strlen(old) + strlen(new));

NULL char . char.

Answer 3

, :

char * append_strings(const char * old, const char * new)
{
    // find the size of the string to allocate
    size_t len = strlen(old) + strlen(new) + 1;

    // allocate a pointer to the new string
    char *out = malloc(len);

    // concat both strings and return
    sprintf(out, "%s%s", old, new);

    return out;
}

:

• , const.
• , , .
• size_t.
• sizeof (char).
• malloc().
• , str, . , !

, strlen() sprintf():

char * append_strings(const char * old, const char * new)
{
    // find the size of the string to allocate
    const size_t old_len = strlen(old), new_len = strlen(new);
    const size_t out_len = old_len + new_len + 1;

    // allocate a pointer to the new string
    char *out = malloc(out_len);

    // concat both strings and return
    memcpy(out, old, old_len);
    memcpy(out + old_len, new, new_len + 1);

    return out;
}

, . , memcpy() new, .

Answer 4

, - strcat , '\ 0' char. realloc, "" . strapp (append_strings) , mathop. :

/**********************************************************************
 * returns a pretty math representation of the calculation op
 **********************************************************************/
 char * mathop(char * previous, char operand, float num)
 {
     char *output, *temp, calculation[50];
     size_t newlen;

     // copy the previous data into the temp string     
     temp = (char*)malloc( strlen(previous) + 1 );
     output = (char*)malloc(2);

     strcpy(temp, previous);
     strcpy(output, "(\0");

     // create the string portion to append to the output
     sprintf(calculation, " %c %.1f)\0", operand, num);

     // reallocate the output to append the additional string
     newlen = strlen(temp) + strlen(calculation) + 1;
     temp = realloc(temp, newlen);

     // append the new data to output
     strcat(temp, calculation);
     output = realloc(output, strlen(temp) + 2);
     strcat(output, temp);
     printf("%s\n", output);

     free(temp);
     return output;  
 }

!