How to dynamically expand a string in C
- by sa125
Hi - I have a function that recursively makes some calculations on a set of numbers. I want to also pretty-print the calculation in each recursion call by passing the string from the previous calculation and concatenating it with the current operation. A sample 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 basically, the second call gets 3 and appends + 2 to it, the third call gets passed (3) + 2 , etc. My recursive function prototype looks like this:
void calc_rec(int input[], int length, char * previous_string);
I wrote a 2 helper functions to help me with the operation, but they implode 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() returns a pointer to newly appended strings (works), and mathop() is supposed to take the old calculation string ("(3)+2"), a char operand ('+', '-', etc) and an int, and return a pointer to the new string, for example "((3)+2)/3". Any idea where I'm messing things up? thanks.
