How to dynamically expand a string in C

Question

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 stri…

Accepted Answer

You should allocate an extra byte for the terminating 0.

And you should use `strlen` instead of `sizeof` here:

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

Sizeof returns the size of `char*` (which is something like 4 on an average system) instead of the length of the string pointed to by `output`.

Moreover, as @Agnel rightly pointed out, `i` is uninitialized, although that does not make your program crash, just execute the loop for a random number of times.

Also, I would recommend `strstr` `strcat` for concatenating your strings.

How to dynamically expand a string in C

Solution

Problem