pointer cannot maintain in c++ function - c++

Question

void concat(char *str, char *ch, int num)
{
*str= *ch; ++str;
while (num>0) {
*str = '0' + num % 10;
num /= 10;
++str;
}
}
concat(runner, 'a', 10);
concat(runner, 'b', 20);
i just want to concat one character like 'a' to 10, the expected result will be a10
the first line works fine. but i just thinking after the first line(concat a10), the runner should point to the end of string, so when i run the second line, it should be a10b20, but actual result is b20 overwrite the a10.
i think it should be pointer problem , can you help me.

Answer 1

I'm changing my answer altogether. Put this in the beginning of your function:
void concat(char * str, const char * ch, int num) {
while (*str) {
++str;
}
Then keep the rest the same. This is really what concat should look like. Just make sure that runner[0] == 0 before calling it the first time! And add the following code to the end of your function, before the final brace:
*str = 0;
}

Answer 2

using & should be ok
or actually in c, you can use **, two ways.

Answer 3

Well, the code does what you ask of it.
For this to work you need to find the end of the first string and then add to it:
void concat(char *str, char *ch, int num)
{
str += strlen(str); /* make sure we start adding at the end of str */
*str= *ch; ++str;
while (num>0) {
*str = '0' + num % 10;
num /= 10;
++str;
}
}
But now you must make sure str[0] is 0 at the beginning

Answer 4

Because every time concat is called, the index of str starts from 0. That's why the content of str is overwritten. Just skip all the filled positions in str before you append any to it.

Answer 5

The problem is that your function is not aware of the end of the string being passed in. To fix this you will need to intialize your char * to all 0's or \0. The other issue is you are converting your number to characters incorrectly. And finally there is nothing safe about the function since the size of the string is not passed in so you just have to make sure you allocate enough space before hand.
void concat(char *str, const char *ch, int num)
{
//This is function not safe since you do not
//know how much space str has allocated
str += strlen(str);
*str = *ch; ++str;
if(num < 0)
{
*str = '-';//Add the -
++str;
num *= -1; //Make the number positive
}
//Determine the number of digits first
//because you need to add characters backwards
int digits = 0, tmpnum = num;
while (tmpnum) {
tmpnum /= 10;
++digits;
}
while(digits--)
{
str[digits] = '0' + num % 10;
num /= 10;
}
}
Usage:
char *runner = new char[20]();
//or
//char *runner = (char*)calloc(20, 1);
concat(runner, "a", 10);
concat(runner, "b", 20);
concat(runner, "c", -30);
delete [] runner;
//or if you used calloc
//free(runner);
I did this assuming this was a homework assignment, there are easier/safer ways to accomplish this especially using C++ which is what your question was tagged.