Appending char to C array - c

Question

I have a string declared as such:
char *mode_s = (char *)calloc(MODE_S_LEN, sizeof(char));
How can I add a char to the end of the array?

Answer 1

Lets assume " first available position " means at index 0.
char *mode_s = (char *)calloc(MODE_S_LEN, sizeof(char));
*mode_s='a';
To store a character at an arbitrary index n
*(mode_s+n)='b';
Use pointer algebra, as demonstrated above, which is equivalent to
mode_s[n]='b';
One sees that the first case simply means that n=0.
If you wish to eliminate incrementing the counter, as specified in the comment bellow, you can write a data structure and a supporting function that fits your needs. A simple one would be
typedef struct modeA{
int size;
int index;
char *mode_s;
}modeA;
The supporting function could be
int add(modeA* a, char toAdd){
if(a->size==a->index) return -1;
a->mode_s[index]=toAdd;
a->index++;
return 0;
}
It returns 0 when the add was successful, and -1 when one runs out of space.
Other functions you might need can be coded in a similar manner. Note that as C is not object oriented, the data structure has to be passed to the function as a parameter.
Finally you code code a function creating an instance
modeA genModeA(int size){
modeA tmp;
tmp.mode_s=(char *)calloc(size, sizeof(char));
tmp.size=size;
tmp.index=0;
return tmp;
}
Thus using it with no need to manually increment the counter
modeA tmp=genModeA(MODE_S_LEN);
add(&tmp,'c');

Answer 2

There is no standard function to concatenate a character to a string in C. You can easily define such a function:
#include <string.h>
char *strcatc(char *str, char c) {
size_t len = strlen(str);
str[len++] = c;
str[len] = '\0';
return str;
}
This function only works if str is allocated or defined with a larger size than its length + 1, ie if there is available space at its end. In your example, mode_s is allocated with a size of MODE_S_LEN, so you can put MODE_S_LEN-1 chars into it:
char *mode_s = calloc(MODE_S_LEN, sizeof(*mode_s));
for (int i = 0; i < MODE_S_LEN - 1; i++) {
strcatc(mode_s, 'X');
}

Answer 3

char newchar = 'a'; //or getch() from keyboard
//realloc memory:
char *mode_sNew = (char *)calloc(MODE_S_LEN + 1, sizeof(char));
//copy the str:
srncpy(mode_sNew, mode_s, MODE_S_LEN);
//put your char:
mode_sNew[MODE_S_LEN] = newchar;
//free old memory:
free(mode_s);
//reassign to the old string:
mode_s = mode_sNew;
//in a loop you can add as many characters as you want. You also can add more than one character at once, but assign only one in a new position