Access to a specific element on an array using pointers C - c

Question

I am working to replace '.' by '::' in a phrase.
I am given a phrase with 30 caracteres, without using other array. I would like to access to the last element using a pointer.
However,
First I count the dots in my phrase.
actualSize= 0; i= 0; dotNumb= 0;
while (i<actualSize){
if (tab[i]=='.') dotNumb++
i++
}
Now I should start by the end; whenever I find an element I move it, whenever I find a '.' I make an operation two times, by copying ':' two times.
Now I need to access to this element tab[dotNumb+actualSize]
Can I do it this way, or should I use pointers .
int newSize = dotNumb+actualSize ;
int j=newSize ;
int cursor=actualSize;
while (j>0){
if (tab[i]!='.') {tab[j]=tab[cursor]; }
else{tab[j]=':';tab[--j]=':';}
cursor--; j--;
}

Answer 1

The code you wrote does not work, (you made some typos, for example actualSize is set to 0 so the loop counting the dots will never execute. and the logic of your code would delete the character preceding a dot in the original array). You probably want something like that:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main(void) {
char *tab = malloc(30 + 1);
if (tab == NULL) {
return -1;
}
strcpy(tab, "123456789.123456789.123456789.");
printf("Original string: %s\n", tab);
size_t actualSize = strlen(tab);
int dotNumb = 0;
for (int i = 0; i < actualSize; i++) {
if (tab[i]=='.') {
dotNumb++;
}
}
const size_t newSize = dotNumb + actualSize + 1;
tab = realloc (tab, newSize);
if (tab == NULL) {
/* leak memory previously allocated in tab */
return -1;
}
tab[newSize] = '\0'; /* termination character */
int j = newSize ;
for (size_t cursor = actualSize; cursor > 0; cursor--) {
if (tab[cursor] != '.') {
tab[j--] = tab[cursor];
}
else {
tab[j--] = ':';
tab[j--] = ':';
}
}
printf("Modified string: %s\n", tab);
return 0;
}
You can test the code here:
http://ideone.com/YOxJKo