Add letter if it's duplicate C [closed] - c

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I work on playfair cipher and I have one problem.
I need to add letter X in my string if the next letter it's duplicate.
Example before: HELLOWORLD
separe the string in 2 pairs (HE LL OW OR LD) and if it's duplicate add X. (I need to add X only if the pair is duplicate.
After: HE LX LO WO RL D -> HELXLOWORLD
My code:
for (j = 0, i = 0; i < len_text; i++, j++) {
if (i % 2 == 0) {
if (my_text[i] == my_text[i+1]) {
text_x[j] = my_text[i];
i++;
text_x[j+1] = 'X';
j++;
}
else {
text_x[j] = my_text[i];
}
}
else if (i % 2 != 0) {
text_x[j] = text[i];
}
}
My code don't works normally. Can you help me? Thank you.

Answer 1

If I'm understanding your problem correctly, you could do something like this. The way you're currently doing it looks like it could potentially go out of bounds with i. Make sure that text_x is a buffer big enough to hold these extra chars you're inserting (i.e. len_text + len_text / 2 at max).
for (j = 0, i = 0; i < len_text - 1; i+=2, j+=2) {
text_x[j] = my_text[i];
if (my_text[i] == my_text[i+1]) {
text_x[j+1] = 'X';
j++;
}
text_x[j+1] = my_text[i+1];
}