How can I shift right after a certain point in the string? - c

Question

My code provides the following parameters requested by the teacher:
But the teacher also wants this: "insertChar function should not overwrite any characters. It should insert the new character and offset remaining characters by one index."
how can I set this up? I mean how can I shift a string right from a certain point?
#include<stdio.h>
#include<string.h>
void insertChar(char string[], char c, int index) {
int len = 0;
int i;
for (i = 0; i > index; i--) {
string[i] = len;
string[i] = string[i-1];
string[i-1] = len;
}
string[index] = c;
}
int main() {
char string[100];
printf("Please input a string \n");
scanf ("%[^\n]%*c", string);
printf("Please input the character to be added to string.\n");
char c;
scanf ("%c", &c);
printf("Please input the index which the function will insert the character. \n");
int index;
scanf("%d", &index);
insertChar(string, c, index);
printf("%s", &string);
return 0;
}

Answer 1

How can I shift right after a certain point in the string?
(OP's insertChar() is too broken for repair.)
Watch out for 2 pitfalls: inserting well past the end of the string and exceeding the size of the buffer:
#include <string.h>
void insertChar(size_t buffer_size, char string[], char c, size_t index) {
size_t size_used = strlen(string) + 1;
if (size_used >= buffer_size) {
fprintf(stderr, "Buffer too small for insertion or existing string.");
} else if (index >= size_used) {
fprintf(stderr, "Inserting well past the end of the string.");
} else {
// Move the right portion of the string by 1 with memmove
// v----------------v Address one past the insertion point
memmove(&string[index + 1], &string[index], size_used - index);
// ^------------^ Insertion location
string[index] = c; // Insert
}
}
Usage
char string[100];
...
insertChar(sizeof string, string, c, index);