c - Add '-' between each character in a string? -

how that? willing add character - after every character in string. specific case assume have string declared in size of 100 filled 3 letters (entered user - value of character str[3] null (\0)).

for(i = strlen(str) ; >= 0 ; i--) {     str[i*2] = str[i]; } for(i = 0 ; < strlen(str) ; i++) {     if(i % 2 != 0)     {         str[i] = '-';     } } 

but it's not working, anyone?

#include <stdio.h> #include <string.h>  int main() {    char str[100];    int len = 0;    int = 0;    strcpy(str, "abcd");     len = strlen(str);    // len = 4     // expected output: "a-b-c-d"    // str[7] = '\0';    // str[6] = 'd';    for(i = len-1; >= 0 ; i--)    {       str[i*2+1] = '-';       str[i*2] = str[i];    }    str[len*2-1] = '\0';     printf("%s\n", str); }