Substring replacement in a string in C [closed] - c

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everyone! I decided to make an account on this site seeking for help because I could not find anything that can help me. I have a problem which requires multiple steps:
1) Read a string from keyboard;
2) Read an integer that represents the number of substrings that you want to modify ( By modifying it means to transform all of the characters of the string into " * " );
3)Read all of the substrings that need to be modified;
4) Print the initial string modified.
I tried to make it with little steps by firstly managing to do it with one substring, but I don't know how to do it when I read multiple substrings. So, here are the versions
VERSION 1
#include<stdlib.h>
#include<string.h>
int main()
{
int n;
char str[5120];
char substr[100];
printf("Type the string:\n");
gets(str);
printf("\nType the substring which you would like to modify:");
gets(substr);
char *p;
p = strstr(str,substr);
if (*p != '\0')
{
for( int j=0 ; j< strlen(substr); j++)
{
*p = '*';
p++;
}
}
printf("\nThe modified string is:\n");
for ( int i=0 ; i< strlen(a) ; i++)
{
printf("%c",a[i]);
}
return 0;
}
AND THE VERSION IN WHICH I TRIED TO MODIFY MULTIPLE
#include<stdlib.h>
#include<string.h>
int main()
{
int n;
char str[5129];
char substr[n][100];
printf("Type the string:\n");
gets(str);
printf("How many substrings to modify?");
scanf("%d",&n);
printf("\nType the substrings:");
for( int i=0 ; i< n ; i++)
{
scanf("%s",substr[i]);
printf(" ");
}
for ( int i=0; i < n; i++)
{
char *p;
p = strstr(str,substr[i]);
if (*p != '\0')
{
for( int j=0 ; j< strlen(cuv[i]); j++)
{
*p = '*';
p++;
}
}
}
printf("\nThe modified string is:\n");
for ( int i=0 ; i< strlen(str) ; i++)
{
printf("%c",str[i]);
}
return 0;
}```
I guess for the n times that I cycle to the string I have to allocate memory somewhere, or I don't know..
Please, I need some help! Thank you!

Answer 1

So I edited my answer based on your and #PaulOgilvie suggestions. I hope that the program now works as you expected.
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#define MAX_SIZE 5129
void replaceSubstring(char *p, int len){
for(int i = 0; i < len; i++, p++){
*p = '*';
}
}
int main() {
int n = 0;
char str[MAX_SIZE];
char substr[n][100];
printf("Type the string:\n");
fgets(str, MAX_SIZE, stdin);
printf("How many substrings to modify?");
scanf("%d",&n);
for( int i=0 ; i< n ; i++) {
printf("\nType the substring number %d:", i+1);
scanf("%s",substr[i]);
}
char *p;
for ( int i=0; i < n; i++){
p = strstr(str, substr[i]);
while(p != NULL) {
replaceSubstring(p, strlen(substr[i]));
p = strstr(str, substr[i]);
}
}
printf("\nThe modified string is:\n");
for ( int i=0 ; i< strlen(str) ; i++)
{
printf("%c",str[i]);
}
return 0;
}