writing to a file

Hello, in the next piece of code I will read the content of one file and write that to another file, accept for a certrain string which I will replace by another string that the user gave as input parameter. Now my question is; in the loop while( (ptr + lengte) + i < (zin + 18)), why do you check for zin + 18 instead of zin + 19? It does not seem correct to me, but the algoritme works, so I must see something wrong. What's wrong with thinking like this"

let's say that the fgets buffer was 5 instead of 19, and the zin buffer 6.
The string you read is: abcde. The substring you want to replace is cd. The begin address of zin is 1000. The length of the substring is 2.

while(zin + i < ptr)
#
{
#
putc(*(zin + i), output);
#
i++;
#
}
#

#
fputs(argv[4], output);

this goes correctly and you write abNEWSUBSTRING but then you come in the new while and:

#
i = 0;
#

#
while( (ptr + lengte) + i < (zin + 4))

1002 (the beginaddress of the substring) + 2 < 1000 + 4. Not true so you will not write e!

Code:

---

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main (int argc, char *argv[])
{

        FILE *input;
        FILE *output;

        char *ptr;

        char zin[20];
        int lengte, i = 0;

        if (argc != 5)
        {
                printf("Aantal bestanden zijn fout\n");
                exit(0);
        }

        if ((input = fopen(argv[1], "r")) == NULL)
        {
                perror("Fout ");
                exit(0);
        }

        if ((output = fopen(argv[2], "w")) == NULL)
        {
                perror("Fout ");
                exit(0);
        }


        while (fgets(zin, 19 ,input) != NULL)
        {
                if (strstr(zin, argv[3]) == NULL)
                        fputs(zin, output);
                else
                {
                        lengte = strlen(argv[3]);
               
                        ptr = strstr(zin, argv[3] );       
                               
       
                        while(zin + i < ptr)
                        {
                                putc(*(zin + i), output);
                                i++;
                        }
       
                        fputs(argv[4], output);
                       
                        i = 0;
               
                        while( (ptr + lengte) + i < (zin + 18))
                        {
                                putc(*(ptr + i + lengte), output);
                               
                                i++;
                        }
               
               
                }
        }

        fclose(input);
        fclose(output);

        return 0;
}

---

I do not think +18 or +19 is correct...

better get rid of the loop and just use

fputs(ptr+lengte);