Automatic Editing

#include <stdlib.h>
#include <stdio.h>
#include <ctype.h>
#include <math.h>
#include <string.h>
#define EPS 1e-8
#define pi 3.1415926535897932384626433832795028841971693993751
#define N 2147483647

int t, i, j, k, l, a, b, c, d, x, y, z, flag, flag2, len, len2, n, m, p;
char ch, str[1000][1000], str2[1000][1000], str3[10000], str4[10000], str5[10000];
char *ptr, *pch;

int main()
{
    /*freopen("sample.txt", "r", stdin);*/
    for (; ;)
    {
        scanf("%d", &n);
        scanf("%c", &ch);
        if (n == 0) break;
        for (j = 0; j < n; j++)
        {
            gets(str[j]);
            gets(str2[j]);
        }
        gets(str4);

        for (j = 0; j < n;)
        {
            pch = strstr(str4, str[j]);
            if (pch != '\0')
            {
                l = 0;
                for (ptr = str4; ptr < pch; ptr++)
                {
                    str3[l] = *ptr;
                    l++;
                }

                len = strlen(str2[j]);
                for (k = 0; k < len; k++)
                {
                    str3[l] = str2[j][k];
                    l++;
                }
                len = strlen(str[j]);
                for (ptr = pch + len; *ptr != '\0'; ptr++)
                {
                    str3[l] = *ptr;
                    l++;
                }
                str3[l] = '\0';
                strcpy(str4, str3);
            }
            else
            {
                j++;
            }
        }
        printf("%s\n", str4);
    }
}