TO REPLACE A WORD BY ANOTHER WORD IN A GIVEN STRING

#include<stdio.h>
#include<conio.h>
int stlen(char str[50])
{
        int len = 0;
        while(str[len]!='\0')
              len++;
        len--;
        return len;
}
void stcat(char str1[50], char str2[50])
{
        int i = 0,len = 0;
        while(str1[len]!='\0')
              len++;
        while(str2[i]!='\0')
        {
              str1[len] = str2[i];
              i++;
              len++;
        }
        str1[len] = '\0';
}
void main()
{
        char str1[50], str2[50], str3[50], temp[50];
        int len1, len2, len3, i, j, match, k;
        clrscr();
        printf("\n\n\t ENTER A SENTENCE...: ");
        gets(str1);
        len1 = stlen(str1);
        printf("\n\n\t ENTER A STRING WHICH YOU WANT TO DELETE...: ");
        gets(str2);
        len2 = stlen(str2);
        printf("\n\n\t ENTER A NEW STRING WHICH YOU WANT TO INSERT ...: ");
        gets(str3);
        len3 = stlen(str3);
        for(i=0;i<=len1;i++)
        {
                match = 1;
                for(j=0;j<=len2;j++)
                       if(str2[j]!=str1[i+j])
                       {
                               match = 0;
                               break;
                       }
                if(match)
                {
                       for(k=0,j=i+len2+1;j<=len1;j++,k++)
                               temp[k] = str1[j];
                       temp[k] = '\0';
                       for(j=0;j<=len3;j++)
                               str1[i+j] = str3[j];
                       str1[i+j] = '\0';
                       stcat(str1,temp);
                       len1 = len1 - len2 +len3;
                       i = i + j;
                }
        }
        printf("\n\n\t OUTPUT IS...: ");
        puts(str1);
        getch();
}

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