String Concatination

char* strcat(char *s, char*t)
{
    if( t == NULL)
      return s;

      char *temp =s;
      while( *s != '')  // don't compare like *s++!='' it lead to errors since wrong pointing.
       s++;
      while( (*s++ = *t++)!='');
      return temp;
}

void main() { char s1[40],s2[20]; int i=0,j=0;
                      printf("enter a string");
                     scanf("%s",s1);
                     printf("enter another string");
                     scanf("%s",s2);
                      while(s1[i]!='')
                        i++;
                      while(s2[j]!='')
                       s1[i++]=s2[j++];
                       s1[i]='';
                      printf("concatenated string is%s",s1);
                  }

void main() { char s[20]; int i,n;
printf("enter a string");
scanf("%s",s);
for(i=0;s[i]!='';i++)
{
if(s[i]=='.')
{
s[i]='';
break;
}}
n=atoi(s);
if(n<255)
printf("%d is less");
else
printf("not less");
}