#include <iostream.h>
#include <stdio.h>

using namespace std;

int main()
{
  char * s;
  cin >> s;

  int a[200] = {5};
  for (int i = 0; i < strlen(s); i++)
    a[i+1] = (s[i] - 'a');
  int n = strlen(s);

  for (int i = n; i >= 1; i--)
  {
     a[i] -= a[i-1];
     if (a[i] < 0) a[i] += 26;
  }

  for (int i = 1; i <= n; i++)
    cout << (char)(a[i]+'a');

}

why compilation error!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Your solution was compiled with the following errors:

ed6aec59-24bf-4ca4-9cb3-cd499eb9bf3b
ed6aec59-24bf-4ca4-9cb3-cd499eb9bf3b(4): error: name must be a namespace
name
  using namespace std;
                  ^

ed6aec59-24bf-4ca4-9cb3-cd499eb9bf3b(9): warning #592: variable "s" is used
before its value is set
  cin >> s;
         ^

ed6aec59-24bf-4ca4-9cb3-cd499eb9bf3b(12): error: identifier "strlen" is
undefined
  for (int i = 0; i < strlen(s); i++)
                      ^

ed6aec59-24bf-4ca4-9cb3-cd499eb9bf3b(14): error: identifier "strlen" is
undefined
  int n = strlen(s);
          ^

ed6aec59-24bf-4ca4-9cb3-cd499eb9bf3b(25): warning #1: last line of file
ends without a newline
  }
   ^

compilation aborted for ed6aec59-24bf-4ca4-9cb3-cd499eb9bf3b (code 2)

Your compiling code--

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


int main()
{
char * s;
cin >> s;

int a[200] = {5};
for (int i = 0; i < strlen(s); i++)
a[i+1] = (s[i] - 'a');
int n = strlen(s);

for (int i = n; i >= 1; i--)
{
a[i] -= a[i-1];
if (a[i] < 0) a[i] += 26;
}

for (int i = 1; i <= n; i++)
cout << (char)(a[i]+'a');

};

and dont forget std namespace.