Why does my program give "NULL used in arithmetic" [closed]

Question

I wrote the following;

string words;
int a=0;
getline(cin,words);

while(words[a]!=NULL)
{                           
     a++;                    
}        
cout<<a<<endl;

It worked correctly, but when I added the line at the end:

string words;
int a=0;
getline(cin,words);

while(words[a]!=NULL)
{
      a++;                       
}
cout<<a<<endl;

if(words[1]=="a") {cout<<"the letter is a";}

It no longer works. Why?

Answer 1

Replace the double quote "" by single quote '' in if conditional expression. Note that NULL macro is used for null pointer constant not for null constant \0.

while(words[a]! = '\0')
{
      cout<<a<<endl;
      a++;                     
}       

if(words[1] == 'a')
    cout<<"the letter is a";

Answer 2

In the line

   while(words[a]!=NULL)

NULL is used not as a pointer but as 0.

It works since word[a] evaluates to a char and you can compare a char with 0.

In the line

if(words[1]=="a")

it fails because you are trying to compare char with "a", whose type is char const*.

You probably meant to use:

if(words[1]=='a')