While Loop Doesn't execute update statement

[juggler22]

I'm trying to initialize a while loop to a single number, going off the Boolean logic, by letting my block code run if the entry is 1. However, when i try to read the update while statement at the end of my code, it won't run it, it skips over it and loops the code forever.

#include<iostream>
using namespace std;
int main(){
	
	char a[50] = { ' ' }; int x;
	cout << "Enter 1 to test for string, 0 otherwise." << endl;
	
	while (cin>>x){
	
		cout << "Enter characters (maximum 50 characters, when done press ctrl+z and enter-key): " << endl; int counter = 0;
		for (int i = 0; i < 50; i++) {
			cin >> a[i];
			counter++;
			if (cin.eof()) break;
		}counter--;
		if ((a[counter - 1] == 'H') && (a[counter - 2] == 'O')) cout << "Entered string is a hydroxide composite." << endl;
		else cout << "Not a hydroxide composite." << endl;
		for (int k = 0; k < counter; k++)cout << a[k];//check
		cout << endl;
		cout << "Enter 1 to test for another string, 0 otherwise." << endl;
	}
	return 0;
}

sorry if I missed something, this is my first time posting

[lastchance]

#include <iostream>
#include <string>
using namespace std;

int main()
{
   string formula;
   cout << "Enter a molecular formula: ";   cin >> formula;
   cout << "This " << ( formula.find( "OH" ) != string::npos ? "contains" : "does not contain" ) << " a hydroxyl group\n";
}

Enter a molecular formula: C2H5OH
This contains a hydroxyl group

If you want it to repeat the query then you can use something like

#include <iostream>
#include <string>
using namespace std;

int main()
{
   string formula;
   while ( true )
   {
      cout << "Enter a molecular formula (or empty string to end): ";
      getline( cin, formula );
      if ( formula == "" ) break;
      cout << formula << ( formula.find( "OH" ) != string::npos ? " contains" : " does not contain" ) << " a hydroxyl group\n\n";
   }
}

Enter a molecular formula (or empty string to end): C2H5OH
C2H5OH contains a hydroxyl group

Enter a molecular formula (or empty string to end): C6H6
C6H6 does not contain a hydroxyl group

Enter a molecular formula (or empty string to end): CH3COOH
CH3COOH contains a hydroxyl group

Enter a molecular formula (or empty string to end):

[juggler22]

That's interesting, thanks! I totally get how that'll work not to mention how it's way less clunkier than mine. The only part I don't understand is the formula.find( "OH" ) != string::npos ? " contains" : " does not contain" part.

I'm only starting out and still not familiar with the type of functions there are. I'm assuming that's a function, at least the formula.find() part, but I don't understand the code after it.

I'm still interested in knowing what my mistake was and why the loop would ignore the cin commands and loop infinite times...

[AbstractionAnon]

Yes, formula.find("OH") is a function. It is a member of the std::string class.
http://www.cplusplus.com/reference/string/string/find/?kw=string%3A%3Afind
It returns string::npos if the argument string was not found.

The rest of the statement is a ternary operator that returns one of two string literals based on whether find() returned string::npos (not found) or not.
http://www.cplusplus.com/forum/articles/14631/

" contains" and " does not contain" are both const char * pointers. The value of the ternary expression becomes one or the other of these pointers. cout has an overload for const char * pointers so the the result of the ternary expression prints the desired text.

[lastchance]

formula is a string here.
find() is a member function of the string class, and so
formula.find( "OH " ) will return the index of the first occurrence of "OH" (i.e. position of letter O here) if "OH" is in the formula. If it isn't found, it will return a characteristic number (which may differ from system to system) but which can always be found as string::npos. Thus
formula.find( "OH" ) != string::npos
will be true if it does genuinely find a hydroxyl group in the formula.

The second part of your issue is the "ternary operator" (google it) which boils down to
result = test ? outcome_if_test_is_true : outcome_if_test_is_false
and it is always shorthand for an if...else block. The first code is equivalent to the longer form

#include <iostream>
#include <string>
using namespace std;

int main()
{
   string formula;
   cout << "Enter a molecular formula: ";
   cin >> formula;

   if ( formula.find( "OH" ) != string::npos )   // if the result of .find() was not equal to the not-found sentinel (string::npos)
   {                                             
      cout << "This contains a hydroxyl group\n";
   }
   else
   {
      cout << "This does not contain a hydroxyl group\n";
   }
}

Your code is a bit obscure, I'm afraid: you would find it much easier to use a std::string than a character array. Testing for eof is highly unreliable as well.

I'll post this, but I can see that I'm such a slow typist that I've been beaten to it!

[juggler22]

Thank you both for the explanation, clears up so much!