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Question

Even i type Pythagoras or Probability it still displays my else output "topic not supported" how do i fix this?

search = "Pythagoras";
pos = sentence.find(search);
if (pos != string::npos)                        
    cout << "pythagoras entry" << endl;
search = "Probability";
pos = sentence.find(search);
if (pos != string::npos)
    cout << "probability entry" << endl; 
else
    cout << "topic not supported" << endl;

Answer 1

If your sentence contains "Pythagoras", it will pass through the first if (outputting what's there), and then go on the "else" check for "Probability" - you don't break the execution after you found "Pythagoras" in the sentence, it will go for the second if , outputting what's in the "topic not found", because "Probability" != "Pythagoras". If you put "Probability" in the sentence, "topic not found" should NOT be outputted, unless you didn't show us the code you're actually using.

To fix this, I suggest using a boolean flag, something like

bool flag = false;
search = "Pythagoras";
pos = sentence.find(search);
if (pos != string::npos)                    
{
    cout << "pythagoras" << endl;
    flag = true;
}
search = "Probability";
pos = sentence.find(search);
if (pos != string::npos)
{                    
    cout << "probability" << endl; 
    flag = true;
}

if (!flag)
    cout << "not supported" << endl;