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Whats wrong with my code?

Aug 11, 2018 at 5:13pm
Seyter (24)
Works at first, now it outputs incorrectly.

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#include <iostream>
using namespace std;
int main ()
{

  int num;
  cout << "Enter a positive integer:";
  cin >> num;

  if (num >= 0)
    if (isalpha (num))
  {
	cout << num << " you entered.";
  }
  else
  {
	cout << "Thats not a positive number.";
  }

}
Edit & run on cpp.sh
Aug 11, 2018 at 5:19pm
Repeater (3046)
If you enter something that isn't a number,
cin >> num; will fail.

if (isalpha (num))
What are you trying to test here? num is an int, yet you're checking to see if it's a letter? If the user tried to enter a letter, then whatever they entered wouldn't be stored in num, because cin >> num; would have failed.
Last edited on Aug 11, 2018 at 5:21pm
Aug 11, 2018 at 5:23pm
Seyter (24)
I tried entering a positive number but the output is "Thats not a positive number."
Aug 11, 2018 at 5:25pm
Seyter (24)
but it really work at first test; input a letter of symbol will output correctly.
Aug 11, 2018 at 5:27pm
Seyter (24)
could be a bug, huh?
Aug 11, 2018 at 5:45pm
Repeater (3046)
isalpha would typically return true if the input value is anything from 65 to 90, and also from 97 to 122.

isalpha(64) returns false.
isalpha(65) returns true.

If you enter 64, you'll get "Thats not a positive number."
If you enter 65, you'll get "65 you entered".

If you enter the letter 'a', that will be interpreted as the number 97


Last edited on Aug 11, 2018 at 5:54pm
Aug 11, 2018 at 5:58pm
Seyter (24)
so how can I avoid this?
Aug 11, 2018 at 6:04pm
Repeater (3046)
Avoid what? It's working as you programmed it. What are you actually trying to do?
Aug 11, 2018 at 6:57pm
Seyter (24)
i want whenever the user input something other than numbers, it comes out an error. So no alphabets or symbols etc.
Aug 11, 2018 at 7:09pm
Repeater (3046)
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#include <iostream>
#include <string>
#include <cctype>
using namespace std;

int main()
{
  string userInput;
  cin >> userInput;

  for ( auto& element : userInput)
  {
     if ( !isdigit(element))
    {
        cout << "Not just numbers";
        return;
    }
  }
cout << "Is just numbers";
return;
}
Edit & run on cpp.sh


Aug 12, 2018 at 6:57pm
Karakuik (9)
so no alphabets or symbols

I'd just keep it simple with a do while loop. I'm kinda newbie but it'd be something to work with. Then with the while at the bottom, do greater than or equal to 0 and if it's anything else, incorrect input.
Aug 12, 2018 at 6:58pm
Repeater (3046)
'a' is greater than 0.
Aug 13, 2018 at 12:41am
clonxy (22)
your {} for the if statement is in the wrong place. try this:
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#include <iostream>
using namespace std;
int main ()
{

  int num;
  cout << "Enter a positive integer:";
  cin >> num;

  if (num >= 0)
  {
    // if (isalpha (num))
    // {
      cout << num << " you entered.";
  // 
  }
  else
  {
    cout << "Thats not a positive number.";
  }

}
Edit & run on cpp.sh


if you want to block off stupid inputs like characters, use this:
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#include <iostream>
using namespace std;
int main ()
{

  int num;
  cout << "Enter a positive integer:";
  cin >> num;

  while (cin.fail())
  {
    cin.clear(); //clears cin
    cin.ignore(256,'\n'); //ignores the first 256 characters until new line.
    cout << "Please make sure it's a number.\n"
    << "Enter a number:";
    cin >> num;
  }

  if (num >= 0)
  {
    // if (isalpha (num))
    // {
      cout << num << " you entered is positive.";
  // 
  }
  else
  {
    cout << "Thats not a positive number.";
  }

}
Edit & run on cpp.sh
Last edited on Aug 13, 2018 at 12:45am
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