Accept only letters

Question

This should accept only letters, but it is not yet correct:

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

int main()
{
    std::string line;
    double d;

    while (std::getline(std::cin, line))
    {
        std::stringstream ss(line);
        if (ss >> d == false && line != "") //false because can convert to double
        {
            std::cout << "its characters!" << std::endl;
            break;
        }
        std::cout << "Error!" << std::endl;
    }
    return 0; 
}

Here is the output:

567
Error!

Error!
678fgh
Error!
567fgh678
Error!
fhg687
its characters!
Press any key to continue . . .

fhg687 should output error because of the numbers in the string.

Accepted output should contain letters only, such as ghggjh.

Do you mean only alphabetical characters, or should it also accept things like punctuation? — Jerry Coffin, Dec 08 '12 at 16:23
In that case, this is *nearly* identical to a couple of [previous](http://stackoverflow.com/q/13080003/179910) [questions](http://stackoverflow.com/q/8888748/179910). The primary change would be substituting `isalpha` for `isdigit`. — Jerry Coffin, Dec 08 '12 at 16:49

score 12 · Answer 1 · answered Dec 08 '12 at 16:30

12

You'd be much better off using std::all_of on the string, with an appropriate predicate. In your case, that predicate would be std::isalpha. (headers <algorithm> and <cctype> required)

if (std::all_of(begin(line), end(line), std::isalpha))
{
    std::cout << "its characters!" << std::endl;
    break;
}
std::cout << "Error!" << std::endl;

answered Dec 08 '12 at 16:30

Benjamin Lindley

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2

+1; googled around for a bit and this looks like the cleanest solution – sampson-chen Dec 08 '12 at 16:42
1

I had to use ::isalpha – jansohn Apr 14 '16 at 13:18

sampson-chen · Accepted Answer · 2012-12-08T17:43:11.483

7

Updated: to show a fuller solution.

The simplest approach would probably be to iterate through each char in the input and check whether that char is within English-letter ranges in ascii (upper + lower):

char c;

while (std::getline(std::cin, line))
{
    // Iterate through the string one letter at a time.
    for (int i = 0; i < line.length(); i++) {

        c = line.at(i);         // Get a char from string

        // if it's NOT within these bounds, then it's not a character
        if (! ( ( c >= 'a' && c <= 'z' ) || ( c >= 'A' && c <= 'Z' ) ) ) {

             std::cout << "Error!" << std::endl;

             // you can probably just return here as soon as you
             // find a non-letter char, but it's up to you to
             // decide how you want to handle it exactly
             return 1;
        }
     }
 }

edited Dec 08 '12 at 17:43

answered Dec 08 '12 at 16:30

sampson-chen

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2

There's also `std::isalpha()` – Cornstalks Dec 08 '12 at 16:33
@Cornstalks oh neat! I saw saw that while reading Benjamin's answer - learned something new today =) – sampson-chen Dec 08 '12 at 16:35
@sampson-chen Could you please check my code here http://ideone.com/2CVw36. Even after I make the change the `fhg687` is still a `character`. Thank you. – sg552 Dec 08 '12 at 17:08
@sg552: The problem is that you were not iterating through the characters in your string (meaning: looking at each character one at a time). See my updated answer for a fuller solution to see what you need to change. – sampson-chen Dec 08 '12 at 17:19
`line` already is a string, so why not just use that directly and eliminate the `stringstream` altogether? – Benjamin Lindley Dec 08 '12 at 17:30
@BenjaminLindley Oh good catch! =p I need to sleep more; fixing that now – sampson-chen Dec 08 '12 at 17:41

Vincenzo Pii · Answer 3 · 2012-12-08T17:17:19.840

You can also use regular expressions, which may come in handy if you need more flexibility.

For this question, Benjamin answer is perfect, but just as a reference, this is how regex could be used (notice that regex is also part of the C++11 standard):

boost::regex r("[a-zA-Z]+");  // At least one character in a-z or A-Z ranges
bool match = boost::regex_match(string, r);
if (match)
    std::cout << "it's characters!" << std::endl;
else
    std::cout << "Error!" << std::endl;

If string contains only alphabetic characters and at least one of them (the +), then match is true.

Requirements:

With boost: <boost/regex.hpp> and -lboost_regex.
With C++11: <regex>.

Accept only letters

3 Answers3

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