Accept only letters | 易学教程

问题

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.

回答1:

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;
        }
     }
 }

回答2:

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;

回答3:

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>.

来源：https://stackoverflow.com/questions/13779321/accept-only-letters

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