Checking for Input Validity?

Question

I am suppose to read in from a text document information written as a name , a space, and a decimal value. For example, a line from a file might have:

Tiger 56.3

What I need to do is validate that first part is a string containing only letters and second part containing only digits including decimals. I have the following basic code so far:

ifstream input("data.txt");
while(!input.eof())
{
    string name;
    double score;

    input >> name;
    input >> score;

}

How do I go about doing this ?

The usual way is to read the file in as text (perhaps a line at a time) and then to manually tokenise and lex it yourself. The C++ istream formatters are not really suitable for robust input validation. — Mankarse
– Mankarse, Commented May 17, 2014 at 15:43
Simple. Input the line as a string. Divide into two parts by the space. Loop over first to check presence of non-letters and second non-digits. — HelloWorld123456789
– HelloWorld123456789, Commented May 17, 2014 at 15:50

Haatschii · Accepted Answer · 2014-05-17 16:02:48Z

1

You may want to have a look at the new C++11 Regular Expressions. They are specifically made for tasks like input validation.

A minimal example to check if a string contains only digits and + or - signs could look like this:

#include <iostream>
#include <regex>
#include <string>

int main()
{
    std::string testString;
    std::regex integer("(\\+|-)?[[:digit:]]+");
    input >> testString;
    if(std::regex_match(input, integer))
        std::cout << "Valid number" << std::endl;
    else
    {
        std::cout << "No valid number" << std::endl;
    }
}

However you need a very recent compiler (GCC 4.9 I think), to use them. If this is unavailable to you, you can use the Boost Regex Library, it provides a very similar interface.

edited May 17, 2014 at 16:02

answered May 17, 2014 at 15:54

Haatschii

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3 Comments

oneCoderToRuleThemAll Over a year ago

Any way to do it w/o C++11? The program I write has to run on my University's machines and I think they are using C++98.

Haatschii Over a year ago

@GdgamesGamers, If they have boost installed, you can use the boost regex library, which is similar to the new C++11 library. If this is unavailable too, I think you will have to hard code the check by iterating of the characters...

Haatschii Over a year ago

@GdgamesGamers Added Link in the answer

David G · Accepted Answer · 2014-05-17 16:16:02Z

0

Read your data into strings and parse it from there:

std::string name, score_str;
while (input >> name >> score_str)
{
    if (!is_alphabetic(name) || !is_decimal_number(score_str)) {
        // error
    }
    else {
        // convert score_str to a double and assign it to score
    }
}

Here's an example of is_alphabetic:

template<class iter>
bool is_alphabetic(iter beg, iter end)
{
     bool found_number = true;
     while (beg != end && (found_number = !std::isdigit(*beg++)))
        ;
     return found_number;
}

template<class container>
bool is_alphabetic(container& c)
{
    return is_alphabetic(c.begin(), c.end());
}

edited May 17, 2014 at 16:16

answered May 17, 2014 at 16:10

David G

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Comments

coder hacker · Accepted Answer · 2014-05-17 16:19:14Z

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

int validate(string,string);

int main(){

    string s;
    string d;

    cin>>s>>d;

    if(validate(s,d)) cout<<"ok";
    else cout<<"not ok";


}


int validate(string s, string d){


    for(int i=0;i<s.size();++i){
            //all either small letter or capital letter else error
        if(!((s[i]>='A' && s[i]<='Z') || (s[i]>='a' && s[i]<='z'))){ 
            return 0;

        }

    }
    int f=0;

    for(int i=0;i<d.size();++i){
            //either digits or decimal
        if(!((d[i]>='0' && d[i]<='9') || d[i]=='.')){
            return 0;
        }

            //if decimal already present and again decimal its an error ex. 123.23.23
        if(d[i]=='.' && f==1) return 0;
            //flag indicating decimal present
        if(d[i]=='.') f=1;

        if(d[i]>='0' && d[i]<='9' && f==1) f=2; // validate decimal like 132. (not valid)
    }

    if(f==1) return 0;

    return 1;
}

user3451749 · Accepted Answer · 2014-05-17 16:19:15Z

0

Simplest code here it is:

while(!input.eof())
{

    string name;
    string score;

    input >> name;
    for (int i=0; i<name.size() ; i++) {
        if ( ( name [i] >= 'A' && name [i] <= 'Z' ) || ( name [i] >= 'a' && name [i] <= 'z' ) || (name [i] == '_') ) {
            continue;
        } else {
            cout <<"Name is not valid!";
            break;
        }
    }

    input >> score;
    for (int j=0; j<score.size() ; j++) {
        if ((score [j] >= '0' && score [j] <= '9') || (score[j]=='.') ) {
            continue;
        } else {
            cout <<"Number is not valid!";
            break;
        }
    }
}

answered May 17, 2014 at 16:19

user3451749

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