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Read any kind of input

barbarian (1)
Hi folks! I'm learning C++ on my own using tutorials here and there. I have a question about input.

Say I want to get input from the user but I don't know what type of input they're going to give me, and I want to do something different depending on the type of input I get.

For example, if they give me an int I want to send it to function X, if they give me a double I'll send it to function Y, if they give me a char it goes to function Z, if they give me a string I want to just print it (or whatever -- I just want to do something different depending on the type).

But in order to save their input via cin, I have to store it somewhere, and to store it somewhere I need to know what type it is. But I can't know that until it's been input. Know what I mean?

Closest I can figure is that I'd have the input go to a string, then send the string to a for-loop and examine each character in the string to see if it's a number, alphabet or period... then if it's all numbers it's an int, if it's all numbers and a single period it's a double, if it's one character of anything else it's a char, and if it's anything else it's a string. Is this the most efficient way to do this though? It seems so clunky to do it this way but I can't think of any other way. I'm still looking in the library for something that might work instead of my clunky way.

Here's my clunky way to do this... All my code does is take an input as a string, cycle thru it and check each character to see if it's a number, alphanum, or period(dot)... then it checks to see if there are multiple dots, and assesses whether the input is an int, char, double or string. If it's a single-digit it is assumed it's an int and not a char. If it's all numbers it's assumed it's an int and not a string.

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#include <iostream>
#include <cstring>
#include <stdio.h>
#include <stdlib.h>

using namespace std;

int main()
{
    string  something   = "";
    int     number      = 0;
    double  decimal     = 0.0;
    char    character   = '0';

    bool    adigit      = false;
    bool    adouble     = false;
    bool    achar       = false;
    bool    astring     = false;
    bool    adot        = false;
    bool    manydots    = false;

    cout << "Enter something: ";
    cin >> something;

    for(int i=0; i<something.length(); i++){

        //loop the entire word
        cout << "Looping thru the string: " << endl;
        //is it a number
        if(isdigit(something[i])){                          //digit?
            adigit  = true;
            adouble = true;
            cout << "Element " << i << " is a digit." << endl;
        } else if ((something[i]=='.') && (adot==false) && (manydots == false)){    //double?
            adigit  = false;
            adouble = true;
            adot    = true;
            cout << "Element " << i << " is a dot." << endl;
        } else if ( (something[i]=='.') && (adot==true)){
            adot     = false;
            manydots = true;
            astring  = true;
        } else if (!isdigit(something[i])){                 //not a number
            adigit  = false;
            adouble = false;
            astring = true;
            cout << "Element " << i << " is not a digit or a dot." << endl;
        }

        cout << "Loop is done." << endl << endl;
    }

    cout << "Assess the results." << endl;
    //is it a char?
    if( (!adigit) && (something.length() <= 1)){
        achar   = true;
        cout << "It's a single char." << endl;
        character = something[0];
    } else if ( (adigit) && (something.length() >1 ) ){
        achar   = false;
        cout << "It's not a single char." << endl;
    }

    //is it a double?
    if( (adouble == true) && (adot == false) && (astring == false)){
        adouble == false;
        cout << "It's an integer." << endl;
        number = atoi(something.c_str());
        something = "";
        character = '\0';
    } else if((adouble == true) && (adot == true)){
        cout << "It's a double." << endl;
        decimal = atof(something.c_str());
        something = "";
        character = '\0';
    } else if( (adigit == true) && (adouble == false)){
        cout << "It's an integer." << endl;
        number = atoi(something.c_str());
        something = "";
        character = '\0';
    } else if (astring == true){
        cout << "It's a string." << endl;
        character = '\0';
        //it's already a string
    } else {
        cout << "Error: Wtf?" << endl;
    }

    cout << "\nFinal info:" << endl;

    cout << "Integer: " << number << endl;
    cout << "Char   : " << character << endl;
    cout << "Double : " << decimal << endl;
    cout << "String : " << something << endl;

}


What is an easier way to do this? Are there any other library functions that could help? Should I do this as a template? Thanks. =)
Last edited on
LB (13399)
Well, that's actually a good way to do it. Java has functions for scanners to determine if the next input is a specific data type or not, and I assume they work exactly like this. You could take each part of your clunky code and put them into funtion such as IsInt() and IsDouble(), and then if it's neither you know it's a string.
kfmfe04 (788)
if you care more about using such functionality rather than implementing it, you may try boost::any

http://www.kuro5hin.org/story/2002/5/1/142321/9513
http://www.boost.org/doc/libs/1_46_1/doc/html/any.html
Computergeek01 (5613)
The problem I see is that a number could be an integer, float or a char and all of those would be valid in C++. If you know how to resolve that then the rest is real easy to write up yourself.
chrisname (7395)
I normally use something like this:
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#include <iostream>
#include <sstream>
#include <string>

std::string getstring(const std::string& prompt, std::ostream& stream = std::cout)
{
	std::string s;
	stream << prompt << std::flush;
	std::getline(std::cin, s);
	std::cin.sync();
	return s;
}

template <typename T>
T getinput(const std::string& prompt, std::ostream& stream = std::cout)
{
	std::stringstream sstrm;
	T value;
	sstrm << getstring(prompt, stream);
	sstrm >> value;
	return value;
}

int main()
{
	std::string name = getstring("What is your name? ");
	std::cout << "Hello, " << name << "!\n";

	int age = getinput<int>("How old are you, " + name + "? ");
	std::cout << "You are " << age << " years old.\n";

	double pi = getinput<double>("What is Pi (roughly)? ");
	std::cout << "Pi = " << pi << std::endl;

	return 0;
}
 
What is your name? Chris
Hello, Chris!
How old are you, Chris? 17
You are 17 years old.
What is Pi (roughly)? 3.1415926535
Pi = 3.14159


The only situation where it doesn't work is if the user enters a character like space for the getinput() function. You could add code to fix that by checking for spaces in the return value of getstring(). You can change how many decimals it prints pi to using std::setprecision() I think.
Last edited on
LB (13399)
The idea is that they could enter both 17, 17.5, Seventeen, and it would still work every time, interpreting it as the type they entered, rather than forcing it to a specific type.
Last edited on
chrisname (7395)
Oh, my mistake :P
manasij7479 (113)
Another way is to take the input string into a 'instringstream' .
and try to extract the data into specific variables of different types in the corresponding 'if' blocks.
Don't forget to clear the streams after each try.
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