"如何在 C++ 中检查输入是整数、字符串还是浮点数"

Question

What I want to know is, If I ask a user to input something, how will I output if the input is a Integer or String or a Float Value. I want some method to check the data type of the input in C++14.

For eg.

If the input is "Hello world"

Output should be : "The input is String"

If the input is "134"

Output should be : "The input is integer"

If the input is "133.23"

Output should be : "The input is float"

Answer 1

Read string.

Answer 2

The user will always input a string, what you can do is try to convert it to a float, if it succeeds then it probably is a float or an int. If the float conversion doesnt succeed then its probably not a number.

"

Answer 3

#include <iostream>
#include <string>
#include <boost/variant.hpp>
#include <sstream>


using myvariant = boost::variant<int, float, std::string>;

struct emit : boost::static_visitor<void>
{
    void operator()(int i) const {
        std::cout << "It's an int: " << i << '\n';
    }

    void operator()(float f) const {
        std::cout << "It's a float: " << f << '\n';
    }

    void operator()(std::string const& s) const {
        std::cout << "It's a string: " << s << '\n';
    }
};

auto parse(const std::string& s) -> myvariant
{
    char* p = nullptr;

    auto i = std::strtol(s.data(), &p, 10);
    if (p == s.data() + s.size())
        return int(i);

    auto f = std::strtof(s.data(), &p);
    if (p == s.data() + s.size())
        return f;


    return s;
}

void test(const std::string& s)
{
    auto val = parse(s);
    boost::apply_visitor(emit(), val);
}

int main()
{
    test("Hello world");
    test("134");
    test("133.23");
}

Answer 4

The input is in a string. Without additional agreements, how could you possibly know if the user intended "1" to be the string containing the character '1' or a string representation of the integer 1?

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


void handleDouble(double d) {
    std::cout << "Double = " << d << "\n";
}

void handleInt(int i) {
    std::cout << "Int = " << i << "\n";
}

void handleString(std::string const & s) {
    std::cout << "String = " << s << "\n";
}

void parse(std::string const& input) {
    static const std::regex doubleRegex{ R"([+\-]?(?:0|[1-9]\d*)(?:\.\d*)?(?:[eE][+\-]?\d+)?)" };
    static const std::regex intRegex{ R"(\d+)"};

    if (std::regex_match(input, intRegex)){
        istringstream inputStream(input);
        int i;
        inputStream >> i;
        handleInt(i);
    }
    else if (std::regex_match(input, doubleRegex)) {
        istringstream inputStream(input);
        double d;
        inputStream >> d;
        handleDouble(d);
    }
    else {
        handleString(input);
    }
}

int main()
{
    parse("+4.234e10");
    parse("1");
    parse("1.0");
    parse("123abc");
}

Answer 5

You can easily wrap string.strof<\/code> for float<\/code> or string.stroi<\/code> for int<\/code> in try-catch block:

#include <string>
bool isFloat(string str) {
    try {
        float floatCheck = stof(str);
        return true;
    }
    catch (...) {
        return false;
    }
}

bool isInt(string str) {
    try {
        int intCheck = stoi(str);
        return true;
    }
    catch (...) {
        return false;
    }
}