How can I convert string to double in C++?

Question

How can I convert string to double in C++? I want a function that returns 0 when the string is not numerical.

Answer 1

See C++ FAQ Lite How do I convert a std::string to a number?

See C++ Super-FAQ How do I convert a std::string to a number?

Please note that with your requirements you can't distinguish all the the allowed string representations of zero from the non numerical strings.

 // the requested function
 #include <sstream>
 double string_to_double( const std::string& s )
 {
   std::istringstream i(s);
   double x;
   if (!(i >> x))
     return 0;
   return x;
 } 

 // some tests
 #include <cassert>
 int main( int, char** )
 {
    // simple case:
    assert( 0.5 == string_to_double( "0.5"    ) );

    // blank space:
    assert( 0.5 == string_to_double( "0.5 "   ) );
    assert( 0.5 == string_to_double( " 0.5"   ) );

    // trailing non digit characters:
    assert( 0.5 == string_to_double( "0.5a"   ) );

    // note that with your requirements you can't distinguish
    // all the the allowed string representation of zero from
    // the non numerical strings:
    assert( 0 == string_to_double( "0"       ) );
    assert( 0 == string_to_double( "0."      ) );
    assert( 0 == string_to_double( "0.0"     ) );
    assert( 0 == string_to_double( "0.00"    ) );
    assert( 0 == string_to_double( "0.0e0"   ) );
    assert( 0 == string_to_double( "0.0e-0"  ) );
    assert( 0 == string_to_double( "0.0e+0"  ) );
    assert( 0 == string_to_double( "+0"      ) );
    assert( 0 == string_to_double( "+0."     ) );
    assert( 0 == string_to_double( "+0.0"    ) );
    assert( 0 == string_to_double( "+0.00"   ) );
    assert( 0 == string_to_double( "+0.0e0"  ) );
    assert( 0 == string_to_double( "+0.0e-0" ) );
    assert( 0 == string_to_double( "+0.0e+0" ) );
    assert( 0 == string_to_double( "-0"      ) );
    assert( 0 == string_to_double( "-0."     ) );
    assert( 0 == string_to_double( "-0.0"    ) );
    assert( 0 == string_to_double( "-0.00"   ) );
    assert( 0 == string_to_double( "-0.0e0"  ) );
    assert( 0 == string_to_double( "-0.0e-0" ) );
    assert( 0 == string_to_double( "-0.0e+0" ) );
    assert( 0 == string_to_double( "foobar"  ) );
    return 0;
 }

Answer 2

Most simple way is to use boost::lexical_cast :

double value;
try
{
    value = boost::lexical_cast<double>(my_string);
}
catch (boost::bad_lexical_cast const&)
{
    value = 0;
}

Answer 3

atof and strtod do what you want but are very forgiving. If you don't want to accept strings like "32asd" as valid you need to wrap strtod in a function such as this:

#include <stdlib.h>
double strict_str2double(char* str)
{
    char* endptr;
    double value = strtod(str, &endptr);
    if (*endptr) return 0;
    return value;
}

Answer 4

If it is a c-string (null-terminated array of type char), you can do something like:

#include <stdlib.h>
char str[] = "3.14159";
double num = atof(str);

If it is a C++ string, just use the c_str() method:

double num = atof( cppstr.c_str() );

atof() will convert the string to a double, returning 0 on failure. The function is documented here: http://www.cplusplus.com/reference/clibrary/cstdlib/atof.html

Answer 5

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

int main()
{
    cout << stod("  99.999  ") << endl;
}

Output: 99.999 (which is double, whitespace was automatically stripped)

Since C++11 converting string to floating-point values (like double) is available with functions:
stof - convert str to a float
stod - convert str to a double
stold - convert str to a long double

I want a function that returns 0 when the string is not numerical.

You can add try catch statement when stod throws an exception.

Answer 6

Must say I agree with that the most elegant solution to this is using boost::lexical_cast. You can then catch the bad_lexical_cast that might occure, and do something when it fails, instead of getting 0.0 which atof gives.

#include <boost/lexical_cast.hpp>
#include <string>

int main()
{
    std::string str = "3.14";
    double strVal;
    try {
        strVal = boost::lexical_cast<double>(str);
    } catch(bad_lexical_cast&) {
        //Do your errormagic
    }
    return 0;
}

Answer 7

这个问题最优雅的解决方案之一是使用 boost::lexical_cast 正如@Evgeny Lazin 提到的那样。

Answer 8

I think atof is exactly what you want. This function parses a string and converts it into a double. If the string does not start with a number (non-numerical) a 0.0 is returned.

However, it does try to parse as much of the string as it can. In other words, the string "3abc" would be interpreted as 3.0. If you want a function that will return 0.0 in these cases, you will need to write a small wrapper yourself.

Also, this function works with the C-style string of a null terminated array of characters. If you're using a string object, it will need to be converted to a char* before you use this function.

Answer 9

There is not a single function that will do that, because 0 is a valid number and you need to be able to catch when the string is not a valid number.

You will need to check the string first (probably with a regular expression) to see if it contains only numbers and numerical punctuation. You can then decide to return 0 if that is what your application needs or convert it to a double.

After looking up atof() and strtod() I should rephrase my statement to "there shouldn't be" instead of "there is not" ... hehe

Answer 10

Sample C program to convert string to double in C

#include <iostream>
#include <comutil.h>
#include <iomanip>

bool ParseNumberToDouble(const std::wstring& strInput, double & dResult)
{
    bool bSucceeded = false;
    dResult = 0;
    if (!strInput.empty() && dResult)
    {
        _variant_t varIn = strInput.c_str();
        if (SUCCEEDED(VariantChangeTypeEx(&varIn, &varIn, GetThreadLocale(), 0, VT_R8)))
        {
            dResult = varIn.dblVal;
            bSucceeded = true;
        }
    }
    return bSucceeded;
}

int main()
{
    std::wstring strInput = L"1000";
    double dResult = 0;

    ParseNumberToDouble(strInput, dResult);

    return 0;
}