C++ and C# Parse a string into a numerical type

exodist · 02-22-2004, 06:11 PM

I need this question answered for both c# and c++, an answer for one or the other is nice, but both are needed.

is there a builtin function that can take a string and parse it into a double/float/int?

for instance if I have a String vaiable called string1, and it contains "1234.567" could I do something like:

double double1 = ParseStringToDouble(string1);

I need this for both C# (CSharp) and C++

jinksys · 02-22-2004, 06:32 PM

I can answer this question with C++.

Code:

#include <cstdlib>
#include <iostream>
#include <string>

using namespace std;

int main()
{
string str = "1.222"; 
double d;			
const char *c_ptr=str.c_str();  // method c_str() returns a read-only C-style Null terminated string.

d=atof(c_ptr);				//atof() converts a C string to a double 
cout << d;                              // EDIT WAS HERE				
return 0;
}

EDIT{ originally had cout << c_ptr; should have been cout <<d; }

exodist · 02-22-2004, 06:53 PM

Thank you, I actually found a list of convert functions for C++, it sais atof converts to a float, that for a double you would do something like this:

d = strtod("1234.567");
however, it does not say anything for a string stored ina variable, would I need to do your:
const char *c_ptr=str.c_str();
and then:
d = strtod(c_ptr); ?
or would this work as is:
d = strtod(str);

and last question for you:
what does "cout << c_ptr;" do? does it simply print to see if anything was left over? or does it print what was placed into "d"? or does it do somethign else entirely? (I am a c++ student, halfway through first of the 3 classes my cc offers, I am also taking java, C#, and bash scripting simeltaniously)

Thank you Very much, anyone else out there have the answer for C#?

deiussum · 02-22-2004, 06:57 PM

For C#, you would use
double d = Convert.ToDouble("1234.56");

exodist · 02-22-2004, 07:00 PM

deiussum:

would I need to add a new using statement to the top of the file?
at the moment I have:

using System;
using System.Collections;

is anything else needed for the conversion?

jinksys · 02-22-2004, 07:18 PM

Quote:

Originally posted by exodist
Thank you, I actually found a list of convert functions for C++, it sais atof converts to a float, that for a double you would do something like this:

d = strtod("1234.567");
however, it does not say anything for a string stored ina variable, would I need to do your:
const char *c_ptr=str.c_str();
and then:
d = strtod(c_ptr); ?
or would this work as is:
d = strtod(str);

and last question for you:
what does "cout << c_ptr;" do? does it simply print to see if anything was left over? or does it print what was placed into "d"? or does it do somethign else entirely? (I am a c++ student, halfway through first of the 3 classes my cc offers, I am also taking java, C#, and bash scripting simeltaniously)

Thank you Very much, anyone else out there have the answer for C#?

atof() returns a double, atoi() returns an int, and atol() returns a long. and yes, if you are
using C++'s string class you will need to convert it to a C-style string for either method to work.
for your last question, (and make sure you notice my edit) cout <<d; prints variable d (to show that d was actually given the numeric value of str).

deiussum · 02-22-2004, 09:23 PM

Quote:

Originally posted by exodist
deiussum:

would I need to add a new using statement to the top of the file?
at the moment I have:

using System;
using System.Collections;

is anything else needed for the conversion?

Nope, the Convert class is in the System namespace, so nothing else should be needed.

BTW, this is in the .Net library from MS. If you are using something like DotGNU, it's possible that it is one of the things that hasn't been implemented yet. I know the last time I took a look at DotGNU it still needed work done on a number of the classes, anyway... An alternative method is the Double.Parse method, which essentially does the same thing as Convert.ToDouble with the exception that Convert.ToDouble takes more data types than just strings...

exodist · 02-22-2004, 09:38 PM

Thank you, I am aware of the limitations, I also have visual studio .net on a windows box I just braught up, I get it free through msdn because I am a student, I use it to test.

dakensta · 02-23-2004, 06:15 AM

Alternatively, in c++

Code:

#include <sstream>

template<typename T>
void string_to_type( const std::string& s, T& t )
{
    std::istringstream iss( s );
    iss >> t;                                    // check this if error handling is required
}