Pulling numbers out of strings so they can be manipulated and returned to the string

[TimL]

We went into greater detail about strings in class the other day, and it brought up a potential problem. I understand that the <string> class does not provide support for converting char and int (and I assume double as well) variables to strings when the string variable is declared, although char variables can be assigned to a string variable after it is declared. I was wondering if there is a library that includes a function(s) that would allow a number to be pulled out of a string, converted to an int (or double) so it can be manipulated in some way, then converted back into a string again.

This isn't something that I need for class. I was just curious about it and my instructor wasn't aware of any existing functionality that could do this. He said I may need to write the code for it myself if I ever needed to do it.

[hamsterman]

see http://cplusplus.com/reference/iostream/stringstream/

[JMJAtlanta]

I'll be curious from other replies, but could strtok be used here?

If this is say, an English sentence, and you can separate the pieces by using the space as a delimiter, you can then convert the element that is a number to a number, manipulate, and put the sentence back together.

[JMJAtlanta]

Ahh yes, stringstream. Something I should be more familiar with, since I should be avoiding the old C calls. Here is a brief post that includes some howto:

http://www.cplusplus.com/forum/beginner/43535/

[TimL]

@hamsterman

see http://cplusplus.com/reference/iostream/stringstream/

Is there any function in stringstream that would support the conversion of numbers separated from a string into int or double variables and back again? I looked through what is available in that class and nothing stands out to me as having that capability.

@JMJAtlanta
strtok is new to me. I think it could be helpful in this instance, since the individual tokens could be examined to see if they were a number or not. However, it still leaves me with the problem of converting the token containing a number to an int or double so it can be manipulated, then converted back into a token and reinserted into the string.

[JMJAtlanta]

For converting the string to the proper data type, you can use the C functions for string conversion found in cstdlib.

http://www.cplusplus.com/reference/clibrary/cstdlib/

[Cubbi]

modern C++ string actually has that capability right in #include <string>

#include <iostream>
#include <string>
int main()
{
    std::string str = "10";
    int n = std::stoi(str);
    n *= 10;
    str = std::to_string(n);
    std::cout << str << '\n';
}

demo: http://ideone.com/Gz6Ji

but it processes the whole string, not a token within a string.

Older C++, as mentioned, uses stringstream for everything:

#include <iostream>
#include <string>
#include <sstream>
int main()
{
    std::string str = "10";
    int n;
    std::istringstream input(str);
    input >> n;
    n *= 10;
    std::ostringstream output;
    output << n;
    str = output.str();
    std::cout << str << '\n';
}

demo: http://ideone.com/HAKMD

or boost.lexical_cast

#include <iostream>
#include <string>
#include <boost/lexical_cast.hpp>
int main()
{
    std::string str = "10";
    int n = boost::lexical_cast<int>(str);;
    n *= 10;
    str = boost::lexical_cast<std::string>(n);
    std::cout << str << '\n';
}

demo: http://ideone.com/vqTLd


If you are examining/modifying individual tokens from the same strnig, stringstream is the tool to use, unless you're dealing with something too complicated

For example,

#include <iostream>
#include <string>
#include <sstream>
int main()
{
    std::string str = "10 11 12 13 14";
    std::istringstream input(str);
    std::ostringstream output;
    for(int n; input >> n; )
        output << n*10 << ' ';
    str = output.str();
    std::cout << str << '\n';
}

demo: http://ideone.com/Ga2Lh

[TimL]

@Cubbi
So stringstream handles the type conversion implicitly through overloading the insertion and extraction operators?

Thanks for the great examples and demos. I was not aware of the ideone site. That is definitely going on my browser favorites toolbar.

@JMJAtlanta
I was aware of some of the cstdlib functions for converting types. What I didn't know was if they could handle numbers with more than one digit. Do they know that, for example, string a = "123"; should be equal to int b = 123; after the value is converted to an int?

[JMJAtlanta]

Yes, cstdlib functions that take in a char* can handle numbers with more than one digit.

I've been playing around with stringstream for similar functionality. It feels more "C++" like.

Using try/catch to handle problem conversions may be cumbersome, but give some flexibility.