String concatenation in C++ problem

Question

everybody I have problem with string concatenation in C++, here is my code

map<double, string> fracs;
for(int d=1; d<=N; d++)
    for(int n=0; n<=d; n++)            
        if(gcd(n, d)==1){
            string s = n+"/"+d;// this does not work in C++ but works in Java
            fracs.insert(make_pair((double)(n/d), s));
            }

How can I fix my code?

Commencing psychic reading, please wait... ERROR: Could not read OP's mind. Post the goddamn error. — the_drow, Jun 16 '11 at 07:21
It is already discussed in : http://stackoverflow.com/questions/191757/c-concatenate-string-and-int — Talha Ahmed Khan, Jun 16 '11 at 07:22
i would like to get map where double is the value of fraction (n/d) and string is ("n/d"), then I want to print it to file — torayeff, Jun 16 '11 at 07:22
@phresnel and @the_drow The source of the error is as obvious as it can be. The OP even commented the line. Maybe the problem is on your end? — Paul Manta, Jun 16 '11 at 07:24
@torayeff: go ahead, vote good answers up, and accept an answer if you find it suitable; otherwise, people in future are not motivated to help you anymore — Sebastian Mach, Jun 16 '11 at 09:02

score 2 · Answer 1 · edited May 23 '17 at 11:48

2

In C++ you have to convert an int to a string before you can concatenate it with another string using the + operator.

See Easiest way to convert int to string in C++.

edited May 23 '17 at 11:48

Community

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answered Jun 16 '11 at 07:21

Assaf Lavie

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You don't have to when you pass non-string types to output streams (they will do the conversion for you). – Sebastian Mach Jun 16 '11 at 07:23
But concatenation. And the canonical C++ way of concatenating non-string values is with streams. – Sebastian Mach Jun 16 '11 at 07:31

score 2 · Answer 2 · answered Jun 16 '11 at 07:21

2

Try like this.

stringstream os;
os << n << "/" << d;
string s =os.str();

answered Jun 16 '11 at 07:21

Prince John Wesley

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Is this save against buffer overflow attacks ? – kiltek Mar 09 '13 at 13:29

Sebastian Mach · Answer 3 · 2011-06-16T07:40:31.770

Use streams, in your case, a stringstream:

#include <sstream>
...
    std::stringstream ss;
    ss << n << '/' << d;

Later, when done with your work, you can store it as an ordinary string:

const std::string s = ss.str();

Important (side-) note: Never do

const char *s = ss.str().c_str();

stringstream::str() produces a temporary std::string, and according to the standard, temporaries live until the end of the expression. Then, std::string::c_str() gives you a pointer to a null-terminated string, but according to The Holy Law, that C-style-string becomes invalid once the std::string (from which you receved it) changes.

It might work this time, and next time, and even on QA, but explodes right in the face of your most valuable customer.

The std::string must survive until the battle is over:

const std::string s = ss.str(); // must exist as long as sz is being used
const char *sz = s.c_str();

score 0 · Answer 4 · answered May 03 '13 at 10:34

I think sprintf(), which is a function used to send formatted data to strings, would be a much clearer way to do it. Just the way you would use printf, but with the c-style string type char* as a first(additional) argument:

char* temp;
sprint(temp, "%d/%d", n, d);
std::string g(temp);

You could check it out at http://www.cplusplus.com/reference/cstdio/sprintf/

score 0 · Answer 5 · answered Jun 16 '11 at 07:21

0

n and d are integers. Here is how you can convert integer to string:

std::string s;
std::stringstream out;
out << n << "/" << d;
s = out.str();

answered Jun 16 '11 at 07:21

Petar Ivanov

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Paul Manta · Answer 6 · 2011-06-16T07:30:10.813

0

Unlike in Java, in C++ there is no operator+ that explicitly converts a number to a string. What is usually done in C++ in cases like this is...

#include <sstream>

stringstream ss;
ss << n << '/' << d; // Just like you'd do with cout
string s = ss.str(); // Convert the stringstream to a string

edited Jun 16 '11 at 07:30

answered Jun 16 '11 at 07:22

Paul Manta

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score 0 · Answer 7 · answered Jun 16 '11 at 07:23

0

You could use a stringstream.

stringstream s;
s << n << "/" << d;
fracs.insert(make_pair((double)n/d, s.str()));

answered Jun 16 '11 at 07:23

jpm

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score 0 · Answer 8 · answered Jun 16 '11 at 07:26

0

No one has suggested it yet but you can also take a look at boost::lexical_cast<>.

While this method is sometimes criticized because of performance issues, it might be ok in your situation, and it surely makes the code more readable.

answered Jun 16 '11 at 07:26

ereOn

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String concatenation in C++ problem

8 Answers8