I thought this would be really simple but it's presenting some difficulties. If I have
std::string name = "John";
int age = 21;
How do I combine them to get a single string "John21"?
转载于:https://stackoverflow.com/questions/191757/how-to-concatenate-a-stdstring-and-an-int
#include <iostream>
#include <string>
#include <sstream>
using namespace std;
string itos(int i) // convert int to string
{
stringstream s;
s << i;
return s.str();
}
Shamelessly stolen from http://www.research.att.com/~bs/bs_faq2.html.
std::ostringstream o;
o << name << age;
std::cout << o.str();
If you have Boost, you can convert the integer to a string using boost::lexical_cast<std::string>(age).
Another way is to use stringstreams:
std::stringstream ss;
ss << age;
std::cout << name << ss.str() << std::endl;
A third approach would be to use sprintf or snprintf from the C library.
char buffer[128];
snprintf(buffer, sizeof(buffer), "%s%d", name.c_str(), age);
std::cout << buffer << std::endl;
Other posters suggested using itoa. This is NOT a standard function, so your code will not be portable if you use it. There are compilers that don't support it.
#include <string>
#include <sstream>
using namespace std;
string concatenate(std::string const& name, int i)
{
stringstream s;
s << name << i;
return s.str();
}
It seems to me that the simplest answer is to use the sprintf function:
sprintf(outString,"%s%d",name,age);
Herb Sutter has a good article on this subject: "The String Formatters of Manor Farm". He covers Boost::lexical_cast, std::stringstream, std::strstream (which is deprecated), and sprintf vs. snprintf.
If you are using MFC, you can use a CString
CString nameAge = "";
nameAge.Format("%s%d", "John", 21);
Managed C++ also has a string formatter.
#include <sstream>
template <class T>
inline std::string to_string (const T& t)
{
std::stringstream ss;
ss << t;
return ss.str();
}
Then your usage would look something like this
std::string szName = "John";
int numAge = 23;
szName += to_string<int>(numAge);
cout << szName << endl;
Googled [and tested :p ]
The std::ostringstream is a good method, but sometimes this additional trick might get handy transforming the formatting to a one-liner:
#include <sstream>
#define MAKE_STRING(tokens) /****************/ \
static_cast<std::ostringstream&>( \
std::ostringstream().flush() << tokens \
).str() \
/**/
Now you can format strings like this:
int main() {
int i = 123;
std::string message = MAKE_STRING("i = " << i);
std::cout << message << std::endl; // prints: "i = 123"
}
In alphabetical order:
std::string name = "John";
int age = 21;
std::string result;
// 1. with Boost
result = name + boost::lexical_cast<std::string>(age);
// 2. with C++11
result = name + std::to_string(age);
// 3. with FastFormat.Format
fastformat::fmt(result, "{0}{1}", name, age);
// 4. with FastFormat.Write
fastformat::write(result, name, age);
// 5. with the {fmt} library
result = fmt::format("{}{}", name, age);
// 6. with IOStreams
std::stringstream sstm;
sstm << name << age;
result = sstm.str();
// 7. with itoa
char numstr[21]; // enough to hold all numbers up to 64-bits
result = name + itoa(age, numstr, 10);
// 8. with sprintf
char numstr[21]; // enough to hold all numbers up to 64-bits
sprintf(numstr, "%d", age);
result = name + numstr;
// 9. with STLSoft's integer_to_string
char numstr[21]; // enough to hold all numbers up to 64-bits
result = name + stlsoft::integer_to_string(numstr, 21, age);
// 10. with STLSoft's winstl::int_to_string()
result = name + winstl::int_to_string(age);
// 11. With Poco NumberFormatter
result = name + Poco::NumberFormatter().format(age);
#include <string>)#include <sstream> (from standard C++)There are more options possible to use to concatenate integer (or other numerric object) with string. It is Boost.Format
#include <boost/format.hpp>
#include <string>
int main()
{
using boost::format;
int age = 22;
std::string str_age = str(format("age is %1%") % age);
}
and Karma from Boost.Spirit (v2)
#include <boost/spirit/include/karma.hpp>
#include <iterator>
#include <string>
int main()
{
using namespace boost::spirit;
int age = 22;
std::string str_age("age is ");
std::back_insert_iterator<std::string> sink(str_age);
karma::generate(sink, int_, age);
return 0;
}
Boost.Spirit Karma claims to be one of the fastest option for integer to string conversion.
If you want to get a char* out, and have used stringstream as per what the above respondants have outlined, then do e.g.:
myFuncWhichTakesPtrToChar(ss.str().c_str());
Since what the stringstream returns via str() is a standard string, you can then call c_str() on that to get your desired output type.
As a Qt-related question was closed in favour of this one, here's how to do it using Qt:
QString string = QString("Some string %1 with an int somewhere").arg(someIntVariable);
string.append(someOtherIntVariable);
The string variable now has someIntVariable's value in place of %1 and someOtherIntVariable's value at the end.
If you'd like to use + for concatenation of anything which has an output operator, you can provide a template version of operator+:
template <typename L, typename R> std::string operator+(L left, R right) {
std::ostringstream os;
os << left << right;
return os.str();
}
Then you can write your concatenations in a straightforward way:
std::string foo("the answer is ");
int i = 42;
std::string bar(foo + i);
std::cout << bar << std::endl;
Output:
the answer is 42
This isn't the most efficient way, but you don't need the most efficient way unless you're doing a lot of concatenation inside a loop.
Another easy way of doing it is:
name.append(age+"");
cout << name;
In C++11, you can use std::to_string, e.g.:
auto result = name + std::to_string( age );
If you have C++11, you can use std::to_string.
Example:
std::string name = "John";
int age = 21;
name += std::to_string(age);
std::cout << name;
Output:
John21
There is a function I wrote, which takes the int number as the parameter, and convert it to a string literal. This function is dependent on another function that converts a single digit to its char equivalent:
char intToChar(int num)
{
if (num < 10 && num >= 0)
{
return num + 48;
//48 is the number that we add to an integer number to have its character equivalent (see the unsigned ASCII table)
}
else
{
return '*';
}
}
string intToString(int num)
{
int digits = 0, process, single;
string numString;
process = num;
// The following process the number of digits in num
while (process != 0)
{
single = process % 10; // 'single' now holds the rightmost portion of the int
process = (process - single)/10;
// Take out the rightmost number of the int (it's a zero in this portion of the int), then divide it by 10
// The above combination eliminates the rightmost portion of the int
digits ++;
}
process = num;
// Fill the numString with '*' times digits
for (int i = 0; i < digits; i++)
{
numString += '*';
}
for (int i = digits-1; i >= 0; i--)
{
single = process % 10;
numString[i] = intToChar ( single);
process = (process - single) / 10;
}
return numString;
}
Here is an implementation of how to append an int to a string using the parsing and formatting facets from the IOStreams library.
#include <iostream>
#include <locale>
#include <string>
template <class Facet>
struct erasable_facet : Facet
{
erasable_facet() : Facet(1) { }
~erasable_facet() { }
};
void append_int(std::string& s, int n)
{
erasable_facet<std::num_put<char,
std::back_insert_iterator<std::string>>> facet;
std::ios str(nullptr);
facet.put(std::back_inserter(s), str,
str.fill(), static_cast<unsigned long>(n));
}
int main()
{
std::string str = "ID: ";
int id = 123;
append_int(str, id);
std::cout << str; // ID: 123
}
Without C++11, for a small integer range, I found this is all I needed:
Declare/include some variant of the following somewhere:
const string intToString[10] = {"0","1","2","3","4","5","6","7","8","9"};
Then:
string str = intToString[3] + " + " + intToString[4] + " = " + intToString[7]; //str equals "3 + 4 = 7"
It works with enums too.
I am a beginner C++ user and found this the easiest way:
cout << name << age;
This will successfully concatenate name and age, the the output will be "John21."
However there has to be a reason nobody said this; I think there may be a flaw in it although I haven't experienced any so far.
EDIT: I have realized that this is not necessarily the right answer, however I will keep it here in case any C++ beginners would like to know how to output concatenated strings.
This is the easiest way:
string s = name + std::to_string(age);
You can concatenate int to string by using the given below simple trick, but note that this only works when integer is of single digit. Otherwise, add integer digit by digit to that string.
string name = "John";
int age = 5;
char temp = 5 + '0';
name = name + temp;
cout << name << endl;
Output: John5
Suggesting an alternate solution for people like me who may not have access to C++ 11 and additional libraries/headers like boost. A simple conversion works like this:
Example the number is 4, to convert 3 into ascii we can simply use the code:
char a='0'+4
This will immediately store 4 as a character in a.
From here, we can simply concatenate a with the rest of the string.
The detailed answer is buried in below other answers, resurfacing part of it:
#include <iostream> // cout
#include <string> // string, to_string(some_number_here)
using namespace std;
int main() {
// using constants
cout << "John" + std::to_string(21) << endl;
// output is:
// John21
// using variables
string name = "John";
int age = 21;
cout << name + to_string(age) << endl;
// output is:
// John21
}
With the {fmt} library:
auto result = fmt::format("{}{}", name, age);
A subset of the library is proposed for standardization as P0645 Text Formatting and, if accepted, the above will become:
auto result = std::format("{}{}", name, age);
Disclaimer: I'm the author of the {fmt} library.
This problem can be done in many ways. I will show it in two ways:
Convert the number to string using to_string(i).
Using string streams.
Code:
#include <string>
#include <sstream>
#include <bits/stdc++.h>
#include <iostream>
using namespace std;
int main() {
string name = "John";
int age = 21;
string answer1 = "";
// Method 1). string s1 = to_string(age).
string s1=to_string(age); // Know the integer get converted into string
// where as we know that concatenation can easily be done using '+' in C++
answer1 = name + s1;
cout << answer1 << endl;
// Method 2). Using string streams
ostringstream s2;
s2 << age;
string s3 = s2.str(); // The str() function will convert a number into a string
string answer2 = ""; // For concatenation of strings.
answer2 = name + s3;
cout << answer2 << endl;
return 0;
}
// Hope it helps
#include <sstream> std::ostringstream s; s << "John " << age; std::string query(s.str());
std::string query("John " + std::to_string(age));
#include <boost/lexical_cast.hpp> std::string query("John " + boost::lexical_cast<std::string>(age));