Multiplying a string by an int in C++

Question

What do I have to do so that when I

string s = ".";

If I do

cout << s * 2;

Will it be the same as

cout << "..";

?

Solution 1

No, std::string has no operator *. You can add (char, string) to other string. Look at this http://en.cppreference.com/w/cpp/string/basic_string

And if you want this behaviour (no advice this) you can use something like this

#include <iostream>
#include <string>

template<typename Char, typename Traits, typename Allocator>
std::basic_string<Char, Traits, Allocator> operator *
(const std::basic_string<Char, Traits, Allocator> s, size_t n)
{
   std::basic_string<Char, Traits, Allocator> tmp = s;
   for (size_t i = 0; i < n; ++i)
   {
      tmp += s;
   }
   return tmp;
}

template<typename Char, typename Traits, typename Allocator>
std::basic_string<Char, Traits, Allocator> operator *
(size_t n, const std::basic_string<Char, Traits, Allocator>& s)
{
   return s * n;
}

int main()
{
   std::string s = "a";
   std::cout << s * 5 << std::endl;
   std::cout << 5 * s << std::endl;
   std::wstring ws = L"a";
   std::wcout << ws * 5 << std::endl;
   std::wcout << 5 * ws << std::endl;
}

http://liveworkspace.org/code/52f7877b88cd0fba4622fab885907313

Solution 2

std::string has a constructor of the form

std::string(size_type count, char c);

that will repeat the character. For example

#include <iostream>

int main() {
   std::string stuff(2, '.');
   std::cout << stuff << std::endl;
   return 0;
}

will output

..

Solution 3

I used operator overloading to simulate this behavior in c++.

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

/* Overloading * operator */
string operator * (string a, unsigned int b) {
    string output = "";
    while (b--) {
        output += a;
    }
    return output;
}


int main() {
    string str = "abc";
    cout << (str * 2);
    return 0;
}

Output: abcabc

Solution 4

There is no predefined * operator that will multiply a string by an int, but you can define your own:

#include <iostream>
#include <sstream>
#include <string>

using namespace std;

string operator*(const string& s, unsigned int n) {
    stringstream out;
    while (n--)
        out << s;
    return out.str();
}

string operator*(unsigned int n, const string& s) { return s * n; }

int main(int, char **) {
    string s = ".";
    cout << s * 3 << endl;
    cout << 3 * s << endl;
}

Solution 5

Strings cannot be multiplied.

If s is a char

'.'     // This has ASCII code 46

then

cout << (char)((int)s * 2);

will give you

'/'     // This has ASCII code 92

View more solutions

Comments

[Andrew]

it will not even compile

[chessweb]

Why don't you give it a try and see for yourself?

[janisz]

Better if you ask how to make it works

[jcoder]

Even though the answer is "no" I don;t see that this is worthy of so many downvotes. It's not a stupid question if you are used to other languages and not easily answered unless you want to read 1000+ pages of specs. It might have been better as "I tried this and it didn't work, so how can I achieve this" instead, but even so...

[Bo Persson]

Oddly enough cout << (s + s) does work, so why not cout << (s * 2)? No particular reason, it just isn't defined by the language.

[James Kanze]

If you do this, you probably want to do operator*(int, std::string const&) as well.

[moooeeeep]

Shouldn't size_t rather be typename std::basic_string<Char, Traits, Allocator>::size_type here?

[ForEveR]

@moooeeeep should be, but it's not very important here.

[Bulletmagnet]

This is a bad implementation. Using a stream is wasteful when std::string has append() and operator+=

[greybeard]

You can, but even as the accepted answer implements string*int pretty much like this: DON'T. For starters, use a double and (conditionally) add approach.

[shayst]

Perfectly elegant

[Peter Mortensen]

Re "system("pause");": That is operating system dependent. What is the intent? On Windows, it will likely be an infinite delay (until a key is pressed). On Linux it will likely be a slight delay before it gives up ("pause: command not found"). Is the intent to avoid a terminal window closing before the result can be seen?

[Peter]

To avoid doing (or potentially doing) multiple reallocations, can use a simple tmp.reserve(n*s.size()) or similar. I wouldn't advise writing an operator*() like this though - a helper function that accepts a string and a repeat count, and returns the result, would arguably be clearer.