Cannot find a reference: What is the best way
to catenate a string multiple times ?
Unfortunately this this does not work ;-)

string tab = "..";
tab = tab * 4; // -> "........"

Peter

On 03/16/2013 10:29 AM, Peter Sommerfeld wrote:
> Cannot find a reference: What is the best way
> to catenate a string multiple times ?
> Unfortunately this this does not work ;-)
>
> string tab = "..";
> tab = tab * 4; // -> "........"
>
> Peter
foreach(0..4)
	write("...");

On 03/16/2013 10:39 AM, 1100110 wrote:
> On 03/16/2013 10:29 AM, Peter Sommerfeld wrote:
>> Cannot find a reference: What is the best way
>> to catenate a string multiple times ?
>> Unfortunately this this does not work ;-)
>>
>> string tab = "..";
>> tab = tab * 4; // -> "........"
>>
>> Peter


foreach(0..4)
> write("...");

or
str = str ~ "...";

I shoulda proofread that.

Peter Sommerfeld:

> Cannot find a reference: What is the best way
> to catenate a string multiple times ?


import std.array;

void main() {
    string tab = "..";
    tab = tab.replicate(4);
}


> Unfortunately this this does not work ;-)

D has vector ops, so I guess it's better to not add that.

Bye,
bearophile

On Saturday, 16 March 2013 at 15:29:03 UTC, Peter Sommerfeld wrote:
> Cannot find a reference: What is the best way
> to catenate a string multiple times ?
> Unfortunately this this does not work ;-)
>
> string tab = "..";
> tab = tab * 4; // -> "........"
>
> Peter

There are very many different ways of doing this, here are a few.

using template recursion:

//Repeats string "s" size_t "num" times at compile-time
template RepeatString (string s, size_t num)
{
    static if(num == 0)
        const RepeatString = "";
    else
        const RepeatString = s ~ RepeatString!(s, num-1);
}


or for fast runtime performance (also works at compile-time), something like this:

//appends any array to itself multiple times.
T[] concat(T)(T[] arr, size_t num_times)
{
    auto app = appender!(T[])(arr);
    app.reserve(num_times);

    foreach(i; 0..num_times)
        app ~= arr;

    return app.data;
}


simpler but slower:

T[] concat_slow(T)(T[] arr, size_t num_times)
{
    auto result = arr;
    foreach(i; 0..num_times)
        result ~= arr;
    return result;
}


If you needed extreme performance there are faster ways, but they are much longer and more complex.

On Saturday, 16 March 2013 at 15:58:32 UTC, John Colvin wrote:
> On Saturday, 16 March 2013 at 15:29:03 UTC, Peter Sommerfeld wrote:
>> Cannot find a reference: What is the best way
>> to catenate a string multiple times ?
>> Unfortunately this this does not work ;-)
>>
>> string tab = "..";
>> tab = tab * 4; // -> "........"
>>
>> Peter
>
> There are very many different ways of doing this, here are a few.
>
> using template recursion:
>
> //Repeats string "s" size_t "num" times at compile-time
> template RepeatString (string s, size_t num)
> {
>     static if(num == 0)
>         const RepeatString = "";
>     else
>         const RepeatString = s ~ RepeatString!(s, num-1);
> }
>
>
> or for fast runtime performance (also works at compile-time), something like this:
>
> //appends any array to itself multiple times.
> T[] concat(T)(T[] arr, size_t num_times)
> {
>     auto app = appender!(T[])(arr);
>     app.reserve(num_times);
>
>     foreach(i; 0..num_times)
>         app ~= arr;
>
>     return app.data;
> }
>
>
> simpler but slower:
>
> T[] concat_slow(T)(T[] arr, size_t num_times)
> {
>     auto result = arr;
>     foreach(i; 0..num_times)
>         result ~= arr;
>     return result;
> }
>
>
> If you needed extreme performance there are faster ways, but they are much longer and more complex.

or, as bearophile points out, use std.array.replicate

I should really read over phobos again, I keep forgetting functions exist...

On Sat, Mar 16, 2013 at 04:29:02PM +0100, Peter Sommerfeld wrote:
> Cannot find a reference: What is the best way
> to catenate a string multiple times ?
> Unfortunately this this does not work ;-)
> 
> string tab = "..";
> tab = tab * 4; // -> "........"
[...]

You could define something like this:

	string x(string a, int n) {
		import std.array;
		auto app = appender!string(a);
		while (--n) {
			app.put(a);
		}
		return app.data;
	}

	void main() {
		// prints "abcabcabc"
		writeln("abc".x(3));
	}

It's not as nice as "abc"*3, but unfortunately opBinary only works when declared inside the class/struct; UFCS doesn't pick it up, so it doesn't work on native types.


T

-- 
Why do conspiracy theories always come from the same people??