What difference between
if ((x = stdin.readln().chomp) is "q")
and
if ((x = stdin.readln().chomp) == "q")

?

On Tuesday, 4 February 2014 at 08:08:30 UTC, Suliman wrote:
> What difference between
> if ((x = stdin.readln().chomp) is "q")
> and
> if ((x = stdin.readln().chomp) == "q")
>
> ?

My interpretation of tdpl p57:

'is' compares for alias equality for arrays and classes.
Otherwise they are the same.

> My interpretation of tdpl p57:
>
> 'is' compares for alias equality for arrays and classes.
> Otherwise they are the same.

So should next code have same behavior if I will use is instead of ==

import std.stdio;
import std.string;

void main()
{
	getchar();
}

void getchar()
{
	string x;
	if ((x = stdin.readln().chomp) == "q")
		writeln("it's is q");
	else
		writeln("Not q");
}

In case I am using is, I have never get first if expression is true.

On Tuesday, 4 February 2014 at 08:25:18 UTC, Suliman wrote:
>> My interpretation of tdpl p57:
>>
>> 'is' compares for alias equality for arrays and classes.
>> Otherwise they are the same.
>
> So should next code have same behavior if I will use is instead of ==
>
> import std.stdio;
> import std.string;
>
> void main()
> {
> 	getchar();
> }
>
> void getchar()
> {
> 	string x;
> 	if ((x = stdin.readln().chomp) == "q")
> 		writeln("it's is q");
> 	else
> 		writeln("Not q");
> }
>
> In case I am using is, I have never get first if expression is true.

My guess is the following:
string is an immutable(char)[]. As string is an array, 'is' checks for alias equality. x is not an alias for the (unnamed?) string literal "q".

Suliman:

> What difference between
> if ((x = stdin.readln().chomp) is "q")
> and
> if ((x = stdin.readln().chomp) == "q")
>
> ?

"is" performs a raw comparison of just the values, and the value of a string is its ptr and length field. While "==" compares their contents. So you want to use "==" here because you are interested to see if x contains the char 'q', because while their lengths could be equal, their ptr is surely different.

Bye,
bearophile

On Tue, 04 Feb 2014 03:08:28 -0500, Suliman <evermind@live.ru> wrote:

> What difference between
> if ((x = stdin.readln().chomp) is "q")
> and
> if ((x = stdin.readln().chomp) == "q")
>
> ?

The first compares the pointer of the arrays. The second compares the contents of the array. Both check length as well for equality.

In other words, the first will always be false (the ROM literal "q" will never have the same address as some heap block), the second will be true if the input was the string "q".

More generally, 'is' should be a bitwise comparison of the variables. '==' should check for logical equality, whatever that means for the variable types.

-Steve