• Suliman (5/5) Feb 04 2014 What difference between
• Martijn Pot (4/9) Feb 04 2014 My interpretation of tdpl p57:
• Suliman (18/21) Feb 04 2014 So should next code have same behavior if I will use is instead
• Martijn Pot (5/27) Feb 04 2014 My guess is the following:
• bearophile (8/13) Feb 04 2014 "is" performs a raw comparison of just the values, and the value
• Steven Schveighoffer (10/15) Feb 04 2014 The first compares the pointer of the arrays. The second compares the

"Suliman" <evermind live.ru> writes:

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

?

"Martijn Pot" <martijnpot52 gmail.com> writes:

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.

"Suliman" <evermind live.ru> writes:

 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.

"Martijn Pot" <martijnpot52 gmail.com> writes:

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".

"bearophile" <bearophileHUGS lycos.com> writes:

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

"Steven Schveighoffer" <schveiguy yahoo.com> writes:

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