Re: Difference between is and ==

2014-02-04 Thread Steven Schveighoffer 

On Tue, 04 Feb 2014 03:08:28 -0500, Suliman  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

Re: Difference between is and ==

2014-02-04 Thread bearophile 

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

Re: Difference between is and ==

2014-02-04 Thread Martijn Pot 

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".

Re: Difference between is and ==

2014-02-04 Thread Suliman 

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.

Re: Difference between is and ==

2014-02-04 Thread Martijn Pot 

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.

Difference between is and ==

2014-02-04 Thread Suliman 

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

?