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digitalmars.D.learn - Why is "array operation without destination memory not allowed"?

reply Lily <yulex.42 gmail.com> writes:
It seems a bit silly that I have to write

int[] a = [1, 2, 300, -29];
int[] b;
b.length = 4;
b[] = a[] * 2;
writeln(b);

to do what I would expect

int[] a = [1, 2, 300, -29];
writeln(a[] * 2);

to do. What am I not understanding?
Jan 06 2018
next sibling parent Adam D. Ruppe <destructionator gmail.com> writes:
On Saturday, 6 January 2018 at 23:08:14 UTC, Lily wrote:

 What am I not understanding?


Where do you expect it to put the result?
Jan 06 2018
prev sibling parent reply =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 01/06/2018 03:08 PM, Lily wrote:

 It seems a bit silly that I have to write
 
 int[] a = [1, 2, 300, -29];
 int[] b;
 b.length = 4;
 b[] = a[] * 2;
 writeln(b);
 
 to do what I would expect
 
 int[] a = [1, 2, 300, -29];
 writeln(a[] * 2);
 
 to do. What am I not understanding?


So, apparently a[] * 2 is not an expression in D. It looks like such 
array-wise operations require a left-hand side with memory for the results.

The reason must be for performance. If a[]*2 were an expression, the 
runtime would have to allocate memory and put the results there. 
Assigning that memory then to b[] would require an additional copy. 
Current rule puts the burden on the programmer to find the memory. No 
need to allocate if there's memory already.

However, idiomatic D delays arrays as much as possible. For example, 
there is no need for destination memory for this example:

import std.stdio;
import std.algorithm;

void main() {
     int[] a = [1, 2, 300, -29];
     writeln(a.map!(x => 2 * x));
}

Ali
Jan 06 2018
parent Lily <yulex.42 gmail.com> writes:
On Saturday, 6 January 2018 at 23:17:53 UTC, Ali Çehreli wrote:

 So, apparently a[] * 2 is not an expression in D.



 The reason must be for performance. If a[]*2 were an 
 expression, the runtime would have to allocate memory and put 
 the results there. Assigning that memory then to b[] would 
 require an additional copy. Current rule puts the burden on the 
 programmer to find the memory. No need to allocate if there's 
 memory already.


That makes sense, thanks!
Jan 06 2018