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digitalmars.D.learn - how does range.put work

reply O.K. <oliverkania googlemail.com> writes:
Hello,
could someone plz clearify what the exact semantics of "put"
are ?
Put works with an appender, but gives me a runtime exception
when using an array.

Best regards,
Oliver
Aug 06 2009
parent reply Daniel Keep <daniel.keep.lists gmail.com> writes:
O.K. wrote:

 Hello,
 could someone plz clearify what the exact semantics of "put"
 are ?
 Put works with an appender, but gives me a runtime exception
 when using an array.
 
 Best regards,
 Oliver


The source code for the standard library comes with the compiler.

If you look in std\array.d, you find this around line 279 (reflowed for
readability):


 void put(T, E)(ref T[] a, E e) {
     assert(a.length);
     a[0] = e; a = a[1 .. $];
 }
Aug 06 2009
parent reply O.K. <oliverkania googlemail.com> writes:
Good point ! Use the [S/F]o[u]rce !
Thx, Oliver


 O.K. wrote:

 Hello,
 could someone plz clearify what the exact semantics of "put"
 are ?
 Put works with an appender, but gives me a runtime exception
 when using an array.

 Best regards,
 Oliver


 The source code for the standard library comes with the compiler.
 If you look in std\array.d, you find this around line 279 (reflowed for
 readability):

 void put(T, E)(ref T[] a, E e) {
     assert(a.length);
     a[0] = e; a = a[1 .. $];
 }
Aug 07 2009
parent reply Jos van Uden <jvu nospam.nl> writes:
Oliver wrote:

 The source code for the standard library comes with the compiler.
 If you look in std\array.d, you find this around line 279 (reflowed for
 readability):

 void put(T, E)(ref T[] a, E e) {
     assert(a.length);
     a[0] = e; a = a[1 .. $];
 }






Would anybody care to explain what this is used for? I find
the example in array.d rather unhelpful.

Example:
----
void main()
{
     int[] a = [ 1, 2, 3 ];
     int[] b = a;
     a.put(5);
     assert(a == [ 2, 3 ]);
     assert(b == [ 5, 2, 3 ]);
}

You're putting an element in a, but then the first element is moved out 
of a and the new one shows up in b? Weird. I guess I don't understand 
what a range is.

Jos
Aug 08 2009
parent reply Daniel Keep <daniel.keep.lists gmail.com> writes:
Jos van Uden wrote:

 Oliver wrote:

 The source code for the standard library comes with the compiler.
 If you look in std\array.d, you find this around line 279 (reflowed for
 readability):

 void put(T, E)(ref T[] a, E e) {
     assert(a.length);
     a[0] = e; a = a[1 .. $];
 }






 
 Would anybody care to explain what this is used for? I find
 the example in array.d rather unhelpful.
 
 Example:
 ----
 void main()
 {
     int[] a = [ 1, 2, 3 ];
     int[] b = a;
     a.put(5);
     assert(a == [ 2, 3 ]);
     assert(b == [ 5, 2, 3 ]);
 }
 
 You're putting an element in a, but then the first element is moved out
 of a and the new one shows up in b? Weird. I guess I don't understand
 what a range is.
 
 Jos


No; read the code.  Before the put, a and b are pointing to the same
span of memory.  a.put(5) puts the value 5 into the front (first
element) of the array, then advances the array.

However, put can't "see" b, so it doesn't get updated along with a.  The
end result is that b = [5,2,3] and a = b[1..3] = [2,3].

Why do it like this?  Here's an example:

void putNumbers(Range)(Range r)
{
    int i = 0;
    while( !r.empty )
    {
        r.put(i);
        ++i;
    }
}

void main()
{
    int[10] ten_numbers;
    putNumbers(ten_numbers);
    assert( ten_numbers = [0,1,2,3,4,5,6,7,8,9] );
}

Note that putNumbers will work with any type that supports the range
API, not just arrays.
Aug 08 2009
parent Jos van Uden <jvu nospam.nl> writes:
Daniel Keep wrote:


 No; read the code.  Before the put, a and b are pointing to the same
 span of memory.  a.put(5) puts the value 5 into the front (first
 element) of the array, then advances the array.
 
 However, put can't "see" b, so it doesn't get updated along with a.  The
 end result is that b = [5,2,3] and a = b[1..3] = [2,3].
 
 Why do it like this?  Here's an example:
 
 void putNumbers(Range)(Range r)
 {
     int i = 0;
     while( !r.empty )
     {
         r.put(i);
         ++i;
     }
 }
 
 void main()
 {
     int[10] ten_numbers;
     putNumbers(ten_numbers);
     assert( ten_numbers = [0,1,2,3,4,5,6,7,8,9] );
 }


I see.

Your example should be in the documentation in my opinion, rather
then the meaningless one that's there now. Something like this
perhaps:

void putNumbers(Range, T)(Range r, T start, T incr) {
     T i = start;
     while( !r.empty )
     {
         r.put(i);
         i += incr;
     }
}

void main() {
    int[10] ten_ints;
    putNumbers!(int[])(ten_ints, 4, 2);
    assert( ten_ints == [4,6,8,10,12,14,16,18,20,22] );
}

Jos
Aug 08 2009