Re: Why is dynamic array length required here?
Stanislav, Ali, Mike -- Thank you all for your thoughtful and helpful replies to my queries. Apologies that it has taken this long to reply to you. I still haven't been able to find time to go through all of the code examples provided but hope to do so later this week. If I have additional questions, I know where to go! Thanks again!
Re: Why is dynamic array length required here?
On Friday, 19 October 2018 at 02:04:37 UTC, Samir wrote:
I would have thought that since this is a dynamic array, I
don't need to pre-assign its length.
Thanks
Just to expand on the previous answers, a dynamic array
declaration with no initializer is an empty array:
int[] arr;
assert(arr.length == 0);
No memory has been allocated to store any elements. Appending
elements will allocate memory. In fact, the runtime will allocate
more than needed so that it doesn't have to allocate on each
append.
arr ~= 1;
assert(arr.length == 1);
writeln(arr.capacity);
When the capacity is 0, the next append will trigger another
allocation. The reserve function can be used to allocate enough
memory for a number of elements, but the array will still be
empty:
int[] arr;
arr.reserve(100);
assert(arr.length == 0);
writeln(arr.capacity);
Now 100+ elements can be appended without triggering an
allocation.
Setting the length directly as Ali's example does means that not
only is the required memory allocated, but also that the array is
not empty.
int[] arr;
arr.length = 100;
assert(arr.length == 100);
writeln(arr.capacity);
And now the array can elements can be written to directly via the
index operator, and a pointer to each element can be referenced
as it is in the call to readf.
Note that it's still possible to write elements directly to a
dynamic array without the append operator after memory has been
reserved, but it completely bypasses the runtime.
void main() {
int[] myArray;
myArray.reserve(10);
foreach(i; 0 .. 10) {
*(myArray.ptr + i) = i;
}
assert(myArray == []);
assert(myArray.length == 0);
foreach(i; 0 .. 10) {
writeln(*(myArray.ptr + i));
}
}
This is essentially treating myArray.ptr as a C array, with no
bounds checking and no automatic memory management. Here be
dragons.
Re: Why is dynamic array length required here?
On 10/18/2018 07:04 PM, Samir wrote:
> myArray.length = noValues; // I get a run-time error if I comment
> this out
It's because the expression that reads the elements below is readf,
which reads on top of an existing element.
> while (i < noValues) {
> write("enter value #", i+1, " ");
> readf(" %s", [i]);
Aside: It may be possible to read like the following without taking the
address, as readf has been improved to take references since that code
was written:
readf(" %s", myArray[i]); // (not tested)
So, the answer to your question is, we need to set the length of the
array so that there are elements to read on top of. If D had function
templates like getResponse() that I use in the Templates chapter,
http://ddili.org/ders/d.en/templates.html
then the best thing to do would be to append the elements directly to
the array:
// No need to set .length beforehand
myArray ~= getResponse!int(/* ... */);
Ali
Re: Why is dynamic array length required here?
On Friday, 19 October 2018 at 02:04:37 UTC, Samir wrote:
I am working my way through the exercises in the "Programming
in D" tutorial (http://ddili.org/ders/d.en/arrays.html). Why
is the line assigning the length of the dynamic array required?
[...]
Without the line:
myArray.length = noValues;
I get the run-time error...
I would have thought that since this is a dynamic array, I
don't need to pre-assign its length.
Even though the array is dynamic, it doesn't have infinite
storage. So you either:
- preallocate required storage, like in that example code (note
that it's not the same as making a fixed-size ('static') array:
in your case you only learn what the required length is at
run-time, as opposed to static arrays where you must know it at
compile time)
- don't preallocate, but instead append new elements like this:
myArray ~= newValue;
The latter isn't a universally "good" advice: it may cause
reallocation every time you do that. That's why it's almost
always best to preallocate in advance if you do know the length,
or use something like std.array.appender that attempts to reduce
memory reallocation. I'm not familiar with Ali's tutorials, maybe
he talks about the appender in some future examples.
Why is dynamic array length required here?
I am working my way through the exercises in the "Programming in
D" tutorial (http://ddili.org/ders/d.en/arrays.html). Why is the
line assigning the length of the dynamic array required?
/*
Write a program that asks the user how many values will be
entered and then
reads all of them. Have the program sort the elements using
sort() and then
reverse the sorted elements using reverse().
*/
import std.stdio;
import std.algorithm;
void main() {
int noValues, i;
int[] myArray;
write("how many values would you like to enter? ");
readf(" %s", );
myArray.length = noValues; // I get a run-time error if I
comment this out
while (i < noValues) {
write("enter value #", i+1, " ");
readf(" %s", [i]);
++i;
}
sort(myArray);
writeln(myArray);
reverse(myArray);
writeln(myArray);
}
Without the line:
myArray.length = noValues;
I get the run-time error:
$ ./exArrays1_1
how many values would you like to enter? 5
core.exception.RangeError@exArrays1_1.d(12): Range violation
??:? _d_arrayboundsp [0x461772]
??:? _Dmain [0x44c284]
I would have thought that since this is a dynamic array, I don't
need to pre-assign its length.
Thanks
