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digitalmars.D.learn - Difference between concatenation and appendation

reply "WhatMeWorry" <kc_heaser yahoo.com> writes:
Ok, I just made up that word. But what is the difference between 
appending and concatenating?  Page 100 of TPDL says "The result 
of the concatenation is a new array..." and the section on 
appending talks about possibly needing expansion and reallocation 
of memory.

But I still don't feel like I have a grasp on the subtleties 
between them. Can someone give a short and sweet "rule of thumb"?

It might be so obvious that I'll regret posting this.

Thanks.
Jan 25 2015
next sibling parent =?UTF-8?B?IuWyqeWAiSDmvqoi?= <mio.iwakura gmail.com> writes:
On Monday, 26 January 2015 at 01:17:17 UTC, WhatMeWorry wrote:

 Ok, I just made up that word. But what is the difference 
 between appending and concatenating?  Page 100 of TPDL says 
 "The result of the concatenation is a new array..." and the 
 section on appending talks about possibly needing expansion and 
 reallocation of memory.

 But I still don't feel like I have a grasp on the subtleties 
 between them. Can someone give a short and sweet "rule of 
 thumb"?

 It might be so obvious that I'll regret posting this.

 Thanks.


I'm no expert, so take what I say with a grain of salt. That 
said, here is my understanding:

When you append to an array with ~=, it attempts to reallocate 
the array in-place, meaning it allocates on top of the already 
used space, but grabs some more space past the end of the array. 
If there isn't enough space after the array then obviously it 
can't do that, so it allocates memory somewhere else that it can 
fit and then it copies the contents of the array to the new 
location. If you were to do myArray = myArray ~ moreStuff; I 
assume this is no different from ~=. Conceptually ~= is just 
syntactic sugar in the same way that += or -= is, you are just 
doing a concatenation and then updating the array to point to the 
new result. The fact that it can reallocate in place if there is 
enough space is just like an optimization, in my mind.
Jan 25 2015
prev sibling next sibling parent reply "Laeeth Isharc" <Laeeth.nospam nospam-laeeth.com> writes:
On Monday, 26 January 2015 at 01:17:17 UTC, WhatMeWorry wrote:

 Ok, I just made up that word. But what is the difference 
 between appending and concatenating?  Page 100 of TPDL says 
 "The result of the concatenation is a new array..." and the 
 section on appending talks about possibly needing expansion and 
 reallocation of memory.

 But I still don't feel like I have a grasp on the subtleties 
 between them. Can someone give a short and sweet "rule of 
 thumb"?

 It might be so obvious that I'll regret posting this.

 Thanks.



At the risk of the blind leading the blind (I am no expert), I 
think concatenation and append are used as synonyms (the same 
meaning is meant).  a~=b or a=a~b

If there isn't enough space then the whole array is reallocated.  
You can see this/change this property by reading capacity or 
calling reserve.

If you want to do lots of appends / concatenates then use 
appender in std.array which is faster and more efficient.
Jan 25 2015
parent reply "bearophile" <bearophileHUGS lycos.com> writes:
Laeeth Isharc:


 I think concatenation and append are used as synonyms (the same 
 meaning is meant).  a~=b or a=a~b


a=a~b always allocates a new array, while a~=b sometimes 
re-allocates in place.

Bye,
bearophile
Jan 25 2015
next sibling parent "Laeeth Isharc" <Laeeth.nospam nospam-laeeth.com> writes:
On Monday, 26 January 2015 at 01:57:04 UTC, bearophile wrote:

 Laeeth Isharc:


 I think concatenation and append are used as synonyms (the 
 same meaning is meant).  a~=b or a=a~b


 a=a~b always allocates a new array, while a~=b sometimes 
 re-allocates in place.

 Bye,
 bearophile


Thanks. That makes sense.
Jan 25 2015
prev sibling parent "WhatMeWorry" <kc_heaser yahoo.com> writes:
On Monday, 26 January 2015 at 01:57:04 UTC, bearophile wrote:

 Laeeth Isharc:


 I think concatenation and append are used as synonyms (the 
 same meaning is meant).  a~=b or a=a~b


 a=a~b always allocates a new array, while a~=b sometimes 
 re-allocates in place.

 Bye,
 bearophile


Perfect! Thank you.  I'll scribble your quote you in the margin 
of my TDPL book.
Jan 25 2015
prev sibling parent Jonathan M Davis via Digitalmars-d-learn writes:
On Monday, January 26, 2015 01:17:15 WhatMeWorry via Digitalmars-d-learn wrote:

 Ok, I just made up that word. But what is the difference between
 appending and concatenating?  Page 100 of TPDL says "The result
 of the concatenation is a new array..." and the section on
 appending talks about possibly needing expansion and reallocation
 of memory.

 But I still don't feel like I have a grasp on the subtleties
 between them. Can someone give a short and sweet "rule of thumb"?

 It might be so obvious that I'll regret posting this.


~ is the concatenation operator. ~= is the append operator. ~ takes two
objects (most typically arrays, but user-defined types can define the same
operators) and returns a new one which contains the data from the first one
followed by the data from the second without affecting either object. ~=
takes two objects and adds the data from the second one to the end of the
first one (without affecting the second object). ~ is forced to allocate,
because it's creating a new object. Whether ~= allocates depends on the
data involved and the implementation, though ideally, it would avoid
allocation if it can.

In the case of arrays rather than user-defined objects, ~= is managed by the
GC. So, whether ~= allocates or not depends on whether there's room/capacity
in the block of memory that the array is a slice of after the array. If
there's enough room, then it will just increase the size of the array, and
put the new data in that memory, but if there isn't enough room (e.g.
because the memory block doesn't have enough room or because another array
refers to a pointer farther in the block of memory than the one being
appended to), then a new block of memory is allocated, the data is then
assigned to there, and the array is changed to be a slice of that memory
block. For user-defined types, it depends entirely on how ~= is implemented,
but it's probably going to either be doing something similar or be forced to
reallocate every time.

In any case, the main difference between ~ and ~= is that ~ creates a new
array or object _every_ time, and ~= mutates the first argument and will
generally only result in an allocation if it has to (particularly if you're
dealing with arrays).

I'd sugges that you read this article on D arrays:

http://dlang.org/d-array-article.html

It mixes up its terminology a bit with regards to dynamic arrays (the
language considers int[] to be a dynamic array, whereas the article refers
to the GC-allocated block of memory that the array refers to as being the
dynamic array), but it should definitely clarify a lot about D arrays for
you.

- Jonathan M Davis
Jan 25 2015