digitalmars.D.learn - Reversing a string
- AndreasDavour (38/38) Jan 11 2019 Hi.
- rikki cattermole (21/21) Jan 11 2019 So strings in D are Unicode. This is both a great thing and a horrible
- JN (7/10) Jan 11 2019 The error message is kind of unfortunate. This is a simple
- AndreasDavour (7/18) Jan 11 2019 That would help a lot, as I got "rikki cattermole"'s answer at
- Seb (3/7) Jan 11 2019 Use .retro - it is also lazy and won't allocate:
- bauss (2/11) Jan 11 2019 What a terrible name.
- AndreasDavour (3/17) Jan 11 2019 Well, it was not the first one I would have searched for, no.
- Mike James (4/18) Jan 11 2019 Check out the origin :-)
- bauss (2/21) Jan 11 2019 I wish I could live my life peacefully, but apparently not.
- AndreasDavour (3/22) Jan 11 2019 Look at that.
- JN (3/5) Jan 11 2019 I guess something like iterReverse, reverseIter, backIterator
- Adam D. Ruppe (3/5) Jan 11 2019 or foreach_reverse, which never actually was removed!
- 0xEAB (7/14) Jan 11 2019 Of course. I would protrude from all other functions with their
- Steven Schveighoffer (5/22) Jan 11 2019 Part of the problem is that arr.reverse already was defined a long time ...
- AlCaponeJr (6/8) Jan 11 2019 Indeed a terrible name, please don't tell me this was chosen by
- Seb (6/15) Jan 12 2019 No. There are zero plans to break a ton of code by changing the
Hi.
I've just started to learn some D, so maybe this question is
extremely stupid, but please bear with me.
I have been trying to reverse a string, and through some googling
found the std.algorithm.mutation.reverse method. But, when I try
to use it I get some very verbose errors that just makes my eyes
glaze over. What on earth do they mean? How do I reverse a
string, really?
This is my code:
----------------------------
import std.stdio;
import std.algorithm;
void main() {
auto test = "This is my text";
string next_test = reverse(test);
writeln(test);
writeln(next_test);
}
----------------------------
This is my error:
----------------------------
[ante tiny ~/src/D/tutorial]$ rdmd string4.d
/usr/home/ante/src/dmd2/freebsd/bin64/../../src/phobos/std/algorit
m/mutation.d(2513): Error: template `std.algorithm.mutation.reverse` cannot
deduce function from argument types `!()(immutable(ubyte)[])`, candidates are:
/usr/home/ante/src/dmd2/freebsd/bin64/../../src/phobos/std/algorit
m/mutation.d(2485): `std.algorithm.mutation.reverse(Range)(Range r) if
(isBidirectionalRange!Range && (hasSwappableElements!Range ||
hasAssignableElements!Range && hasLength!Range && isRandomAccessRange!Range ||
isNarrowString!Range && isAssignable!(ElementType!Range)))`
/usr/home/ante/src/dmd2/freebsd/bin64/../../src/phobos/std/algorit
m/mutation.d(2521): Error: template `std.algorithm.mutation.reverse` cannot
deduce function from argument types `!()(immutable(ubyte)[])`, candidates are:
/usr/home/ante/src/dmd2/freebsd/bin64/../../src/phobos/std/algorit
m/mutation.d(2485): `std.algorithm.mutation.reverse(Range)(Range r) if
(isBidirectionalRange!Range && (hasSwappableElements!Range ||
hasAssignableElements!Range && hasLength!Range && isRandomAccessRange!Range ||
isNarrowString!Range && isAssignable!(ElementType!Range)))`
string4.d(6): Error: template instance
`std.algorithm.mutation.reverse!string` error instantiating
Failed: ["/usr/home/ante/src/dmd2/freebsd/bin64/dmd", "-v",
"-o-", "string4.d", "-I."]
------------------------------
Can some unpack that for me, please?
I might add that the most intuitive way to reverse a string, I
think, would be to consider it a range and just do "newstring =
oldstring[$ .. 0]" but that gives an error about being out of
bounds, so I guess the compiler expect values to increase from
left to right, and does not want to increment beyond $. Makes
sense, but that syntax would be very neat.
Jan 11 2019
So strings in D are Unicode. This is both a great thing and a horrible
thing.
To reverse a Unicode string correctly you need to take into account BiDi
and graphemes, in other words it gets rather complex. However I suspect
that this isn't what you want.
Now a (w/d)string is defined as:
alias string = immutable(char)[];
alias wstring = immutable(wchar)[];
alias dstring = immutable(dchar)[];
Note the immutable, it means you cannot modify individual values. Which
is a problem for reverse because it modifies in place.
Which means:
writeln("Hello D".reverse);
Won't work, but:
writeln("Hello D".dup.reverse);
Will. A simple duplication (char[]) makes it work.
Finally, arrays in D are absolutely brilliant. They are what we call
slices. A slice is a pointer + a length. That is it. Hence they cannot
be reversed in place. Of course this is great for interacting with e.g.
C, since its just a matter of slicing any data back to get your bounds
checking ext.
Jan 11 2019
On Friday, 11 January 2019 at 08:15:01 UTC, rikki cattermole wrote:Note the immutable, it means you cannot modify individual values. Which is a problem for reverse because it modifies in place.The error message is kind of unfortunate. This is a simple usecase and the error message is undecipherable already. It'd be cool if the compiler could try to strip immutability, and if the type matches then, throw an error something like "Cannot pass immutable char[] to reverse, did you mean char[]?".
Jan 11 2019
AndreasDavour <andreas.davour protonmail.ch> On Friday, 11 January 2019 at 08:45:12 UTC, JN wrote:On Friday, 11 January 2019 at 08:15:01 UTC, rikki cattermole wrote:That would help a lot, as I got "rikki cattermole"'s answer at once, when my eyes were brought to the "immutable" part. I come from the lisp world, so I'm kind of familiar with the idea of copying and/or modifying in place to limit consing. I guess I should I have realised this would be a perfect example of that kind of situation.Note the immutable, it means you cannot modify individual values. Which is a problem for reverse because it modifies in place.The error message is kind of unfortunate. This is a simple usecase and the error message is undecipherable already. It'd be cool if the compiler could try to strip immutability, and if the type matches then, throw an error something like "Cannot pass immutable char[] to reverse, did you mean char[]?".
Jan 11 2019
On Friday, 11 January 2019 at 08:05:39 UTC, AndreasDavour wrote:Hi. I've just started to learn some D, so maybe this question is extremely stupid, but please bear with me. [...]Use .retro - it is also lazy and won't allocate: https://run.dlang.io/is/A6bjrC
Jan 11 2019
On Friday, 11 January 2019 at 08:25:41 UTC, Seb wrote:On Friday, 11 January 2019 at 08:05:39 UTC, AndreasDavour wrote:What a terrible name.Hi. I've just started to learn some D, so maybe this question is extremely stupid, but please bear with me. [...]Use .retro - it is also lazy and won't allocate: https://run.dlang.io/is/A6bjrC
Jan 11 2019
On Friday, 11 January 2019 at 09:41:30 UTC, bauss wrote:On Friday, 11 January 2019 at 08:25:41 UTC, Seb wrote:Well, it was not the first one I would have searched for, no. Good to know it exists as well, though. Thanks!On Friday, 11 January 2019 at 08:05:39 UTC, AndreasDavour wrote:What a terrible name.Hi. I've just started to learn some D, so maybe this question is extremely stupid, but please bear with me. [...]Use .retro - it is also lazy and won't allocate: https://run.dlang.io/is/A6bjrC
Jan 11 2019
On Friday, 11 January 2019 at 09:41:30 UTC, bauss wrote:On Friday, 11 January 2019 at 08:25:41 UTC, Seb wrote:Check out the origin :-) https://forum.dlang.org/thread/hl8345$2b1q$1 digitalmars.com?page=1 -=mike=-On Friday, 11 January 2019 at 08:05:39 UTC, AndreasDavour wrote:What a terrible name.Hi. I've just started to learn some D, so maybe this question is extremely stupid, but please bear with me. [...]Use .retro - it is also lazy and won't allocate: https://run.dlang.io/is/A6bjrC
Jan 11 2019
On Friday, 11 January 2019 at 11:15:05 UTC, Mike James wrote:On Friday, 11 January 2019 at 09:41:30 UTC, bauss wrote:I wish I could live my life peacefully, but apparently not.On Friday, 11 January 2019 at 08:25:41 UTC, Seb wrote:Check out the origin :-) https://forum.dlang.org/thread/hl8345$2b1q$1 digitalmars.com?page=1 -=mike=-On Friday, 11 January 2019 at 08:05:39 UTC, AndreasDavour wrote:What a terrible name.Hi. I've just started to learn some D, so maybe this question is extremely stupid, but please bear with me. [...]Use .retro - it is also lazy and won't allocate: https://run.dlang.io/is/A6bjrC
Jan 11 2019
On Friday, 11 January 2019 at 11:15:05 UTC, Mike James wrote:On Friday, 11 January 2019 at 09:41:30 UTC, bauss wrote:Look at that. Incidentally, I kind of like "foreach(i; 99 .. 0 : -1)".On Friday, 11 January 2019 at 08:25:41 UTC, Seb wrote:Check out the origin :-) https://forum.dlang.org/thread/hl8345$2b1q$1 digitalmars.com?page=1 -=mike=-On Friday, 11 January 2019 at 08:05:39 UTC, AndreasDavour wrote:What a terrible name.Hi. I've just started to learn some D, so maybe this question is extremely stupid, but please bear with me. [...]Use .retro - it is also lazy and won't allocate: https://run.dlang.io/is/A6bjrC
Jan 11 2019
On Friday, 11 January 2019 at 11:15:05 UTC, Mike James wrote:Check out the origin :-) https://forum.dlang.org/thread/hl8345$2b1q$1 digitalmars.com?page=1I guess something like iterReverse, reverseIter, backIterator would be too simple...
Jan 11 2019
On Friday, 11 January 2019 at 13:51:04 UTC, JN wrote:I guess something like iterReverse, reverseIter, backIterator would be too simple...or foreach_reverse, which never actually was removed! lol
Jan 11 2019
On Friday, 11 January 2019 at 13:51:04 UTC, JN wrote:On Friday, 11 January 2019 at 11:15:05 UTC, Mike James wrote:Of course. I would protrude from all other functions with their weird names then. Just kidding. Although... I have to admit it happened more than just once that I thought, "oh... another function that's named exactly different from what I'd have gone with."Check out the origin :-) https://forum.dlang.org/thread/hl8345$2b1q$1 digitalmars.com?page=1I guess something like iterReverse, reverseIter, backIterator would be too simple...
Jan 11 2019
On 1/11/19 1:25 PM, 0xEAB wrote:On Friday, 11 January 2019 at 13:51:04 UTC, JN wrote:Part of the problem is that arr.reverse already was defined a long time ago. The thing about retro, is that even though it's not what I would think of for this operation, I have never forgotten the name since ;) -SteveOn Friday, 11 January 2019 at 11:15:05 UTC, Mike James wrote:Of course. I would protrude from all other functions with their weird names then. Just kidding. Although... I have to admit it happened more than just once that I thought, "oh... another function that's named exactly different from what I'd have gone with."Check out the origin :-) https://forum.dlang.org/thread/hl8345$2b1q$1 digitalmars.com?page=1I guess something like iterReverse, reverseIter, backIterator would be too simple...
Jan 11 2019
On Friday, 11 January 2019 at 11:15:05 UTC, Mike James wrote:Check out the origin :-) https://forum.dlang.org/thread/hl8345$2b1q$1 digitalmars.com?page=1Indeed a terrible name, please don't tell me this was chosen by vote. By the way currently is there any vote system for naming these things? Al.
Jan 11 2019
On Friday, 11 January 2019 at 19:16:05 UTC, AlCaponeJr wrote:On Friday, 11 January 2019 at 11:15:05 UTC, Mike James wrote:No. There are zero plans to break a ton of code by changing the name of a symbol in the standard library. It's more likely that someone comes along with a new general purpose library which hasn't seen much usage and thus can define slightly better names.Check out the origin :-) https://forum.dlang.org/thread/hl8345$2b1q$1 digitalmars.com?page=1Indeed a terrible name, please don't tell me this was chosen by vote. By the way currently is there any vote system for naming these things? Al.
Jan 12 2019