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digitalmars.D.learn - Char[] confusing

reply Qian Xu <quian.xu stud.tu-ilmenau.de> writes:
Hi,

I am confusing with getting sub-string of a char[].

------------------------- code ---------------------------------
module main;

import tango.io.Console;
import tango.text.convert.Integer;

void main()
{
   char[] s = "ABCDE"; // 5 chars
   int len = s.length;
   Cout("s='" ~ s ~ "', length=" ~ toString(len)).newline;
   Cout("s[" ~ toString(len-1) ~ "]= " ~ s[len-1]).newline;
   Cout("s[0 .. " ~ toString(len-1) ~ "]= " ~ s[0 .. len-1]).newline;
   Cout("s[0 .. " ~ toString(len) ~ "]= " ~ s[0 .. len]).newline;
   Cout("s[1 .. " ~ toString(len-1) ~ "]= " ~ s[1 .. len-1]).newline;
   Cout("s[1 .. " ~ toString(len) ~ "]= " ~ s[1 .. len]).newline;
}
------------------------- code ---------------------------------

The result is (dmd + windowsxp)

s='ABCDE', length=5
s[4]= E
s[0 .. 4]= ABCD
s[0 .. 5]= ABCDE
s[1 .. 4]= BCD
s[1 .. 5]= BCDE

-------------------------------------------------------

My question is: why s[4]=E, but s[0..4]=ABCD (without E)







-- 
Xu, Qian (stanleyxu)
  http://stanleyxu2005.blogspot.com
Mar 02 2009
next sibling parent reply Lutger <lutger.blijdestijn gmail.com> writes:
Qian Xu wrote:


 Hi,
 
 I am confusing with getting sub-string of a char[].
 
 ------------------------- code ---------------------------------
 module main;
 
 import tango.io.Console;
 import tango.text.convert.Integer;
 
 void main()
 {
    char[] s = "ABCDE"; // 5 chars
    int len = s.length;
    Cout("s='" ~ s ~ "', length=" ~ toString(len)).newline;
    Cout("s[" ~ toString(len-1) ~ "]= " ~ s[len-1]).newline;
    Cout("s[0 .. " ~ toString(len-1) ~ "]= " ~ s[0 .. len-1]).newline;
    Cout("s[0 .. " ~ toString(len) ~ "]= " ~ s[0 .. len]).newline;
    Cout("s[1 .. " ~ toString(len-1) ~ "]= " ~ s[1 .. len-1]).newline;
    Cout("s[1 .. " ~ toString(len) ~ "]= " ~ s[1 .. len]).newline;
 }
 ------------------------- code ---------------------------------
 
 The result is (dmd + windowsxp)
 
 s='ABCDE', length=5
 s[4]= E
 s[0 .. 4]= ABCD
 s[0 .. 5]= ABCDE
 s[1 .. 4]= BCD
 s[1 .. 5]= BCDE
 
 -------------------------------------------------------
 
 My question is: why s[4]=E, but s[0..4]=ABCD (without E)



s[4] means the fifth element of s[]
s[0..4] is a slice from the first to the fifth, but not including the fifth 
element. The last element in a slice is always one past the end of that 
slice.
Mar 02 2009
parent reply Qian Xu <quian.xu stud.tu-ilmenau.de> writes:
Lutger wrote:

 s[4] means the fifth element of s[]
 s[0..4] is a slice from the first to the fifth, but not including the fifth 
 element. The last element in a slice is always one past the end of that 
 slice. 
 



Thank you both.
I have to do math in mind in order to keep my code correct ;-)


IMO, it does not make any sense.
s[start..end]

   end - is neither the count of characters in the array slice,
            nor the end index of the slice.
         it is just the index after the real end character.

-- 
Xu, Qian (stanleyxu)
  http://stanleyxu2005.blogspot.com
Mar 02 2009
next sibling parent reply grauzone <none example.net> writes:
Qian Xu wrote:

 Lutger wrote:

 s[4] means the fifth element of s[]
 s[0..4] is a slice from the first to the fifth, but not including the 
 fifth element. The last element in a slice is always one past the end 
 of that slice.



 Thank you both.
 I have to do math in mind in order to keep my code correct ;-)
 
 
 IMO, it does not make any sense.
 s[start..end]
 
   end - is neither the count of characters in the array slice,
            nor the end index of the slice.
         it is just the index after the real end character.
 



Think of it as "everything in the string before this."
Mar 02 2009
parent Rainer Deyke <rainerd eldwood.com> writes:
grauzone wrote:

 Think of it as "everything in the string before this."



I tend to think of a indexes as referring to the positions between the
characters instead of the characters themselves.

"ABCD" -> 0 'A' 1 'B' 2 'C' 3 'D' 4

's[a..b]' = the elements betweens positions 'a' and 'b'.
's[a]' = the element to the right of position 'a'.


-- 
Rainer Deyke - rainerd eldwood.com
Mar 02 2009
prev sibling next sibling parent Robert Fraser <fraserofthenight gmail.com> writes:
Qian Xu wrote:

 Lutger wrote:

 s[4] means the fifth element of s[]
 s[0..4] is a slice from the first to the fifth, but not including the 
 fifth element. The last element in a slice is always one past the end 
 of that slice.



 Thank you both.
 I have to do math in mind in order to keep my code correct ;-)
 
 
 IMO, it does not make any sense.
 s[start..end]
 
   end - is neither the count of characters in the array slice,
            nor the end index of the slice.
         it is just the index after the real end character.



It's the same way it is in Python, etc. Also, it makes a lot of things 
easier, i.e.:

s[0..x] is x characters long
s[x..$] is everything from index x to the end of the string
Mar 03 2009
prev sibling next sibling parent bearophile <bearophileHUGS lycos.com> writes:
Qian Xu:

 I have to do math in mind in order to keep my code correct ;-)
 IMO, it does not make any sense.



At the beginning you have to think a bit about it, but you quickly learn it,
and you find it's the best way to design it :-)
Several languages use this same convention.

It allows you to split an array in two parts with very little troubles:
s[0 .. $] == s[0 .. n] ~ s[n .. $]

It's especially good when all the language uses this idea, for example, in D2,
this loops ten times, and x never becomes 10:
foreach (x; 0 .. 10) {...}

Bye,
bearophile
Mar 03 2009
prev sibling parent Stewart Gordon <smjg_1998 yahoo.com> writes:
Qian Xu wrote:
<snip>

 IMO, it does not make any sense.
 s[start..end]
 
   end - is neither the count of characters in the array slice,
            nor the end index of the slice.
         it is just the index after the real end character.



It might help to think of the indexes in a slice as numbering the 
boundaries between the array elements, rather than the elements themselves.

Stewart.
Mar 05 2009
prev sibling next sibling parent BCS <ao pathlink.com> writes:
Reply to Qian,


 Hi,
 
 I am confusing with getting sub-string of a char[].
 




 
 My question is: why s[4]=E, but s[0..4]=ABCD (without E)
 



Having the fist number be included and the second not works better than the 
other options.

Consider what would have to change to make these work for the the other options:

arr[0 .. n] and arr[n .. arr.length] cover the full array

arr[0 .. 0] is empty

arr[0 .. arr.length] is the full array
Mar 02 2009
prev sibling parent "Denis Koroskin" <2korden gmail.com> writes:
On Tue, 03 Mar 2009 02:03:23 +0300, BCS <ao pathlink.com> wrote:


 Reply to Qian,


 Hi,
  I am confusing with getting sub-string of a char[].




  My question is: why s[4]=E, but s[0..4]=ABCD (without E)



 Having the fist number be included and the second not works better than  
 the other options.

 Consider what would have to change to make these work for the the other  
 options:

 arr[0 .. n] and arr[n .. arr.length] cover the full array

 arr[0 .. 0] is empty

 arr[0 .. arr.length] is the full array



That and also

assert(arr[a..b].length == (b - a)); // evaluates to true always
Mar 02 2009