Go has a different understanding of slice with python, why go think like this?

[Dean Sinaean]

In go :
A is an array
B:=A[:]
or
B:=A
then B is a copy of A

B:=A[2:5]
then B is a pointer to A[2]  (I mean when an element in B is modified, the corresponding element in A is also modified.)

in python, its just the  opposite way. why does go think  its way is better?

ps:
what is the difference between B:=A[0:]  and B:=A[0:6] when A has seven elements? in the latter case, I found A modified when B is modified, while in the former case, A is not.

my go version is 1.02 for windows.

thanks and regards,
dean

[David DENG]

if len(A) == 7, A[:]/A[0:] is a slice of length 7, while A[0:6]'s length == 6. They are all not hard copy.

http://play.golang.org/p/Ly5x2GdrH2

David

[David Leimbach]

On Wednesday, February 27, 2013 6:49:34 PM UTC-8, Dean Sinaean wrote:

In go :
A is an array
B:=A[:]
or
B:=A
then B is a copy of A

B := A[:] makes a slice of A, containing all of a and assigns it to B.  These two structures can have potentially different indexing, but share the same backing memory storage of the original array in A.  It's not true to say "B is a copy of A".  I guess you can call it a shallow copy in some instances, but I don't think of "shallow copies" as real copies very often. 

See this:

http://play.golang.org/p/OxHP8kEgGI

 

B:=A[2:5]
then B is a pointer to A[2]  (I mean when an element in B is modified, the corresponding element in A is also modified.)

in python, its just the  opposite way. why does go think  its way is better?

Opposite?   Can you demonstrate?  I really don't remember this part of Python very well.

 

ps:
what is the difference between B:=A[0:]  and B:=A[0:6] when A has seven elements? in the latter case, I found A modified when B is modified, while in the former case, A is not.

I don't see that at all...

http://play.golang.org/p/08OzPRP1cS

[John Asmuth]

On Wednesday, February 27, 2013 9:49:34 PM UTC-5, Dean Sinaean wrote:

then B is a copy of A

 

what is the difference between B:=A[0:]  and B:=A[0:6] when A has seven elements? in the latter case, I found A modified when B is modified, while in the former case, A is not.

 Your experiments are not working. Nothing you did ever copied any arrays.

[Andrew Gerrand]

On 28 February 2013 18:20, John Asmuth <jas...@gmail.com> wrote:

 Your experiments are not working. Nothing you did ever copied any arrays.

Not true. He mentioned

A is an array

B := A

which is an array copy.

http://play.golang.org/p/fWthD17Axx

Andrew

[David DENG]

I think Dean is confused with array and slice.

David

[Andrew Gerrand]

You might want to read this doc:

http://golang.org/doc/articles/slices_usage_and_internals.html

It explains how arrays and slices work in Go. Perhaps from that you'll gather some idea of why they were designed that way. The main thing is giving the programmer explicit control over memory allocations.

Andrew

[Dean Sinaean]

Sorry for late reply, I found the problem at work, where I can't access gmail via a computer and I sent this mail using my phone.

Back at home, I can't reproduce the case in which B is a copy of A in go anyhow. Maybe I made a mistake( sorry..).

With go, B:=A, and I make some changes to B, for example B[1]=6, the corresponding element in A is changed, too.
package main

import(

    "fmt"

)

func main(){

    var A=[]int{1,2,3,4,5}//or var A=[5]int{1,2,3,4,5}

    B:=A// or B:=A[0:] or B:=A[:]

    B[1]=6

    fmt.Println(A)

}

the result is [1 6 3 4 5]。

while with Python:
>>> A=[1,2,3,4,5]
>>> B=A[2:]
>>> B[1]=6
>>> A
[1, 2, 3, 4, 5]
>>> B=A
>>> B[1]=6
>>> A
[1, 6, 3, 4, 5]

I think I'm clear with the difference now, and I better use copy function to copy a slice.
Thank you all for the conversation.

Nice weekend,
Dean