Question

Help me with this part of code:

class Ooo
 attr_accessor :class_array
end

def func(ctx)
 local_array = ctx.class_array
 local_array = [4,3,5,5,6]
 return
end

aaa = Ooo.new
func(aaa)
aaa.class_array => not [4,3,5,5,6] :-(

I supposed that ruby use addresses when operates with arrays…
Why does this code not work?
I want to do this (in C):

struct ctx
{
uint class_array[10000]
}

void func(struct *ctx)
{
 uint* local_array = &ctx->class_array
 local_array[0] = 4;
 ctx->class_array[0] => 4
}

Answer 1

The problem is with this part of code:

local_array = ctx.class_array
local_array = [4,3,5,5,6]

This doesn’t do what you think it does. The second line creates a new list and assigns it to the local variable, thus replacing the reference to ctx.class_array. It will not touch ctx.class_array. An equivalent piece of C code would work the same way, so I think you don’t only have a Ruby problem here.

In C you could use pointers to solve this. In Ruby you probably want either:

local_array = ctx.class_array
local_array.replace [4,3,5,5,6]

or simply (much better!)

ctx.class_array = [4,3,5,5,6]

By the way, the direct translation of your C program would also work:

def func(ctx)
  local_array = ctx.class_array
  local_array[4] = 4
end