Hi Eric,
I think you’re being confused by the distinction between the bindings of
variables and values, which can be bound to variables.
If w is an Array, then an expression like
w = [1, 2, 3]
assigns a value (namely the value of an array containing 1, 2 and 3) to the
variable w.
In contrast, an expression like
w[1] = 4
does not refer to a variable called w[1]. It refers to a position in memory
whose value is being mutated to 4.
— John
On Jan 24, 2014, at 9:01 PM, Eric Ford <ericbf...@gmail.com> wrote:
> Hi Ivar,
>
> Thanks for the idea. But replacing w += 10 with w = w + 10 still gives the
> same problem. (As does w = w .+ 10).
> Either way, after calling f2, the values of x are modified.
>
> Even stranger, the values of x are not modified by the following function
> function f3(x::Array)
> w = x + 0.
> for i in 1:length(w)
> w[i] = w[i] + 10.0
> end
> return w
> end
> So the behavior of future lines of code depends on whether w is initialized
> as w = x or w = x + 0.
>
> It appears that for some reason, julia is doing different things when the
> right hand side is just an array, versus when it is an expression.
> Maybe somebody considers that a feature, but it's definitely non-intuitive.
>
> I can't find any mention of this in the documentation. Amusingly, there
> appears to be basically no documentation for what "=" does (despite it being
> the most common symbol in most every julia program).
> I thought = might be implemented as a generic function with different
> behaviors for Arrays and expressions, but methods(=) and help(=) turned up
> nothing.
> On one hand, it seems, like = should be fairly obvious, but evidently there
> are currently important differences in the behavior of = that need to be
> documented and/or corrected.
>
> Cheers,
> Eric
>
>
> On Friday, January 24, 2014 2:38:53 PM UTC-5, Ivar Nesje wrote:
> It becomes more obvious if you write "a += 3" as the longer form "a = a + 4"
>
> kl. 19:57:22 UTC+1 fredag 24. januar 2014 skrev Eric Ford følgende:
> Sorry. (Evidently, downloading a notebook is based on the last saved version
> and what's currently on your screen.) IJulia notebook attached. Readable
> version below.
>
> function f1(x::Array)
> w = x
> w += 10.0
> return w
> end
>
> function f2(x::Array)
> w = x
> for i in 1:length(x)
> w[i] += 10.0
> end
> return w
> end
>
> x=randn(10)
> x_orig = deepcopy(x)
> f1_of_x = f1(x)
> println("After f1: ",sum((x.-x_orig).^2));
> x = deepcopy(x_orig)
> f2_of_x = f2(x)
> println("After f2: ",sum((x.-x_orig).^2));
>
> After f1: 0.0
> After f2: 1000.0
>
> Thanks,
> Eric
>
>
> On Friday, January 24, 2014 1:10:59 PM UTC-5, Kevin Squire wrote:
> In what you posted, `f1` and `f2` are identical (except for the name). Can
> you share the output of a Julia or IJulia session showing the problem?
>
> Cheers,
> Kevin
>
>
> On Fri, Jan 24, 2014 at 9:58 AM, Eric Ford <eric...@gmail.com> wrote:
> I don't understand why the first function doesn't change x, but the second
> function does.
> Is the = calling deepcopy in f1, but copy in f2?
> If so, why?
>
> function f1(x::Array)
> w = x
> for i in 1:length(x)
> w[i] += 10.0
> end
> w += 10.0
> return w
> end
>
> function f2(x::Array)
> w = x
> for i in 1:length(x)
> w[i] += 10.0
> end
> w += 10.0
> return w
> end
>
> x=randn(10)
> x_orig = deepcopy(x)
> f_of_x = f(x)
> sum((x.-x_orig).^2)
>
> After f1: 0.0
> After f2: 1000.0
>
> Thanks,
> Eric (on behalf of an Astro 585 student)
>
>
