# Difference between element-wise allocation and copy()
**URL:** https://discourse.julialang.org/t/difference-between-element-wise-allocation-and-copy/111803
**Category:** New to Julia
**Created:** [March 19, 2024, 7:36am UTC](https://discourse.julialang.org/t/difference-between-element-wise-allocation-and-copy/111803 "2024-03-19T07:36:19Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@TongTongYee](https://discourse.julialang.org/u/TongTongYee)
#### Post date: [March 19, 2024, 7:36am UTC](https://discourse.julialang.org/t/difference-between-element-wise-allocation-and-copy/111803/1 "2024-03-19T07:36:19Z")
Hi!
I was using the elemet-wize allocation ( .= ) as the copy function. But I found that in certain cases, it’s not. I want to know in which case I can use element-wize allocation as the copy function.
for example
```julia
C11 = fill(0.0im,(2,10))
C12 = fill(0.0im,(2,10))
for i = 1: 2
C13 = fill(i,10)
C11[i,:,:] = copy(C13)
C12[i,:,:] .= 1*C13
end
C12[1,:] # ten of 1.0
C12[2,:] # ten of 2.0
C11[1,:] # ten of 1.0
C12[2,:] # ten of 2.0
C15 = fill(1.0im*fill(0.0,(10)),(2))
C14 = fill(1.0im*fill(0.0,(10)),(2))
for i = 1: 2
C13 = fill(i,10)
C14[i] = copy(C13)
C15[i] .= 1*C13
end
C14[1] # ten of 1.0
C14[2] # ten of 2.0
**C15[1] # ten of 2.0**
C15[2] # ten of 2.0
```
I found strange results
```julia
C14 = fill(1.0im*fill(0.0,(10)),(2))
for i = 1: 2
C13 = fill(i,10)
C15[i] **.=** C13
end
C15[1] # 2.0
for i = 1: 2
C13 = fill(i,10)
C15[i] **=** C13
end
C15[1] #1.0
for i = 1: 2
C13 = fill(i,10)
C15[i] **.=** C13
end
C15[1] # 1.0
```
---
### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [March 19, 2024, 12:18pm UTC](https://discourse.julialang.org/t/difference-between-element-wise-allocation-and-copy/111803/3 "2024-03-19T12:18:32Z")
> [@TongTongYee](#):
>
> `C15 = fill(1.0im*fill(0.0,(10)),(2))`
You are having a [`fill` array-of-arrays problem](https://stackoverflow.com/questions/41072991/why-do-these-two-different-ways-of-constructing-an-array-produce-different-behav). Hint: do `C15[1][1] = 1` and then look at `C15[2]`.
TDLR: You are initializing your arrays so that every element “points” to the _same_ array, so that mutating one element’s array mutates them all.
> [@TongTongYee](#):
>
> ```julia
> C14[i] = copy(C13)
> C15[i] .= 1*C13
>
> ```
This is compounding your `fill` confusion with [assignment vs. mutation](https://docs.julialang.org/en/v1/manual/variables/#man-assignment-expressions) confusion.
TLDR: `= copy` assigns that element to a new array, without affecting the other elements, whereas `.=` mutates the existing array (which affects all your other elements due to the `fill` confusion).
---
### Author:  [@stevengj](https://discourse.julialang.org/u/stevengj)
#### Post date: [March 19, 2024, 12:28pm UTC](https://discourse.julialang.org/t/difference-between-element-wise-allocation-and-copy/111803/4 "2024-03-19T12:28:14Z")
> [@stevengj](#):
>
> You are having a [`fill` array-of-arrays problem](https://stackoverflow.com/questions/41072991/why-do-these-two-different-ways-of-constructing-an-array-produce-different-behav).
I’ve often wanted a package that could take code like this:
```julia
a = fill([3,1,4], 4)
```
and draw a diagram like this:
as opposed to
```julia
b = [[3,1,4] for i=1:4]
```
for which it would draw:
