# How to increase a matrix?

**URL:** <https://discourse.julialang.org/t/how-to-increase-a-matrix/17912>\
**Category:** General Usage\
**Created:** [November 23, 2018, 3:45pm UTC](https://discourse.julialang.org/t/how-to-increase-a-matrix/17912 "2018-11-23T15:45:07Z")\
**Posts on this page:** 11\
**Page:** 1

<div class="post-metadata">

**Author:** ![joa-quim](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/joa-quim/32/227_2.png) [@joa-quim](https://discourse.julialang.org/u/joa-quim)\
**Post date:** [November 23, 2018, 3:45pm UTC](https://discourse.julialang.org/t/how-to-increase-a-matrix/17912/1 "2018-11-23T15:45:07Z")

</div>

I need to increase a matrix inside a function. But `append!` works for vectors but doesn’t work for matrices

```julia
arg1=[1 2;3 4];

julia> append!(arg1, [5,6])
ERROR: MethodError: no method matching append!(::Array{Int64,2}, ::Array{Int64,1})
Closest candidates are:
  append!(::Array{#s58,1} where #s58, ::AbstractArray{T,1} where T) at array.jl:895
  append!(::Array{T,1} where T, ::Any) at array.jl:902

```

So I thought in this trick via `reshape` but no luck either

```julia
julia> arg2=reshape(append!((reshape(arg1,:)), [5,6]), (2,3))
2×3 Array{Int64,2}:
 1 2 5
 3 4 6

julia> arg1
2×2 Array{Int64,2}:
 1 2
 3 4

```

So, even all those operations are _inplace_ the input `arg1` was not modified. This happens apparently because `reshape` creates a new object that in spite sharing the same memory is different thing and is that new object that gets appended.

---

<div class="post-metadata">

**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [November 23, 2018, 4:08pm UTC](https://discourse.julialang.org/t/how-to-increase-a-matrix/17912/2 "2018-11-23T16:08:34Z")

</div>

`Matrix` is fixed size. Consider storing a `Vector` and just reshape where it is used as a Matrix.

---

<div class="post-metadata">

**Author:** ![bennedich](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bennedich/32/4894_2.png) [@bennedich](https://discourse.julialang.org/u/bennedich)\
**Post date:** [November 23, 2018, 4:11pm UTC](https://discourse.julialang.org/t/how-to-increase-a-matrix/17912/3 "2018-11-23T16:11:57Z")

</div>

What I’ve done in the past is simply:

```julia
function resize_matrix(A, rows, cols)
    resized = similar(A, rows, cols)
    resized[1:size(A, 1), 1:size(A, 2)] = A
    return resized
end

```

This is quite inefficient, so if you continuously resize the matrix, it’s more efficient to resize it by chunks at a time (e.g. double it in size whenever capacity is reached).

You could also use a `Vector{Vector{Int}}()` and use `append!`.

---

<div class="post-metadata">

**Author:** ![joa-quim](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/joa-quim/32/227_2.png) [@joa-quim](https://discourse.julialang.org/u/joa-quim)\
**Post date:** [November 23, 2018, 4:20pm UTC](https://discourse.julialang.org/t/how-to-increase-a-matrix/17912/4 "2018-11-23T16:20:15Z")

</div>

Thanks but it won’t work for me. I need that all occurs inside a function and that the argument is modified and not to create a new object for returning.

```julia
function foo(mat, args...)
    if (some_condition_in_args)
        increase!(mat)
    end
end

```

so that if I call `foo()` the matrix `mat` gets (or not) increased.  
And I need it to be a matrix, not a vector.

---

<div class="post-metadata">

**Author:** ![bennedich](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bennedich/32/4894_2.png) [@bennedich](https://discourse.julialang.org/u/bennedich)\
**Post date:** [November 23, 2018, 4:33pm UTC](https://discourse.julialang.org/t/how-to-increase-a-matrix/17912/5 "2018-11-23T16:33:14Z")

</div>

Then I’m afraid you will have to rethink. As Kristoffer points out, `Matrix` is fixed in size, so what you’re asking for is not possible with the `Matrix` type. Perhaps you can create your own resizable `AbstractMatrix` type using any of the ideas suggested above.

---

<div class="post-metadata">

**Author:** ![foobar\_lv2](https://avatars.discourse-cdn.com/v4/letter/f/ee59a6/32.png) [@foobar\_lv2](https://discourse.julialang.org/u/foobar_lv2)\
**Post date:** [November 23, 2018, 4:37pm UTC](https://discourse.julialang.org/t/how-to-increase-a-matrix/17912/6 "2018-11-23T16:37:27Z")

</div>

You can’t use reshape either, since that fixes the size forever. The correct solution is to define your own `resizeMatrix <:AbstractMatrix` that wraps an underlying `Vector` for storage.

Luckily, [https://github.com/JuliaArrays/ElasticArrays.jl](https://github.com/JuliaArrays/ElasticArrays.jl) does exactly that.

---

<div class="post-metadata">

**Author:** ![joa-quim](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/joa-quim/32/227_2.png) [@joa-quim](https://discourse.julialang.org/u/joa-quim)\
**Post date:** [November 23, 2018, 4:45pm UTC](https://discourse.julialang.org/t/how-to-increase-a-matrix/17912/7 "2018-11-23T16:45:53Z")

</div>

OK, thanks. I’ll keep trying till I get it.

---

<div class="post-metadata">

**Author:** ![bennedich](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bennedich/32/4894_2.png) [@bennedich](https://discourse.julialang.org/u/bennedich)\
**Post date:** [November 23, 2018, 4:54pm UTC](https://discourse.julialang.org/t/how-to-increase-a-matrix/17912/8 "2018-11-23T16:54:41Z")

</div>

The underlying-Vector/ElasticArrays solution only works for resizing the last dimension, right? So I guess it wouldn’t work for what OP is trying to do here, unless also transposing the data:

```julia
arg1=[1 2;3 4]
append!(arg1, [5,6])

```

---

<div class="post-metadata">

**Author:** ![joa-quim](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/joa-quim/32/227_2.png) [@joa-quim](https://discourse.julialang.org/u/joa-quim)\
**Post date:** [November 23, 2018, 4:59pm UTC](https://discourse.julialang.org/t/how-to-increase-a-matrix/17912/9 "2018-11-23T16:59:37Z")

</div>

I haven’t look at `ElasticArrays` yet but tried playing with the vector conversions. And I get this new bizarre behavior (the column being appended is a 5x1 col

```julia
		@show(arg1)
		arg1 = reshape(arg1, :)
		cmd = add_opt(cmd, 'E', d, [:E :error_bars],
					  (x="|x",y="|y",xy="|xy",X="|X",Y="|Y",asym="+a",cap="+w",pen="+p"), false, arg1)
		@show(arg1)
		reshape(arg1, (5, 3))
		@show(arg1)

```

but last reshape refuses to … reshape. Running the code that calls the above chunk

```julia
arg1 = [1.175 25.0; 2.175 32.0; 3.175 34.0; 4.175 20.0; 5.175 25.0]
arg1 = [1.175, 2.175, 3.175, 4.175, 5.175, 25.0, 32.0, 34.0, 20.0, 25.0, 2.0, 3.0, 4.0, 1.0, 2.0]
arg1 = [1.175, 2.175, 3.175, 4.175, 5.175, 25.0, 32.0, 34.0, 20.0, 25.0, 2.0, 3.0, 4.0, 1.0, 2.0]

```

Is it related to @foobar_lv2 statement that reshape fixes the size forever?  
(it didn’t, but it doesn’t work either)

---

<div class="post-metadata">

**Author:** ![bennedich](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bennedich/32/4894_2.png) [@bennedich](https://discourse.julialang.org/u/bennedich)\
**Post date:** [November 23, 2018, 5:01pm UTC](https://discourse.julialang.org/t/how-to-increase-a-matrix/17912/10 "2018-11-23T17:01:52Z")

</div>

You’re not assigning the last `reshape` to `arg1`.

---

<div class="post-metadata">

**Author:** ![joa-quim](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/joa-quim/32/227_2.png) [@joa-quim](https://discourse.julialang.org/u/joa-quim)\
**Post date:** [November 23, 2018, 5:06pm UTC](https://discourse.julialang.org/t/how-to-increase-a-matrix/17912/11 "2018-11-23T17:06:13Z")

</div>

Ah, still assuming that reshape works on place and does keeps the input object.  
Thanks, convoluted but it seems to be working now.