Eachrow() error?

New to Julia
1 
julia> Random.seed!(1);
julia> x = randn(3, 5);
julia> x
3×5 Matrix{Float64}:
  0.297288  -0.0104452  2.29509   -2.26709   0.583708
  0.382396  -0.839027   0.408396   0.529966  0.963272
 -0.597634   0.311111   0.22901    0.431422  0.458791

julia> [maximum(eachrow(x)) [maximum(x[:, i]) for i in 1:5] ]
5×2 Matrix{Float64}:
  0.382396  0.382396
 -0.839027  0.311111
  0.408396  2.29509
  0.529966  0.529966
  0.963272  0.963272

the above example shows 2 problems:

  1. some of the results not matched ???
  2. it’s called “eachrow” but it should be called “eachcol” instead ???
    using v1.6.1. Thanks.
2

my misunderstanding. I should use eachcol() with vectorized maximum():

julia> [maximum.(eachcol(x)) [maximum(x[:, i]) for i in 1:5] ]
5×2 Matrix{Float64}:
 0.382396  0.382396
 0.311111  0.311111
 2.29509   2.29509
 0.529966  0.529966
 0.963272  0.963272
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3

In fact, the docs for ?eachcol is very confusing:

Example
  ≡≡≡≡≡≡≡≡≡

  julia> a = [1 2; 3 4]
  2×2 Matrix{Int64}:
   1  2
   3  4
  
  julia> first(eachcol(a))
  2-element view(::Matrix{Int64}, :, 1) with eltype Int64:
   1
   3

reading the example above would be confused with the naming of eachrow vs eachcol. It also gives an improper usage !!

actually the example should be provided as:

julia> a = [1 2; 3 4]
2×2 Matrix{Int64}:
 1  2
 3  4

julia> first.(eachcol(a))
2-element Vector{Int64}:
 1
 2

hope it helps.

4

Think of eachrow and eachcol as Vectors of Vectors. first(eachcol(a)) just gives the first column. I think the docs are pretty understandable.

Maybe part of the confusion is maximum on Vector of Vectors. Julia compares vectors elementwise, so that [1, 100] > [1, 2] (I always forget the name for this)

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5

isn’t that a column? it shouldn’t match what eachrow gives you right?

2×2 Matrix{Int64}:
 1  2
 3  4

1 and 3 form a column, are you from a culture that calls that a row? Eachcol gives you a collection, think of it as [col1, col2], so when you call first(), you’re getting the first column, col1, which is 1 and 3 in the above example.

On the other hand, calling first.() is getting first element of EACH of the columns, of course you get the first row then.

6

yes. If we replace first() with maximum() in the docs example, my concern would be apparent.

I think at least we should include the use of first.() in the docs example for clarification and comparison.

7

Feel free to make a PR, then, to improve the docs.

8

If you add something to the docs, it should probably be map(first, eachcol(a)). Broadcasting over the generator collects it first, which isn’t necessary.

1 Like
9

I want to do so… but I just don’t know how… it says “You must be on a branch to propose or make changes to this file”… what could I do? How to be “on” a branch???

10

You might like this:

But for doc PRs, you can also just click the pencil-like button on the github page, and it will (usually) make a fork and make a branch on that, called patch-1 or something.

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