Meshgrid function in Julia

Specific Domains Numerics
1

hello everyone I am new to Julia. Earlier I was using Matlab but now I have started to work in Julia. I would like to know is there any similar function available in julia like Meshgrid which is available in matlab. Please help.

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2

Yup, see VectorizedRoutines.jl

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3

Also worth saying: as you use Julia more and more, you’ll probably use meshgrid less and less. Here’s a nice explanation of why (for an outdated Julia version, but other than suggesting you ignore the comments about performance—all those issues were fixed long ago—the syntax seems pretty applicable now): https://groups.google.com/g/julia-users/c/83Pfg9HGhGQ/m/9G_0wi-GBQAJ

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4

A few months ago, while teaching conformal mapping, I rolled my own

"""
duplicate of matlab meshgrid function
"""
function meshgrid(xin,yin)
nx=length(xin)
ny=length(yin)
xout=zeros(ny,nx)
yout=zeros(ny,nx)
for jx=1:nx
    for ix=1:ny
        xout[ix,jx]=xin[jx]
        yout[ix,jx]=yin[ix]
    end
end
return (x=xout, y=yout)
end
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5

Modern Julia is better:

x = 1:3
y = 1:5
x' .* ones(5)

#=
5Ă—3 Array{Float64,2}:
 1.0  2.0  3.0
 1.0  2.0  3.0
 1.0  2.0  3.0
 1.0  2.0  3.0
 1.0  2.0  3.0
=#
 
ones(3)' .* y

#=
5Ă—3 Array{Float64,2}:
 1.0  1.0  1.0
 2.0  2.0  2.0
 3.0  3.0  3.0
 4.0  4.0  4.0
 5.0  5.0  5.0
=#
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6

neat! when was that?

also, can you do 3D arrays with the same bcast trick?

7

The only reason to make a meshgrid in Julia now is that you can do it as a lazy object without an array backing, so it could be like a 2D or 3D Range, meaning a 10,000,000 x 10,000,000 grid is representable in like 500 bytes.

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8

You don’t need meshgrid in Julia. In fact, it is rarely needed even in Matlab these days, after they introduced a type of broadcasting.

I would recommend to not use meshgrid, and instead learn idiomatic Julia.

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10

Same thing with repmat. Never needed in Julia, virtually never needed in Matlab. It’s old-style Matlab.

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11

old-style Matlab.

This was the moment I realized I am now old.

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12

I meant “classic Matlab” :wink:

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13

Can you help with how to plot a structured grid using these points in Julia?

14

I am also confused. I use this for contour in PyPlot, where the help files tell me to use meshgrid. How can I avoid meshgrid without finding a different plotting package?

1 Like
15

You don’t need a meshgrid for contour plotting with PyPlot — you can pass 1d arrays for the axes and it knows how to broadcast them, and you write 2d functions of the axes variables by broadcast operations as well:

using PyPlot
x = range(0,2,length=100)'  # note ': this is a row vector
y = range(0,4,length=200)
z = @. sin(x) * cos(y)  # broadcasts to 2d array
contour(x, y, z)

image

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16

Thanks. I’m even older than @jlchan and transcribed my stuff from (no joke) 20 year old matlab.

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17

This is the corresponding Matlab code, btw:

x = linspace(0, 2, 100);
y = linspace(0, 4, 200).';
z = sin(x) .* cos(y);  % broadcasts to 2d array
contour(x, y, z)

figure1
figure12559Ă—1884 103 KB

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18

I have seen the error in my ways and will go forth and sin in a different way in the future.

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19

Remember to cos as well!

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20

I will do that as I sit in the sun and work on my tan.

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21

Instead of sinning, sec and ye shall find!

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