Meshgrid and double for-loops does not result in the same matrix, why?

Question

I am trying to evaluate all values the expression f = 2y-exp(z) can take for different values of z and y. Were y and z are two vectors of length M. I am wondering why the two approaches for generating the expression f yields different results.

Using meshgrid:

    [Y,Z] = meshgrid(y,z);
argument = 2*Y-exp(Z);

and with double for-loops

argument_new = zeros(M,M);
for i = 1:length(y)
    for j = 1:length(z)
    argument_new(i,j) = 2*y(i)-exp(z(j));
    end
end

Any hints will be highly appreciated!

[What Luis Mendo said](http://stackoverflow.com/a/27495521/2778484). But here's [my 2 cents](http://stackoverflow.com/a/22461766/2778484). — chappjc, Dec 16 '14 at 01:02
so @user3748876 does any of the answer below helped you? If so please mark them as accepted. Thanks! — Benoit_11, Dec 17 '14 at 14:09

Luis Mendo · Answer 1 · 2014-12-16T00:24:23.563

2

Blame that on meshgrid:

MESHGRID is like NDGRID except that the order of the first two input and output arguments are switched (i.e., [X,Y,Z] = MESHGRID(x,y,z) produces the same result as [Y,X,Z] = NDGRID(y,x,z)).

Solution: use ndgrid, which doesn't do that switching, and is thus more "natural":

[Y,Z] = ndgrid(y,z);
argument = 2*Y-exp(Z);

Or in your code, after meshgrid, add a transpose operation: argument = argument.';)

edited Dec 16 '14 at 00:24

answered Dec 16 '14 at 00:18

Luis Mendo

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I think @chappjc would have liked to answer this :-) – Luis Mendo Dec 16 '14 at 00:23
Bah! Haha, alas, I was chugging through a pointless review queue! – chappjc Dec 16 '14 at 00:28
Yup... `meshgrid` has truly messed me up quite a few times. +1. – rayryeng Dec 16 '14 at 00:29
@chappjc Hmmm... looking for hats, are you? – Luis Mendo Dec 16 '14 at 00:29
Looking and found. I now have a soccer ball on my head... or should I say fútbol. ;) – chappjc Dec 16 '14 at 00:31
@chappjc Deffinitely nicer than the android you had :-P (and the hat tilting feature is a nice addition this year). I'm a great fan of fútbol by the way! – Luis Mendo Dec 16 '14 at 00:40
@LuisMendo - ... but I like Android! I changed my hat to that! BTW, this post is interesting: http://meta.stackexchange.com/questions/245170/winter-bash-2014-secret-hats – rayryeng Dec 16 '14 at 01:24
@LuisMendo - Never mind... Joga Bonito! https://www.youtube.com/watch?v=IjMHJCKGBzg – rayryeng Dec 16 '14 at 01:32
1

@rayryeng I do like Android a lot... but not so much the hat :-) Nice ad, I didn't know it! – Luis Mendo Dec 16 '14 at 10:43

score 2 · Accepted Answer · answered Dec 16 '14 at 00:19

That's because of the way meshgrid creates 'inverted' directions. I don't find the right words, but here is an example illustrating with your code.You see that if you uncomment option 2 and use argument_new(j,i) instead of argument_new(i,j) both matrices are equal (as obtained with isequal).

clear
clc

M = 20;
y = 1:M;
z = 1:M;

[Y,Z] = meshgrid(y,z);
argument = 2*Y-exp(Z);

argument_new = zeros(M,M);
for i = 1:length(y)
    for j = 1:length(z)
    %// 1)    
    argument_new(i,j) = 2*y(i)-exp(z(j));
    %// 2)
    %// argument_new(j,i) = 2*y(i)-exp(z(j));
    end
end

isequal(argument,argument_new) %// Gives 0 for option 1 and 1 for option 2.

score 0 · Answer 3 · answered Dec 16 '14 at 00:20

0

They are the same, you should just transpose either one (' in Matlab), or you can replace i by j and vice versa in the for loops

answered Dec 16 '14 at 00:20

Mihon

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Meshgrid and double for-loops does not result in the same matrix, why?

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