Matrix/array manipulation in MATLAB

Question

M1 = 

     1   --> Position 1
     2   --> Position 2
     1   --> Position 3
     2   --> Position 4

M2 =

     2     1     3     4

I need help in coding for a simple program. I have two matrices (or arrays, samples given above) and I want to update M2 according to M1.

For instance, whatever number is present in M1 third position gets updated in every index of M2 where number 3 is present. And the same goes for all positions of M2.

So, my desired result would be like this: M2 = [2 1 1 2]

How can I accomplish this?

@Hazem : Thanks..but this isn't what i have mentioned. i want to replace the numbers in M2 according to M1, not sort them. — Gavanpreet Singh
– Gavanpreet Singh, Commented Jan 30, 2018 at 14:11

gnovice · Accepted Answer · 2018-01-30 16:33:50Z

2

This can be done with a simple indexing operation:

M2 = M1(M2);

edited Jan 30, 2018 at 16:33

answered Jan 30, 2018 at 16:28

gnovice

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1 Comment

gnovice Over a year ago

@GavanpreetSingh: Happy to help. Don't forget that you can mark answers as accepted.

Hans Lehnert · Accepted Answer · 2018-01-30 15:39:02Z

0

You could try something like

for i = 1:length(M1)
    M2(M2 == i) = M1(i);
end

M2 == i uses the logic equality operator ==, and it returns a vector of the same size of M2 with ones in the positions where the elements are equal to i and zeros otherwise. So for example if M2 = [1 2 2 3], M2 == 2 would return [0 1 1 0].

Using this vector as the index argument select only the elements where the indexing vector is 1. Following the previous example M2(M2 == 2) is equal to M2([0 1 1 0]) and will select the second and third elements, which are the ones that are equal to 2.

If we now put it in a loop and iterate over all positions of M1, M2(M2 == i) selects all elements in M2 that match M2 == i and you want to assign the value M1(i) to those elements.

edited Jan 30, 2018 at 15:39

answered Jan 30, 2018 at 14:22

Hans Lehnert

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2 Comments

Gavanpreet Singh Over a year ago

It is working perfectly fine...but i haven't understand how. Maybe you can explain a little bit if it doesn't bother you...Many thanks for code, anyway....

Hans Lehnert Over a year ago

I tried to explain how it works. I hope that's better :)

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