How to check if the beginning elements of array in matlab is the same

Question

I would like to see if an array starts with the same elements as another array without having to write a bunch of for loops going through each element individually.

For example if I had the arrays below

Array1 = [1 2 3 4]
Array2 = [1 2 3 4 5 3 2 5 7]

Array3 = [1 2 3 5]

Then comparing Array1 with Array2 would return true. and comparing Array3 with Array2 would return false.

Is there any quick and easy way of doing this. I would not know the lengths of the arrays I would be comparing. The number of elements I want to compare equals the length of the shortest vector.

Thanks!

How many of the first elements must be equal?

Stewie Griffin
– Stewie Griffin

2014-07-15 07:16:29 +00:00
Commented Jul 15, 2014 at 7:16 — Stewie Griffin
– Stewie Griffin, Commented Jul 15, 2014 at 7:16

Community · Accepted Answer · 2017-05-23 11:50:19Z

You can check if all elements in two vectors are the same using isequal. To check only the first n elements, you can do Array(1:n), thus the entire function will be like this:

Array1 = [1 2 3 4]
Array2 = [1 2 3 4 5 3 2 5 7]
Array3 = [1 2 3 5]

n = 4;    % Compare the first n elements
isequal(Array1(1:n), Array2(1:n))
ans =  1

isequal(Array2(1:n), Array3(1:n))
ans = 0

If you use Array1(1:n) == Array2(1:n) you will get a piece-wise comparison resulting in 1 1 1 1. Of course, this means you could also do:

all(Array1(1:n) == Array2(1:n))
ans =  1

all(Array2(1:n) == Array3(1:n))
ans =  0

If you want n to be the number of elements in the smallest vector (per your comment), as Chris and Ben interpret the question, you can solve it this way:

isequal(Array1(min([numel(Array1) numel(Array2)])), Array2(min([numel(Array1) numel(Array2)])))

or a bit cleaner:

n = min([numel(Array1) numel(Array2)])
isequal(Array1(1:n), Array2(1:n))

Chris Taylor · Accepted Answer · 2014-07-15 07:20:42Z

2

Here's a function that will compare the initial segments of any two vectors, up to the length of the shortest vector. It returns true if they are identical, and false if they are not identical.

Note that

It only works correctly with vectors, not with matrices (although you could extend it to deal with matrices)
If any entries are NaN then it will always return false, since NaN == NaN is false.

Here it is -

function result = equal_initial_segment(x, y)

  N = min(length(x), length(y));

  result = isequal(x(1:N), y(1:N));

end

answered Jul 15, 2014 at 7:20

Chris Taylor

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Comments

Fantastic Mr Fox · Accepted Answer · 2014-07-15 07:21:00Z

0

It seems like you are just comparing the all of the elements of the shorter list to the first elements of the longer list, in which case you can just do this:

function same = compareLists(list1, list2)
    if length(list1) > length(list2)
         same = isequal(list2, list1(1:length(list2));
    else if
         same = isequal(list1, list2(1:length(list1));
    end
end

answered Jul 15, 2014 at 7:21

Fantastic Mr Fox

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Comments

Luis Mendo · Accepted Answer · 2014-07-15 09:15:12Z

0

You can use strmatch for that:

~(isempty(strmatch(Array1, Array2)) && isempty(strmatch(Array2, Array1)))

answered Jul 15, 2014 at 9:15

Luis Mendo

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