How can I subtract elements in an array from the element before it.

Hi guys,

I want to know how to subtract every element in an array from the element before it, for example element 2-element1 and element 3-element2 and so on.

Answer 1

[John D'Errico]

help diff

 DIFF Difference and approximate derivative.
    DIFF(X), for a vector X, is [X(2)-X(1)  X(3)-X(2) ... X(n)-X(n-1)].
    DIFF(X), for a matrix X, is the matrix of row differences,
       [X(2:n,:) - X(1:n-1,:)].
    DIFF(X), for an N-D array X, is the difference along the first
       non-singleton dimension of X.
    DIFF(X,N) is the N-th order difference along the first non-singleton 
       dimension (denote it by DIM). If N >= size(X,DIM), DIFF takes 
       successive differences along the next non-singleton dimension.
    DIFF(X,N,DIM) is the Nth difference function along dimension DIM. 
       If N >= size(X,DIM), DIFF returns an empty array.
 
    Examples:
       h = .001; x = 0:h:pi;
       diff(sin(x.^2))/h is an approximation to 2*cos(x.^2).*x
       diff((1:10).^2) is 3:2:19
 
       If X = [3 7 5
               0 9 2]
       then diff(X,1,1) is [-3 2 -3], diff(X,1,2) is [4 -2
                                                      9 -7],
       diff(X,2,2) is the 2nd order difference along the dimension 2, and
       diff(X,3,2) is the empty matrix.
 
    See also GRADIENT, SUM, PROD.

    Documentation for diff
       doc diff

    Other uses of diff

       codistributed/diff    iddata/diff      tall/diff
       datetime/diff         sym/diff         tireModel/diff
       duration/diff         symbolic/diff    umat/diff
       gpuArray/diff         tabular/diff

[Aman Kumar]

so for a matrix I simply need to use diff(function name)

[John D'Errico]

Essentially so. You say matrix however, but earlier you said vector. Diff can be applied to any desired dimension of a matrix.

And now you say function name. If a function returns an array of any size, then you can apply diff to that result.

So, yes.

[Aman Kumar]

Yes by function name I meant a function which returns an array. Thank you for all the help it is appreciated

[John D'Errico]

magic(5)

ans = 5x5

17 24 1 8 15 23 5 7 14 16 4 6 13 20 22 10 12 19 21 3 11 18 25 2 9

Difference of pairs of columns:

diff(magic(5),[],2)

ans = 5x4

7 -23 7 7 -18 2 7 2 2 7 7 2 2 7 2 -18 7 7 -23 7

Difference between consecutive rows.

diff(magic(5),[],1)

ans = 4x5

6 -19 6 6 1 -19 1 6 6 6 6 6 6 1 -19 1 6 6 -19 6

[Aman Kumar]

That makes it more clear thank you. I have a function which returns an array of the size 639x1. So I need to do row 2-1 and row 3-2 etc until the final row.

[John D'Errico]

Then you can just call diff, wih no other arguments. For example...

primes(20)

ans = 1x8

2 3 5 7 11 13 17 19

So the MATLAB function which returns a list of primes.

diff(primes(20))

ans = 1x7

1 2 2 4 2 4 2

[Aman Kumar]

That makes a lot of sense, but why is there is a 2 at the end of the result array if 11 is not being subtracted by anything?

[John D'Errico]

Um, 19-17 == 2. That seems right to me. There are 8 numbers returned by primes in that call. There will be 7 differences.

[Aman Kumar]

Right my apologies I didn’t scroll right on my phone so I didn’t see the rest of the numbers, but that all makes perfect sense. Thank you for all the help and guidance.

[Dyuman Joshi]

Hello @Aman Kumar, if this answer solved your problem, please consider accepting the answer.

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[Aman Kumar]

Yes I will definitely accept it sorry about that