How to get an array "complement" of another?

Question

the easiest way for me to explain what i want is with an example:

a = 1:20

b = [2,7,12,18]

Now I want c to be [1,3,4,5,6,8,...,19,20] with length 16: length(a) - length(b) of course.

Is there a way for me to get c?

score 5 · Answer 1 · answered Feb 09 '14 at 23:57

5

You can delete array elements using x(3)=[]

c=a;
c(b)=[];

answered Feb 09 '14 at 23:57

Daniel

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works for this simple case. But for the general case (where `b` contains elements to be removed from `a` instead of indices), use `setdiff`, as in the other answer. – Rody Oldenhuis Feb 10 '14 at 09:14

score 4 · Accepted Answer · edited Feb 10 '14 at 09:15

4

What you want is called set difference in most languages. In MATLAB, you can use the setdiff function:

a=1:20;
>> b=[2,7,12,18];
>> setdiff(a,b);

ans =

Columns 1 through 11

 1     3     4     5     6     8     9    10    11    13    14

Columns 12 through 16

15    16    17    19    20

edited Feb 10 '14 at 09:15

Rody Oldenhuis

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answered Feb 10 '14 at 00:00

Bartomeu Galmés

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function is useful for complicated cases, thx will take note for future use. – Antonio Craveiro Feb 10 '14 at 00:06
complement and set difference are synonymous. (at least applied to sets) – Daniel Feb 10 '14 at 00:07
@Daniel: Indeed. Still it's useful to mention that most programming languages call it *difference*, to avoid any confusion. – Rody Oldenhuis Feb 10 '14 at 09:13

How to get an array "complement" of another?

2 Answers2