Matlab Indexing Cheat Sheet - Duke University
Matlab Indexing Cheat Sheet
John Pearson
Matrix Indexing
A(1, 1)
1.
Matlab uses (row,column) subscripts to label matrices:
A(2, 1) A(3, 1)
A(4, 1)
A(1, 2) A(2, 2) A(3, 2) A(4, 2)
A(1, 3) A(2, 3) A(3, 3) A(4, 3)
A(1, 4)
A(2, 4)
A(3, 4)
A(4, 4)
A(1) A(5) A(9) A(13)
2.
Matlab numbers down each column in turn:
A(2) A(3)
A(6) A(7)
A(10) A(11)
A(14)
A(15)
A(4) A(8) A(12) A(16)
3. Get a single element: A(2,3)
4. Multiple elements: A([2 5],3)
5. Get subrows and subcolumns: A(2:4,3)
,
A(2,1:2)
1
6. Disconnected subrows: A([2 4],2:4)
7. Get sub-blocks: A(2:5,3:4)
8. Get single row, column: A(2,:)
, A(:,3)
9. Multiple rows, columns: A(2:4,:)
, A(:,3:4)
10. Whole matrix: A(:,:)
11. Entire array (as column vector): A(:)
...
2
Logical Indexing
1. For logical indexing, if L is a logical vector with the same dimensions as A, you can always treat L as being equivalent to the indices returned by find(L): [0 1 0 1 1 ] [2 4 5]
2. This means that we can replace any expression involving logical indices by the expression involving regular indices:
A(L, M) A(find(L), find(M))
3. Example: Let L=[0 1 0 1 1] and M = [0 1 0 1]. What is A(L,M)? From 1, we have
L
[2 4 5]
M
[2 4]
so
A(L, M)
A([2 4 5], [2 4])
.
3
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