question about indexing of logic matrix

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Yu Li 2019년 10월 2일
댓글: Yu Li 2019년 10월 2일
Hi:
there is an operation using command:
Where hel.faces is a m*3 matrix, and mask is a n*1 logic matrix. I could not understand how could this operation happens because it can not reproduce in my side. my test commands are:
A=rand(20,3);
B=rand(20,1);
C=B>0.5;
C(A)
could anyone give me some suggestions?
Thanks!
Yu

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채택된 답변

Stephen23 2019년 10월 2일
편집: Stephen23 2019년 10월 2일
Your mistake is on this line:
A=rand(20,3);
You correctly wrote that "hel.faces is a m*3 matrix", but you seem to have missed that its values are all indices, i.e. positive integers, with values from 1 to numel(mask):
ans =
121368
>> max(hel.faces(:))
ans =
121368
>> min(hel.faces(:))
ans =
1
>> find(mod(hel.faces(:),1)) % only integers
ans =
Empty matrix: 0-by-1
That is why it can be used as an index into another array (e.g. mask): because it it contains indices.
But you generated A with random numbers between 0 and 1, which are not indices (indices must be positive integers). Once you generate A with positive integers, with values from 1 to numel(B), then your code will work:
>> B = rand(20,1);
>> A = randi([1,20],20,3);
>> C = B>0.5;
>> C(A)
ans =
0 1 0
1 1 0
0 0 0
1 0 0
0 1 1
0 1 1
1 1 0
1 1 0
0 1 0
1 0 0
1 0 0
1 0 0
0 0 0
1 1 1
1 0 1
0 1 0
0 1 1
0 0 0
1 1 1
1 1 1
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편집: Adam Danz 2019년 10월 2일
" Where hel.faces is a m*3 matrix, and mask is a n*1 logic matrix. I could not understand how could this operation happens"
In that blog article, mask is a [121368 x 1] logical vector and hel.faces is a [239778, 3] matrix of integers. Note that the range of values in hel.faces is
min(hel.faces(:)) % = 1
max(hel.faces(:)) % = 121368 = numel(mask)
These values are linear indices of the mask vector. Here's a demo with a smaller amount of data to illustrate this point:
a = -3:6;
b = [4 7 7
5 6 3
6 3 1];
a(b)
ans =
0 3 3 % a(4) a(7) a(7)
1 2 -1 % a(5) a(6) a(3)
2 -1 -3 % a(6) a(3) a(1)
Yu Li 2019년 10월 2일
thank you all very much!
Bests,
Yu

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추가 답변 (1개)

Steven Lord 2019년 10월 2일
This is linear indexing into a logical array.
A = [true false]
B = randi(2, 5, 5)
C = A(B)
The elements of C that are true (A(1)) correspond to elements in B that are 1.
The elements of C that are false (A(2)) correspond to elements in B that are 2.
Perhaps a different example, one that has a few more unique values from which to select, would illustrate the technique more clearly.
D = ["Ace of spades"; "Queen of hearts"; "7 of diamonds"; "Jack of clubs"]
selection = randi(numel(D), [4 4])
selectedCards = D(selection)
Element 11 of selectedCards is the element in D whose index is stored in element 11 of selection.
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Yu Li 2019년 10월 2일
Hi Steven:
thank you all the same!
Bests,
Yu

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