Counting specific element in one column corresponding to unique elements in an another column

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Hari krishnan 2020-2-20

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编辑： Stephen23 2020-2-20

I have two vectors a and b. I want to count how many times the value '-1' appears in b corresponding to unique elements in a. Is there an efficient way to do this without loops?

a = [1,1,1,1,1,2,2,2,2,2,3,3,4,4,4,5,5,5,5,5,5]';
b = [1,1,-1,1,-1,-1,-1,-1,1,1,-1,1,1,-1,-1,1,1,-1,-1,1,1]';

desired output:

1--2
2--3
3--1
4--2
5--2

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Answer 1

Jakob B. Nielsen 2020-2-20

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You can use logic indexing. Example:

length(b(a==1 & b==-1));

You get those entries from b which fulfils the condition that the corresponding index position in a must equal 1, and also that the same index position in b must equal -1.

& is logic AND - both conditions must be true to get an output. If you need a logic OR, the matlab operator is |

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Jakob B. Nielsen 2020-2-20

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Other than manual work, you probably wont get out of a loop entirely. But it is a very simple one;

%where N is your highest integer of a that you want the comparison for;
N=5;
for i=1:N
    output(i)=length(b(a==i & b==-1));
end

Stephen23 2020-2-20

编辑：Stephen23 2020-2-20

"Other than manual work, you probably wont get out of a loop entirely"

Basic MATLAB functions like histc don't require a loop.

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Answer 2

Stephen23 2020-2-20

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>> a = [1;1;1;1;1;2;2;2;2;2;3;3;4;4;4;5;5;5;5;5;5];
>> b = [1;1;-1;1;-1;-1;-1;-1;1;1;-1;1;1;-1;-1;1;1;-1;-1;1;1];
>> u = unique(a);
>> n = histc(a(b==-1),u);
>> m = [u,n]
m =
     1     2
     2     3
     3     1
     4     2
     5     2