Finding a number in an array less than 2

Question

Kyle Donk 2020-1-13

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https://ww2.mathworks.cn/matlabcentral/answers/500106-finding-a-number-in-an-array-less-than-2

回答： Dominik Mattioli 2020-1-16

I'm trying to find a count of all the numbers in an array that are less than 2. I set up my code like this:

(y was already defined earlier)

%Determine the number of y-values less than the number 2.0.

%Display only the number of y-values less than the number 2.0.

count=0;

N=length(y);

for i=1:N

if y<2

count=[y<2]

sum(count)

end

disp('The number of values less than two is')

disp(sum(count))

end

Yet whenever I try to run this, Matlab always says:

"Array indices must be positive integers or logical values."

Error in BasicSorts1 (line 35)

disp(sum(count))

Why is Matlab telling me this?

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Answer 1

Dominik Mattioli 2020-1-16

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The error message is telling you that you are trying to index into a variable using an index that is not positive or a logical. It looks like you created a variable called 'sum' and are indexing into it using another variable called 'count', which is - for whatever reason - not a valid index.

As the other answered mentioned, your code is needlessly complicated, anyway. You can vectorize the operation of finding the number of y-values that are less than 2 like so:

clear sum % make sure you don't have a variable name overriding a function.
y = randn( 10000, 1 )
count = sum( y < 2 );
disp( ['The number of y-values < 2 is: ', num2str( count ) ] )

Or, if you need to use a for-loop, you need to index into the y-array and evaluate that element of y individually, not as a whole (that is what your code is doing):

count = 0;
for idx = 1:length( y )
    if y( idx ) < 2
        count = count + 1;
    end
end
disp( ['The number of y-values < 2 is: ', num2str( count ) ] )