Why doesn't my code return the correct value when I try to find the average of the values greater than 0?

Hi,

I'm trying to find the average of the values that are greater than 0 and the average of values less than 0. I wrote the following code:

sample = xlsread('sample.xlsx', 1); %Download spreadsheet to MATLAB
  avgGreaterThan0 = mean(sample(:)>0); %finds the average of values > 0 
  avgLessThan0= mean(sample(:)<0); %finds the average of values < 0

The outputs are, 0.232876712328767 (avg. when values > 0) and 0.767123287671233 (avg. when values < 0). However I preformed the task manually to find it should be 0.132517148 (avg. when values > 0) and -0.043091451 (avg. when values < 0.)

Could you please help me with my code?

Best, A

Answer 1

[Jan]

sample(:)>0 replies a logical array. Then mean(sample(:)>0) calculates the average number of the elements greater than 0, not the average of the values itself. For this use:

avgGreaterThan0 = mean(sample(sample(:) > 0));
avgLessThan0    = mean(sample(sample(:) < 0));

[Alexandra Brian]

I received the following error:

"Subscript indices must either be real positive integers or logicals."

[Image Analyst]

Then you didn't use Jan's code. Here's proof Jan's code works:

sample = randi(100, 1, 1000) - 50
avgGreaterThan0 = mean(sample(sample(:) > 0))
avgLessThan0    = mean(sample(sample(:) < 0))

See results:

avgGreaterThan0 =
       25.729
avgLessThan0 =
      -24.485

[Alexandra Brian]

You're right Image Analyst. Thanks guys! I had an error with my parenthesis.