Trying to find all the average of negative numbers under certain conditions

Question

Alexandra Brian 2017-2-4

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https://ww2.mathworks.cn/matlabcentral/answers/323300-trying-to-find-all-the-average-of-negative-numbers-under-certain-conditions

编辑： per isakson 2017-2-5

采纳的回答： per isakson

xy.zip

在 MATLAB Online 中打开

Hi,

I'm trying to find the average of numbers less than 0 in one column that correspond with positive numbers in the rows of another column. I have the following code written. The avg for positives works, but the negatives return a positive number. There is obviously something going on. Could you please help? I attached the spreadsheet I used to this post.

clear all 
    xy= xlsread('xy_2.3.17.xlsx'); %Download spreadsheet to MATLAB
    x = xlsread('xy_2.3.17.xlsx','A:A'); %Choose row
    y = xlsread('xy_2.3.17.xlsx', 'B:B');%Choose row
    rows = xy(:,2)>0; %Logical vector with the rows which satisfy all conditions.
    if any(rows) % True if there is at least 1 row which meats the condition.
         avgx= mean(x(rows)); %Avg. when x > 0
     avgNeg = mean(y>0 & x<0); %Avg Negatives

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Answer 1

per isakson 2017-2-4

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https://ww2.mathworks.cn/matlabcentral/answers/323300-trying-to-find-all-the-average-of-negative-numbers-under-certain-conditions#answer_253290

编辑：per isakson 2017-2-4

在 MATLAB Online 中打开

Replace

avgNeg = mean(y>0 & x<0); %Avg Negatives

by

avgNeg = mean( x(y>0 & x<0) ); %Avg Negatives

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Alexandra Brian 2017-2-4

编辑：Alexandra Brian 2017-2-4

Thanks per isakson! Would you mind explaining why the second code works over the first? I have an idea, but I would appreciate clarification.

per isakson 2017-2-5

编辑：per isakson 2017-2-5

在 MATLAB Online 中打开

Background

Matlab has some different ways of indexing, see Matrix Indexing
Matlab has many different data types, see Fundamental MATLAB Classes.
Matlab converts between types when "appropriate", see Valid Combinations of Unlike Classes

This is "legal" with Matlab

>> 'a'+true+ones(true,true,'single')
ans =
    99
>>
>> mean([true,true,false])
ans =
    0.6667

With Matlab there is a tendency to save on keystrokes, write compact code and sacrifice on readability.

Intermediate Values says: "When calculating complex formulae, it is useful to break up the calculation into intermediate temporary variables."

Break up &nbsp mean( y>0 & x<0 )

is_y_gt_zero = y>0;                         % logical vector
is_x_lt_zero = x<0;                         % logical vector
is_avg_neg   = is_y_gt_zero & is_x_lt_zero; % logical vector
num_avg_neg  = double( is_avg_neg );        % double vector (type convertion)
avgNeg       = mean( num_avg_neg );         % double scalar

Break up &nbsp mean( x( y>0 & x<0 ) )

is_y_gt_zero = y>0;                         % logical vector
is_x_lt_zero = x<0;                         % logical vector
is_avg_neg   = is_y_gt_zero & is_x_lt_zero; % logical vector
num_avg_neg  = x( is_avg_neg );             % double vector (logical indexing)
avgNeg       = mean( num_avg_neg );         % double scalar

Execute these two and compare the values of the intermediate temporary variables.

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