counting the number of data in the csv file

Hello MATLAB community

temps is a bunch of numbers ranging from 20 to 70 in a column. So what I am struggling to do is that I want to sum the number of data points that are below 32 (just the number of data points not the actual data, which are numbers ranging from 20 to 70, summed up). Below is my code and I just don't know how to continue.

temps = load('40819DecJanLancasterTemp.csv');
plot(temps)
xlabel('Dec2018                 Jan2018                 Dec2019                 Jan2019')
ylabel('Temperature (F)')
title('Hourly Temperature')
for i = 1:length(temps)
    if temps(i) <= 32
        
    elseif temps(i) > 32
        
    end
end

Answer 1

[Image Analyst]

Try this

indexesBelow32 = temps < 32
countBelow32 = sum(indexesBelow32)

You could of course do it all in one shot:

countBelow32 = sum(temps < 32)

No for loop needed to do the count.

[Ireedui Ganzorig]

Thank you very much Image Anaylyst. I literally spent solid 4 hours trying to figure it out. I knew I had to use the built-in sum command but just didn't think the inequality signs go inside the parenthesis. Again Thank you.

[Image Analyst]

Yes, the trick is to understand the difference between logical indexes and linear indexes.

Logical indexes are either true or false (1 or 0) if the condition is met. So we can just sum those to get a count.

Linear indexes are the actual indexes, like 1,2,3,etc. So like if we had

temps = [1,99, 2, 88, 3, 77] 
logicalIndexes = temps < 32

then logicalIndexes would be [1, 0, 1, 0, 1, 0]. If we sum that we get 3.

Now the linear indexes

linearIndexes = find(logicalIndexes)    % Or, equivalently find(temps < 32)

would be [1, 3, 5], which is a list of only where the logical indexes are "true" (meaning they're 1).

To get the actual numbers from the array, rather than their indexes/locations, you can use either the logical or linear indexes.

smallNumbers = temps(logicalIndexes)
smallNumbers = temps(linearIndexes)

smallNumbers would be [1,2,3] no matter which line of code above that you used. This is one of the wonderful things about a vectorized language like MATLAB.