Finding the mean of all negative elements in the first column of a matrix?

Hello my friends! Happy New Year!!

This is what got so far:

X=[-4  8 -2 -6  9;
    1  2  7  3 -5;
   -9 -5 -3  4  5]
if(X<0)]
    M=Mean(X)
end;

I'm new to Matlab, so sorry if it's easy to answer. Many thanks in advance

Answer 1

[Star Strider]

This works:

Xm1 = mean(X(X(:,1)<0,1));

[steve]

thanks for the answer!

what do the (X(X(:,1)<0,1) do?

[Star Strider]

My pleasure!

I’ll break it down into its component parts:

1. ‘X(:,1)<0’ creates a logical vector that is 1 where the first column of ‘X’ is less than zero,
2. then ‘X(X(:,1)<0,1)’ uses that logical vector to ‘see’ only those rows in the first column of ‘X’,
3. lastly, the mean function takes the mean of those values.

In this instance,

Xm1 = mean(X(X(:,1)<0,1))

evaluates to:

Xm1 =

      -6.5

(You likely can’t Accept my Answer, but it would be nice if you Voted for it, especially if it helped you!)

[Micheal]

Hello my comrads! I accept answer, could not find button until now. Thank you Star Strider for your help!!

[Star Strider]

My pleasure!

Thank your for your extra effort in order to Accept my Answer!