Problem with mean function

Okey, so I have the two nx2 vectors "day" and "comp_r".

I want to calculate the mean for a certain day, say day 8.

condition = day(:,1) == 8;
avg_day_8 = mean( comp_r( condition, 1) );

The problem is that I get that the mean equals NaN, and I am sure that's a wrong answer.

So what am I doing wrong?

Answer 1

[James Tursa]

What does this show:

sum(condition)

If sum(condition) is 0, then you will get a NaN result in the mean calculation. E.g.,

>> x = reshape(1:6,3,2)
x =
     1     4
     2     5
     3     6
>> condition = x(:,1) < 1
condition =
     0
     0
     0
>> mean(x(condition,1))
ans =
   NaN

[Hampus]

Yeah, but the "funny thing" is that i get an answer > 0, so there seems to be something wrong. Maybe I should re-install Matlab?

[James Tursa]

What does this show:

any(isnan(comp_r( condition, 1)))