How can I display a value of a vector for a specific value?

I have two vectors:

p = [5 0 6 7 0];
w = [6 2 5 6 7];

How can I display in matlab that for every 0 in vector p, the program shows me the two elements in w, namely w1 = 2 and w2 = 7?

Answer 1

[Adam Danz]

p = [5 0 6 7 0];
w = [6 2 5 6 7];

% Show the values in w that correspond to p==0
w(p==0) 
ans =
     2     7

[ntclrz]

Thank you!

My vectors are anyhow larger than my example, they are 1x5000 double.

I tried w(p==0) but I get:

1×0 empty double row vector

[madhan ravi]

Attach your data as .mat file.

[ntclrz]

Attached the data :)

[Stephen23]

@ntclrz: none of your p data are equal to zero:

>> any(p==0)
ans = 0
>> [~,idx] = min(abs(p))
idx =  123
>> p(idx) % the closest value to zero
ans =  0.00056900

[ntclrz]

Thank you for the answer!

I'm realizing now the problem:

If I plot w (x-axis) and p (y-axis) I get this graph:

I know there are three values where p is approximately equal to zero (roots of the function). How can I determinate this roots when no p-value is equal to zero?

[Adam Danz]

That gets a little tricky. One way is to define a threshold such that any value that is within the 'threshold' distance from 0 is considered to be 0. Here's an example.

p = [5 0.01 6 7 -0.03];
w = [6 2 5 6 7];
thresh = 0.1; 

%index of all values that are within 'thresh' distance to 0
isNearZero = abs(p) <= thresh; 
w(isNearZero)
ans =
     2     7

Another method is to use ismembertol(); play around with the 3rd input to suit your needs.

p = [5 0.001 6 7 -0.03];
w = [6 2 5 6 7];

% index of all values near 0
isNearZero = ismembertol(p, 0, .1); 
w(isNearZero)
ans =
     2     7

In either of these methods, if your data are finely sampled (which appears to be the case) expect that many values near 0 will be considered to be zero - not just the ones prior to crossing zero. There are more complicated ways around that if that turns out to be a problem.

Answer 2

[Bob Thompson]

This can be done with logic indexing.

w2 = w(p==0);