Finding minima using for loop and if construct

I have a 205x1 matrix of data points called zpoint. I need to write a script using a 'for' loop and an 'if' construct to identify all minima in zpoint.

I want the script to utilize the fact that the previous and next points around the minima points will be greater than it. (possibly using localmin)

This is the start of my script:

n = length(zpoint)
for i = 1:n
    if zpoint(i) .... 
end

[Rik]

You will need to use a comparator (so < and >). You also need to consider what will happen with the first and last points. You need to treat them differently (if you want to consider them as minima at all).

You forgot to tag your question as homework, so I'll do that for you.

[Elijah L]

This is the route I'm trying to take... Obviously it doesn't work but is there an easier way to write this script?

zmn = zpoint
for i=1:length(zpoint)
if zpoint(i) < zpoint(i+1) && zpoint(i-1)
end

[Rik]

It might make intuitive sense that you would write it like that, but you're missing several important points:

• The order of operations: Matlab is first checking if zpoint(i) is smaller than zpoint(i+1). That will return a logical. Next Matlab will evaluate true && zpoint(i-1), which not what you mean. You need to split this into two comparisons.
• If that statement is true, then what? What should happen? Also, an if in Matlab is a code block: it requires an end statement, just like the for. So your code is missing an end statement. It is even difficult to write what you did in the editor here, because it will automatically add the closing end for you.

Regarding the easier way: there is a way to factor out the loop by using the diff function and looking at the sign of the resulting values.

[Star Strider]

Note that one of the two duplicate postings (three total) of this has an Accepted Answer: Finding minima using if and for loops

Answer 1

[Image Analyst]

You also need to start at 2 and end at the end-1

for i = 2 : length(zpoint)-1
    if zpoint(i) < zpoint(i+1) && zpoint(i) < zpoint(i-1)
    end
end

And your zmn is not really used for anything so you can get rid of it.

Also, you might want to take a look at movmin(), and imregionalmin() which do the same thing.