How to use an if statement to substitute NaN

I am trying to use a combination of if and for statements to find and replace certain cells in my matrix with NaN.

My goal for this code is to develop an algorithm that looks through my final vector (in this case p4) and replaced all values in p4 with Nan when the percent difference between p1 and p2 (calculated in percdif) is greater than 10%. However, I want it to skip every cell where p4 is less than 20.

Here is my current code

%create example data
p1=[11 15 7 40 50 100];
p2=[8 14 6 38 35 95];
%Combine and take row mean
p3=[p1' p2'];
p4=mean(p3,2);
%Determine percent difference of each row
percdif=nan(size(p3,1),1);
for i=1:size(p3,1);
     prep(i,1)=(p3(i,1)-p3(i,2))/p3(i,1);
end
%Determine which percent differences are greater than 10%
badpercdif=abs(percdif)>0.1;
%for all data where the p4 value is greater than 20, 
%if the corresponding percdiff is greater than .1, then replace that value with NaN

This was my attempt:

cp1=p4;
for i = 1:size(cp1,1)
    if cp1(i)>20
         cp1(badpercdif)=NaN;
    else
        continue
    end
end

this gives the result:

cp1=[NaN 14.5 6.5 39.0 42.5 97.5]

My intended result was

cp1=[9.5 14.5 6.5 39 NaN 97.5]

What do I need to change in my for and if loops to be able to get this result?

Answer 1

[KALYAN ACHARJYA]

p1=[11 15 7 40 50 100];
p2=[8 14 6 38 35 95];
%Combine and take row mean
p3=[p1' p2'];
p4=mean(p3,2);
%Determine percent difference of each row
percdif=nan(size(p3,1),1);
for i=1:size(p3,1);
     prep(i,1)=(p3(i,1)-p3(i,2))/p3(i,1);
end
%Determine which percent differences are greater than 10%
badpercdif=abs(percdif)>0.1;
cp1=p4;
cp1(round(cp1)==43)=NaN

Please note: Here it is forcefully manipulated and here loop can be avoided.

[Jonathan Gingrich]

Sorry, I wasn't clear on my goal.

My goal for this code is to develop an algorithm that looks through my final vector (in this case p4) and replaced all values in p4 with Nan when the percent difference between p1 and p2 (calculated in percdif) is greater than 10%. However, I want it to skip every cell where p4 is less than 20.

[KALYAN ACHARJYA]

cp1(badpercdif & p4>20)=NaN