MATLAB: How to use certain values to write a for loop

MATLAB

X = [67;89;78;56;55;75;99];
f = [4;5];
for i = 1:length(X);
    if i == f;
        X1(i,1) = 5*X(i,1);
    else
        X1(i,1) = X(i,1);
    end
end
I know that the way i have written the if expression is wrong but can someone tell how do I write the if expression such that it uses the values of f.. basically if i equals the value in the f matrix or array the first formula is used else the second.
Thanks for help.
Sid

Best Answer

Hi Siddharth,
My understanding from your problem statement that you want to check whether the values of the variable 'i' is present in the vector 'f' in the if-statement. You can use in-built 'find' function for this purpose. 'find' function returns the indices of the vector which meet certain condition. If no match then it returns a empty vector. So you can write the if statement in the following way:
 >>if(~isempty(f==i))
So, the whole script as below:
if true
  % code
end
X = [67;89;78;56;55;75;99];
f = [4;5];
for i = 1:length(X);
%if i == f
if(~isempty(find(f==i)))
    X1(i,1) = 5*X(i,1);
else
    X1(i,1) = 1*X(i,1);
end
end
For more detail about 'find', refer to the following link:
http://www.mathworks.com/help/matlab/ref/find.html
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