how to use certain values to write a for loop?

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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
  2 Comments
Siddharth Parmar
Siddharth Parmar on 25 Feb 2016
I understand that it looks simple because I have modified my code and asked a simple question. Imagine if you have the array with 1000 rows in X and f. Then the loop simplifies it.

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Accepted Answer

Arnab Sen
Arnab Sen on 25 Feb 2016
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:
  2 Comments
Siddharth Parmar
Siddharth Parmar on 25 Feb 2016
Thanks guys this is exactly what I was looking for.

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More Answers (1)

Walter Roberson
Walter Roberson on 25 Feb 2016
Edited: Stephen on 25 Feb 2016
ismember(i, f)
Or, you can use
X1 = X;
X1(f) = 5 * X(f);
with no loop.

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