how to use certain values to write a for loop?

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

[Stephen23]

There is absolutely no point in using a slow and ugly loop. Here is Walter Roberson's much better solution in full:

>> X = [67;89;78;56;55;75;99];
>> f = [4;5];
>> X1 = X;
>> X1(f) = 5 * X1(f)
X1 =
    67
    89
    78
   280
   275
    75
    99

[Siddharth Parmar]

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.

Answer 1

[Arnab Sen]

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

[Jos (10584)]

Use the single command any(..) rather than the three commands ~empty(find(..))

any(f==i)

[Siddharth Parmar]

Thanks guys this is exactly what I was looking for.

Answer 2

[Walter Roberson]

ismember(i, f)

Or, you can use

X1 = X;
X1(f) = 5 * X(f);

with no loop.