How do you replace vector values?

Consider the following vector:

 A=[6 8 12 -9 0 5 4 -3 7 -1]

Write a program using a for loop to produce a new vector, B which is related to A in the following way: All values of A which are not less than 1 should be replaced with the natural logarithm of that number, all numbers that are less than 1 should be replaced with the original number plus 20. Output the new vector.

[Thomas]

Sounds like a homework question.. What have you done so far?

[Madeline]

This is what I have so far:

A = [6 8 12 -9 0 5 4 -3 7 -1];
k=1;
p=20;
for A
    true(A>1)
    Bvector=k.*log(A);
    for A
    false(A>1)
    Bvector=p+A;
    end
end

I know it's incorrect but one of the people in my group has the correct code in a while loop.

A=[6,8,12,-9,0,5,4,-3,7,-1];
n=1;
while n<=length(A)
    if A(n)<1
        A(n)=A(n)+20;
    else
        A(n)=log(A(n));
    end
    n=n+1;
end
disp(A)

So now I can't figure out how to write the code using a for loop.

Answer 1

[Matt Tearle]

What you have inside the while loop is basically what you need:

    if A(n)<1
        A(n)=A(n)+20;
    else
        A(n)=log(A(n));
    end

You just need to convert the while structure to for. The strange thing is that for is much simpler. You want to loop over n from 1 to length(A). I'm not sure I can say much more than that without just doing it for you.

However, one other thing: the assignment says to produce a new vector B.

And while I'm here... I hate these kinds of assignments. I really hope that your instructor makes it absolutely clear that you should not use a loop for this kind of operation. Here it is done properly:

A = [6,8,12,-9,0,5,4,-3,7,-1];
B = A;
idx = (B<1);
B(idx) = B(idx) + 20;
B(~idx) = log(B(~idx));