# Beginner question about for loops and indexing

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Callum Taylor on 20 Nov 2018
Commented: Callum Taylor on 21 Nov 2018
I want to perform a function on each number in a series 'a' depending on whether it's odd or even. Then output this series of numbers 'b' as an array.
I don't know what to do next. So far my script doesn't output anything.
a = 1:5:150; % array before function applied
b = zeros(1,30); % used to store numbers from a after functions
counter = 0;
for series = [1:30]
r = rem(a,2); % r = 0 if number is even, r = 1 if number is odd
counter = counter + 1;
if r == 0
b = a(counter)*4 % if number in b is even, multiply it by 4
elseif r == 1
b = a(counter)^3 % if number in a is odd, cube it
end
end

Brian Hart on 20 Nov 2018
Hi Callum,
You're really close.
First, you can move the "r = ... " line outside the loop to do the operation on the whole array at once.
Then inside the loop, you need to index the other variables the same way you did with "a(counter)".
So it looks like this:
a = 1:5:150; % array before function applied
b = zeros(1,30); % used to store numbers from a after functions
counter = 0;
r = rem(a,2); % r = 0 if number is even, r = 1 if number is odd
for series = [1:30]
counter = counter + 1;
if r(counter) == 0
b(counter) = a(counter)*4; % if number in b is even, multiply it by 4
elseif r(counter) == 1
b(counter) = a(counter)^3; % if number in a is odd, cube it
end
end
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Callum Taylor on 21 Nov 2018
Thank you so much! It works just fine now. I didn't know you had to index every variable, including 'r'.
I definitely need more practice, thanks for the help!