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How can I use loops to perform different functions on every other array element?

Asked by Joe Williams on 20 Apr 2019
Latest activity Commented on by Image Analyst
on 21 Apr 2019
Hello. If I want to create a column vector where I specify the value of the first element and then use loops to automatically generate values for the rest based on two different functions that are carried out alternatively until the vector is a specific size, how would I do this?
For instance, my input could be 100, and the code would generate a vector:
100
60
70
42
52
31.2
41.2
24.72
etc. for a given number of times where the two different functions are:
n = (n-1)*0.6
for all even values of n, and:
n = (n-1)+10
for all odd values of n apart from 1.
I'm sorry that I don't have any code to show but I am just starting with matlab and I honestly don't know where to start with this even after extensive googling.
Thank you in advance for helping :)

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R2019a

1 Answer

Answer by Image Analyst
on 20 Apr 2019

Try this:
v = [100;60;70;42;52;31.2;41.2;24.72]
nOut = zeros(length(v), 1); % Instantiate column vector for output n.
for k = 1 : length(v)
if k == 1 || rem(k, 2) == 0
% It's even so use the even formula
nOut(k) = (k-1)*0.6; % For 1, 2, 4, 6, 8, ...
fprintf('For k = %d, we are using the even formula and the output = %f\n', k, nOut(k));
else
% It's odd so use the odd formula
nOut(k) = (k-1)+10; % For 3, 5, 7, ...
fprintf('For k = %d, we are using the odd formula and the output = %f\n', k, nOut(k));
end
end
% Show all values
nOut
You'll see in the command window:
For k = 1, we are using the even formula and the output = 0.000000
For k = 2, we are using the even formula and the output = 0.600000
For k = 3, we are using the odd formula and the output = 12.000000
For k = 4, we are using the even formula and the output = 1.800000
For k = 5, we are using the odd formula and the output = 14.000000
For k = 6, we are using the even formula and the output = 3.000000
For k = 7, we are using the odd formula and the output = 16.000000
For k = 8, we are using the even formula and the output = 4.200000
nOut =
0
0.6
12
1.8
14
3
16
4.2

  2 Comments

Thank you very much for your reply, It's really close but not quite what I'm looking to do. I'm not finding it very easy to explain but I would like to input a single number and the code generate a vector of a specified length with values each being calculated using alternative formulae from the value before.
My mistake in explaining is that when I say
n = (n-1)*0.6
I don't mean (the value of n) - 1, I mean the value of the element before the one being calculated. With an input of 100 I am trying to program matlab to do the following:
nOut(1) = 100 % Input value
nOut(2) = (100*0.6) = 60 % Even formula
nOut(3) = (60 + 10) = 70 % Odd formula
nOut(4) = (70*0.6) = 42 % Even formula
nOut(5) = (42 + 10) = 52 % Odd formula
% etc etc a given number of times.
And to save each of those values as elements in a vector of given size.

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