# How to apply this formula into a matlab loop?

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Lu Da Silva on 25 Jan 2022
Hi, I have a table that looks like this:
% have: % wanted:
X Y Z
____________________________
A 1 2
B 2 2
C 3 -2
D 4 -2
A 5 2
B 6 2
C 7 -2
D 8 -2
A 9 2
B 10 2
C 11 -2
D 12 -2
I need to create a vector Z that only uses values when X=B or X=D and is solved as follows:
Z(1) = Y(4)-Y(2)
Z(2) = Y(4)-Y(2)
Z(3) = Y(4)-Y(6)
Z(4) = Y(4)-Y(6)
Z(5) = Y(8)-Y(6)
Z(6) = Y(8)-Y(6)
Z(7) = Y(8)-Y(10)
Z(8) = Y(8)-Y(10)
Z(9) = Y(12)-Y(10)
Z(10) = Y(12)-Y(10)
etc...
(note that the actual data has different numbers so it's not actually as easy as just creating a vector of 2s and -2s)
I'm struggling to implement this into matlab, e.g. in a loop... I hope someone can help me out!
Thanks

Esha Chakraborty on 28 Jan 2022
I understand that Z(i) = Y(b) - Y(a), where variables a and b are incremented by 4 after every 4 iterations. For this, you can use "rem" function inside "for" loop and iterate over the rows of the table. Then, embed the “if” statement inside to choose the values for subtraction.
Check the documentation for the "rem" here.
Use the below code for reference:
a = 0;
b = 2;
rows = 10;
for i = 1:rows %Can be modified as per requirements
if rem(i,4) == 3
b = b + 4;
end
if rem(i,4) == 1
a = a + 4;
end
Z(i) = Y(b) - Y(a);
disp(Z);
end
##### 2 CommentsShowHide 1 older comment
Esha Chakraborty on 1 Feb 2022
The code is only indexing for the values instead of using the values itself. Hence, it will work for any set of numbers other than what is provided. You can modify the initial variables and the increment as per requirements.