# Finding difference of array using alternative indexes

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shane watson on 4 Jun 2021
Commented: Adam Danz on 9 Jun 2021
Hello everyone,
I hope you're doing, I've simple question I've array (1, 24), now I want to findout the difference and divide, like [element1-element2 element2- element3 element3-element4...............element24-element23], what I did as follows
for i=1:24
a=1x24
%first case
a1(i)=a(i)-a(i+1)./i-(i+1),
% Second case
a1(i)=a(i)-a(i-1)./i-(i-1)
end
However, it is clear the index causing error (first case: Index exceeds the number of array elements (24). second case: Array indices must be positive integers or logical values.)
Adam Danz on 7 Jun 2021
I'm trying to show you that the loss of one value when numerically differentiating is not a problem - it's exactly the expected behavior. Carefully look at my previous comment again to understand why you're losing a value.

Adam Danz on 7 Jun 2021
Edited: Adam Danz on 7 Jun 2021
The loss of 1 value when differentiating with diff(x,1) is the expected behavior. This function computes the difference between adjacent values in a matrix [a b c d] and there is n-1 comparisons.
Perhaps you're looking for the numeric gradient.
y = exp([1:.1:3]);
d = diff(y);
size(y)
ans = 1×2
1 21
size(d)
ans = 1×2
1 20
size(g)
ans = 1×2
1 21
x = 1:numel(y)
x = 1×21
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
hold on
plot(x(2:end),d,'b-','DisplayName','diff')
legend ##### 2 CommentsShowHide 1 older comment
Adam Danz on 9 Jun 2021
I need to know more about your goal. In your original question, the indexing error was caused by the loss of 1 value due to differentiating using diff(). Maybe you don't need to differentiate. Maybe you need to differentiate using a different method. Or maybe what you're doing is fine and you need to understand the output.

KSSV on 4 Jun 2021
Edited: KSSV on 4 Jun 2021
n = length(a) ;
iwant = (a(2:n)-a(1:n-1))./((2:n)-(1:n-1)) ;
Also have a look on geadient.
shane watson on 4 Jun 2021
@KSSVIt's not showing the right results unfortunately. :(