What does the expression x(ind -1) mean?

I recently saw the following code and I'm curious as to what it is actually doing:

x(ind -1)

where "ind" is a vector of indices for x.

I played around with x = 1:10, ind = 2:2:10:

x(ind -1) = 2 4 6 8
x(ind -2) = 2 4 6 8
x(ind -3) = 2 4 6
x(ind -4) = 2 4 6
x(ind +1) = 2 4 6 8 10 12

I clearly see a pattern, but I can't explain in plain words what the expression x(ind -1) is really "saying." Can someone please explain?

Thanks!

--R

[Guillaume]

I would recommend that you use different style for your expressions, such as

x(ind-1)

or

x(ind - 1)

This is important because [space minus number] usually means a negative number rather than a subtraction.

When I see your formatting, I wonder: "did the writer forget an operator between the ind and the negative number? Is there a bug?".

The less confusion you throw around, the easier the code is to debug and maintain.

[Ryan]

I completely agree! That is exactly what they were doing, and it is confusing. I had a dumb moment and for whatever reason thought (ind -1) was somehow different from (ind - 1) or (ind-1).

Thanks!

Answer 1

[the cyclist]

This article about indexing arrays explains how it works.

By the way, it is not correct that

x(ind -2) == [2 4 6 8]

for x = 1:10 and ind = 2:2:10.

In fact, x(ind-2) will give an error, because it will try to access the 0th element of the vector, which does not exist (because MATLAB has 1-based indexing).

Answer 2

[Stephen23]

You are getting confused because you are mixing indexing with vector generation and you are not looking at the intermediate results. Lets just look at the vector generation:

>> 2:2:10-1
ans =
   2   4   6   8
>> 2:2:10-2
ans =
   2   4   6   8
>> 2:2:10-3
ans =
   2   4   6
>> 2:2:10-4
ans =
   2   4   6
>> 2:2:10-5
ans =
   2   4

You can see that the vector is created after the subtraction, so the last example is equivalent to this:

>> 2:2:5
ans =
   2   4

You can read about the colon operator here:

http://www.mathworks.com/help/matlab/ref/colon.html

and operator precedence here:

http://www.mathworks.com/help/matlab/matlab_prog/operator-precedence.html

Note that the colon operator is listed with a lower priority than the subtraction operator.