Why doesn't this command work (vector addition)?

 for x=1:0.5:10
    y(x) = x + 1
end

Why doesn't this above command work, but the one below does? What is it about the "0.5" that prevents this from running? X is still a vector in both of these commands, so what's wrong here?

 for x=1:10
    y(x) = x + 1
end

Why is it that I need a counting variable, as shown below, for the first command above to work?

x=1:0.5:10
  for i=1:length(x)
    y(i) = x( i ) + 1
 end

Answer 1

[Jan]

Did you read the error message?

   for x = 1:0.5:10
      y(x) = x + 1
   end

produces:

 Subscript indices must either be real positive integers or logicals.

This hits the point already. In the expression y(x) the x is the index related to the array y. It means: set the x.th element of the variable y to the value x + 1 . But if x is 1.5, Matlab is instructed to address the 1.5-th element and this is not possible for trivial reasons. There is only a 1st and a 2nd element, but no element between them.

Do you know Excel? A vector is like a column there. You have elements with the address "A1" and "A2", but no "A1.5".

The "counting variable" solves this problem easily: Now the indices are positive integers and not directly related to the used values.

By the way, an efficient solution does not need a loop here:

x = 1:0.5:10;
y = x + 1;

[Rachel Dawn]

I just want to make sure I understand this response. So, what you're saying is that x= 1, 1.5, 2, 2.5, etc. right? And y is a vector, that copies each x entry but adds a 1 to each. However, by writing y(x)=x+1, we are trying to say that each x VALUE is an entry/grid position. And we can't have a grid position such as "1.5". So, by making i=1:length(x), this assigns each entry in x a positive integer. Then we say y(i)=x(i)+1, which means that at i=1 in x (the vector's first entry/grid position), 1 is added, and this becomes the y vector's first entry/grid position. And so on. is this correct? Sorry. I'm new to programming and just trying to make sense of all these errors/instructions

[Stephen23]

"is this correct? "

Yes, your understanding is correct.

Note that in MATLAB the "entry/grid position" of any vector (or matrix) is called an element, so the loop index i specifies the first element, the second element, etc. The term element applies to matrices, vectors, and arrays, so use it when referring to an "entry/grid position" and we will always understand each other :)

You might like to read this too:

https://www.mathworks.com/matlabcentral/answers/228557-experts-of-matlab-how-did-you-learn-any-advice-for-beginner-intermediate-users

Answer 2

[Star Strider]

You are defining ‘y’ as a vector. In MATLAB, subscripts are defined as integers greater than zero.

So this will not work because the subscripts are not integers:

for x=1:0.5:10
    y(x) = x + 1
end

while this one will because they are:

for x=1:10
    y(x) = x + 1
end

See Matrix Indexing (link) for a comprehensive discussion.