Why .+ arithmetic operations fails on vector of same size

matlab

Solution

The problem is that the operator `.+` does not exist:

>> help ops
  Operators and special characters.

  Arithmetic operators.
    plus       - Plus                               +    
    uplus      - Unary plus                         +    
    minus      - Minus                              -    
    uminus     - Unary minus                        -    
    mtimes     - Matrix multiply                    *    
    times      - Array multiply                    .*    
    mpower     - Matrix power                       ^    
    power      - Array power                       .^    
    ...

Note that for multiplications, there are two operators: `.*`, which is element-wise multiplication, and `*`, which is matrix multiplication. There is no such thing as matrix addition, so there is only one operator `+`, which is element-wise addition.

When you type `p .+ p1`, the Matlab parser does not recognize a valid operator, so it probably assumes you are using the command syntax and tries to make the function call with string literals `p('.+', 'p1')`. Since `p` is not a function, you get the error message you see.

This 'command syntax' is convenient in that it can save you typing a few characters (i.e. `load data.mat` instead of `load('data.mat')`. The problem is that this leads to ambiguity in interpreting statements, see the page that was linked directly by your error message. This can give surprising results, as your question shows. This is one of the shady sides of Matlab's syntax.

Problem

``` >> p=[1;2;3] p = 1 2 3 >> p1 = [2;3;4] p1 = 2 3 4 >> p + p1 ans = 3 5 7 ``` But ``` >> p .+ p1 Error: "p" was previously used as a variable, conflicting with its use here as the name of a function or command. See "How MATLAB Recognizes Command Syntax" in the MATLAB documentation for details. ``` Also ``` >> p .* p1 ans = 2 6 12 >> p * p1 Error using * Inner matrix dimensions must agree. ```

Original source