Loren on the Art of MATLAB

Turn ideas into MATLAB


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Indexed Assignment

Recently some folks at The MathWorks wondered about the difference of behavior of variable creation versus assigning values into an existing array. I thought I'd address this topic live from The MathWorks Aerospace and Defense Conference '07.


Here's a picture of the audience.

The Question

Why doesn’t changing an element of a logical array change its logicalness? Most arithmetic operations remove the logicalness from an array; it would seem to me that changing an element of the array (at least to something other than 0 or 1) should either throw an error or similarly modify its logicalness.

An Example

Let's show an example. First create a logical array.

clear all
A = magic(3);
B = A > 6;
  Name      Size            Bytes  Class      Attributes

  A         3x3                72  double               
  B         3x3                 9  logical              

Now let's do some arithmetic with logical arrays mixed with doubles.

C = B + 1;
D = B;
D(1:5) = 2
whos A B C D
D =
     1     1     0
     1     1     1
     1     1     0
  Name      Size            Bytes  Class      Attributes

  A         3x3                72  double               
  B         3x3                 9  logical              
  C         3x3                72  double               
  D         3x3                 9  logical              

What you see is that A and C are double arrays, while B and D are logical.

The Answer

Indexed assignment, that is, D(1:5) = 2 has always preserved the type of the array being indexed. Only values may change, not the type. You'd only get an error if there was not a valid conversion routine from the type on the right-hand side to the type stored on the left.

If you simply concatenate values, you also find that the output is logical:

E = [D ; B]
E =
     1     1     0
     1     1     1
     1     1     0
     1     0     0
     0     0     1
     0     1     0

To see this with another datatype, let's look at string array.

firstname = 'Loren'
lastname = [83   104   117   114   101]
name = firstname
name(end+2:end+1+length(lastname)) = lastname
firstname =
lastname =
    83   104   117   114   101
name =
name =
Loren Shure

If I just use straight concatenation, here's what happens

newname = [firstname, ' ', lastname]
newname =
Loren Shure


If I perform arithmetic between mixed types, where one is numeric and one is not, I instead get a numeric answer.

name - 32
ans =
  Columns 1 through 8
    44    79    82    69    78   -32    51    72
  Columns 9 through 11
    85    82    69


The fact that you get results with different datatypes depends on whether you are doing a numerical calculation or assigning to an array. Has the behavior been what you need or have you found it confusing? Let me know here.

Published with MATLAB® 7.4

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