Why am I getting a red message when trying to run this code.

I ran this code earlier, I can not find out why it is giving me an error.

Answer 1

[Dyuman Joshi]

Based on the picture of the code you have attached, there seems to be a typo while defining x0.

%typo
%            v
x0 = randn(10:1);

The most obvious correction is using a comma -

x0 = randn(10,1);

Update the code and check again.

If you still encounter an error, copy and paste your code and the full error message (i.e. all of the red text).

[Dyuman Joshi]

Reproducing the error -

A = rand(11,10);
x0 = randn(10:1);
ATA = A'*A;

ATA*x0

Error using *
Incorrect dimensions for matrix multiplication. Check that the number of columns in the first matrix matches the number of rows in the second matrix. To operate on each element of the matrix individually, use TIMES (.*) for elementwise multiplication.

[Walter Roberson]

x0 = randn(10,1);

Answer 2

[Steven Lord]

In the future, please don't post a picture of your code, post your code itself (formatted using the first button of the Code section of the message editor's toolstrip.)

Your line 39 is incorrect. Compare what you wrote (with the semicolon removed, so you can see what it created):

x0 = randn(10:1)

x0 =

     []

with what you likely meant:

x0 = randn(10,1)

x0 = 10×1

0.0797 -0.1291 0.8241 0.5541 -0.4408 0.5157 0.5580 1.3633 -1.3299 0.3437

Using 10:1 attempts to create an array of that size, which results in you getting an empty array.

10:1

ans =

  1×0 empty double row vector

If you'd flipped the two sizes it would "work" but you'd be creating a much larger array.

x1 = rand(1:10); % 10-dimensional!
x2 = rand(1,10);
whos x0 x1 x2

  Name       Size               Bytes  Class     Attributes

  x0        10x1                   80  double              
  x1        10-D             29030400  double              
  x2         1x10                  80  double              

For generality, I recommend you also avoid hard-coding sizes like 10. If you modified your A you'd also have to modify everywhere your code assumes A has 10 columns. If instead you computed the sizes of x0 and other intermediate arrays using the size of A your code is more flexible.

A = rand(5, 6);
numrows = size(A, 1) % or could use height(A)

numrows = 5

numcols = size(A, 2) % or could use width(A)

numcols = 6