'Matrix dimensions must agree' error, when using the backslash operator to solve Ax = b,

Hi there!

I am getting a 'matrix dimensions must agree' error message when using the backslash operator \ to solve Ax = b. I went to check my matrix A and vector b, and see that A is 3x3 square, with full rank = 3. My vector b is 3x1, with rank = 1. Thus, the inner dimensions 'cancel', and the outputs are 3x1 vectors, as the vector x varies. So, my math doesn't seem to be wrong. So, out of curiosity, I go to the Command Window and just typed in A \ b, to see what the vector x looks like. As expected (I know that our equations of motion are going to be very, very long), I get back the vector x, but also this error message: 'Output truncated. Text exceeds maximum line length for Command Window display.'

Could it be that my 3x1 vector x has entries that are so 'long' that it disrupts the backslash operator \ ?

Or do you think my error is something else?

Thanks in advance,

Answer 1

[Ruchika Parag]

Hi @Noob, the error message "matrix dimensions must agree" typically indicates a mismatch in dimensions between matrices or vectors involved in an operation. However, since you've confirmed that matrix ( A ) is 3 x 3 and vector ( b ) is 3 x 1, the dimensions should indeed be compatible for solving the equation ( Ax = b ) using the backslash operator.

The error message you mentioned, "Output truncated. Text exceeds maximum line length for Command Window display," is not related to dimension mismatches but rather to the display of results in the Command Window. This indicates that the output is too lengthy to be displayed in a single line, which is common when dealing with very large numbers, symbolic expressions, or complex calculations.

Here are a few things you can check or try:

1. Check Variable Definitions: Ensure that both ( A ) and ( b ) are correctly defined as numeric arrays. Sometimes, mistakes in loading or defining these variables can lead to unexpected errors.
2. Use format long: If you suspect that the issue is related to how the output is displayed, you can use the format long command to get a more detailed view of the numbers without truncation. However, this won't change the content of the vector, just its display format.
3. Check for Symbolic Variables: If any of the entries in ( A ) or ( b ) are symbolic, the computation might result in symbolic expressions that are too lengthy. You can convert symbolic results to numeric using double() if appropriate.
4. Verify the Error Location: Double-check that the "matrix dimensions must agree" error isn't arising from another part of your code. Sometimes, these errors can be mistakenly attributed to the wrong line.
5. Simplify the Output: If the output is too complex, consider simplifying the expressions using symbolic math functions like 'simplify' before displaying them.

Hope this helps!