Programming and Data Types

Errors and Warnings

In many cases, it is desirable to take specific actions when different kinds of errors occur. For example, you may want to prompt the user for more input, display extended error or warning information, or repeat a calculation using default values. The error handling capabilities in MATLAB let your application check for particular error conditions and execute appropriate code depending on the situation.

This section covers the following topics:

• Checking for Errors with try-catch
• Handling and Recovering from an Error
• Warnings
• Message Identifiers
• Using Message Identifiers with lasterr
• Warning Control
• Debugging Errors and Warnings

Checking for Errors with try-catch

No matter how carefully you plan and test the programs you write, they may not always run as smoothly as expected when run under different conditions. It is always a good idea to include error checking in programs to ensure reliable operation under all conditions.

When you have statements in your code that could possibly generate unwanted results, put those statements into a try-catch block that will catch any errors and handle them appropriately. The example below shows a try-catch block within a sample function that multiplies two matrices:

• function matrix_multiply(A, B)
  try
     X = A * B
  catch
     disp '** Error multiplying A * B'
  end
  

A try-catch block is divided into two sections. The first begins with try and the second with catch. Terminate the block with end:

• All statements in the try segment are executed normally, just as if they were in the regular code flow. But if any of these operations result in an error, MATLAB skips the remaining statements in the try and jumps to the catch segment of the block.
• The catch segment handles the error. In this example, it displays a general error message. If there are different types of errors that can occur, you will want to identify which error has been caught and respond to that specific error. You can also try to recover from an error in the catch section.

When you execute the above example with inputs that are incompatible for matrix multiplication (e.g., the column dimension of A is not equal to the row dimension of B), MATLAB catches the error and displays the message generated in the catch section of the try-catch block.

• A = [1  2  3; 6  7  2; 0  1  5];
  B = [9  5  6; 0  4  9];
  
  matrix_multiply(A, B)
  ** Error multiplying A * B
  

Nested try-catch Blocks

You can also nest try-catch blocks, as shown here. You can use this to attempt to recover from an error caught in the first try section.

• try
     statement1                    % Try to execute statement1
  catch
     try
        statement2                 % Attempt to recover from error
     catch
        disp 'Operation failed'    % Handle the error
     end
  end
  

Empty Matrices

Handling and Recovering from an Error