Program Debugging and Error Handling

Checking Errors with try-catch Statements

Section 3.3 briefly introduced the try-catch statement, which can be used to catch unknown errors.

[Example 3-26] There is a function for calculating the product of two matrices. It will throw an error when it cannot correctly calculate the result.

function matrixMultiply(A, B)
try
    X = A * B
catch
    disp '** Error calculating A * B.'
end

In the try-catch statement above, the statement in the try block can execute normally, and the catch block throws an error message. The following statement first defines two matrices A and B, then calls the matrixMultiply function to calculate their product. Since A and B do not satisfy the conditions for matrix multiplication (i.e., the number of columns in A does not equal the number of rows in B), an error occurs.

A = [1  2  3; 6  7  2; 0  1  5];
B = [9  5  6; 0  4  9];
matrixMultiply(A, B)
** Error calculating A * B.

Error and Warning Messages

Using MATLAB's error and warning functions can report error and warning messages and delay the program's execution.

[Example 3-27]

Such as:

if n < 1
    error('n must be greater than or equal to 1.')
end

If n is less than 1, an error message "n must be greater than or equal to 1." is given.

Similarly, the warning function is used below to give a warning message.

warning('Input content must be a string')

The warning indicates that the input content must be a string.

Debugging with the "Editor" Toolbar

Using the button options in the MATLAB "Editor" toolbar, you can conveniently perform program debugging. Specific debugging methods can be broadly divided into three types: breakpoint debugging, block execution, and section-by-section debugging.

1. Breakpoint Debugging

During breakpoint debugging, the program stops running when it reaches a breakpoint, which helps quickly find the location of errors. This debugging method requires setting breakpoints first. To set a breakpoint, in the editor, left-click before the code where you want to set the breakpoint; a red dot will appear, which is the breakpoint; click again to cancel the breakpoint setting.

2. Block Execution

Select the content you want to execute in the editor, right-click the mouse, and click the "Evaluate Selection" or "Evaluate Section" command in the pop-up shortcut menu to execute the block.

3. Section-by-Section Debugging

Click the "Run and Advance" button in the "Editor" toolbar to execute the current section of the program and advance to the next section.