Error Handling in Programs

Keystroke: [Ctrl] ' Do not type the words "on error." They do not produce the operator.

x  on error y

Evaluates y. If y produces an error, evaluates and returns x. Otherwise returns the results of y. The on error operator is a distinct type of conditional operator, guiding execution only in the event of an error during calculation.

Operands:

• x, y are any valid Mathcad expressions or sequence of expressions. For example, a local assignment, a function call, or a sequence of programming steps are allowed here. Use the Add Line operator to insert placeholders for additional statements.

Example:

Here, on error is used to avoid dividing by zero; when x is 2, the program evaluates as ∞, rather than returning a singularity.

The left placeholder should contain code that always evaluates properly. You can use on error to return a variable for debugging purposes, perform an alternate calculation, or exit the program and return a custom error message.

Notes:

• You cannot use on error to trap problems with type (function versus scalar) or units (mass versus unitless). Mathcad interprets an expression in a function such as

  x←x*kg on error x + kg

  to imply that all inputs x to the function must have quantity mass, and using a unitless value for x results in an error before the function is ever executed.

• You cannot return values of different types on either side of an on error statement. Mathcad returns an error when interpreting an expression such as

  g(f, x) := (f + x) on error f(x)

User-defined error messages

You can also return custom errors using if statements and the error function. This function allows you to return a Mathcad-style error tooltip from a program and halts the program before any additional steps are performed.