• Click “OK” to stop the calculations or “Cancel” to resume calculations. If you click “OK,” Mathcad displays a message on the message line to indicate that processing has been interrupted. The equation that was being processed when you pressed [Ese] is marked with an error message indicating that calculation has been

interrupted. To resume an interrupted calculation, first click in the equation having the error message, then choose Calculate from the Math menu. If you find yourself frequently interrupting calculations to avoid having to wait for Mathcad to recalculate as you edit your worksheet, you may wish to switch to manual mode. To do so, disable automatic mode by choosing Automatic Calculation from the

Math menu. This will remove the checkrnark from the menu. In manual mode, Mathcad recalculates only when you choose Calculate from the Math menu. The calculation mode-either manual or automatic-is a property saved in your Mathcad worksheet. If you often work in manual calculation mode, simply save your worksheets with Mathcad configured to work in manual mode; this way you do not have to disable automatic calculation each time you open your worksheets The calculation mode of Mathcad is also one of the properties saved in Mathcad template (MeT) files. As described in Chapter 4, “Worksheet Management,” you use Mathcad templates to save layout and formatting information to re-use in worksheets you later create. When you create a Mathcad template file, the template will be in manual mode if you previously removed the checkrnark from Automatic Calculation on the Math menu; otherwise it will be in automatic mode. You may also change the calculation mode of a template file from automatic to manual,

or vice versa. For example, to change the calculation mode of an existing Mathcad template file from automatic to manual, do the following:

• Open the Mathcad template file, as described in the section “Worksheets and

templates” in Chapter 4.

• Choose Automatic Calculation from the Math menu. This command puts Mathcad into manual mode. The checkrnark beside this menu item should now be gone.

• Save the template file. Thereafter, whenever you create a Mathcad worksheet based on this template, Mathcad will be in manual mode.

An erros.

•message is visible only when you click on the associated expression. Figure 7-12 shows how an error message looks when you click on an expression. You’ll always be able to get on-line help about the error message by clicking on it and pressing [Shift][Fl].

**Fixing errors**

Mathcad cannot process an expression containing an error. If the expression is a definition, the variable or function it is supposed to define will remain undefined. This can cause any expressions that reference that variable to be undefined as well. Mathcad indicates undefined variables and functions by displaying their names in red. In the example shown in Figure 7-12, an error in the definition of xl causes the variable to be undefined in three different places in the worksheet. Note that in an expression in which zero is either a pre- factor or numerator (for example o .x or 0/ x ), Mathcad computes the result as zero, without evaluating or checking for errors in the x expression your worksheet contains several expressions with errors, as shown in Figure 7-12, this is what to do:

• Determine which expression with an error is closest to the top of the worksheet.

This error is probably the cause of many of the other errors .

• If necessary, click on the error and press [Shift][Fl] for help.

• If you anticipate time-consuming calculations, switch to manual mode as described in “Controlling calculations” on page 144. This will allow you to make numerous changes without having to wait for Mathcad to recalculate. When you are ready to recalculate, choose Calculate from the Math menu. Once you have determined which expression caused the error, edit that expression to fix the error, or change the variable definitions that led to the error. When you click in the expression and begin editing, Mathcad removes the error message. When you click outside the equation (or in manual calculation mode, when you recalculate), Mathcad recomputes the expression. If you have fixed the error, Mathcad then recomputes the other expressions affected by the expression you changed. Note that when you see an error message attached to an expression, it doesn’t necessarily mean that you should edit that expression. More often than not, the error arises as a result of functions or variables defined farther up in the worksheet. Edit these other definitions to fix the error. For example, in Figure 7-12, all five errors are caused by a division by zero. To fix all five error messages at once, change the definition for a as shown in Figure 7-13.