Conditional statements are how you make a program behave differently based on comparisons. First of all, know that an equation can return true or flase. (ex. 5=5 returns true, 3=5+-3 returns false, sin(x)^2+cos(x)^2=1 returns true even if x is undefined).

A. Booleans

Booleans are the values true and false. We have a couple of commands to combine these: bool AND bool, bool OR bool, bool XOR bool, and NOT bool. NOT inverts the boolean. The following explains the others.

Condition	Result	Condition	Result	Condition	Result
true and true	true	true or true	true	true xor true	false
true and false	false	true or false	true	true xor false	true
false and true	false	false or true	true	true xor true	true
false and false	false	false or false	false	false xor false	false

B. The IF statement

Suppose we wanted our program to do different things depending on a boolean. We can use IF bool to compare. If the boolean is true, the IF statement will execute the next line of code. Otherwise, the next line is skipped. Now suppose we wanted it to execute a few lines. Well, we add a THEN statement after the IF and add an ENDIF after the block we wanted executed. Ex:


Now, what if we wanted it to tell us if out answer was wrong. Well, we could put it in another IF statement:



But that's bulky. Forunately, there's an easier way. The ELSE statement runs if the boolean after the IF is false:



It's a little shorter, and easier. We can also use WHILE bool to loop until is true. See the GETKEY() section for an example. Up to here, everything is similar to other programming languages. TI-BASIC has another nice tid-bit: TRY statements.

C. TRY Statements

Now, in some cases, our user might enter something that's invalid. Now, we could use IF statements to check the validity of what they enter, but that's time consuming. We can use a TRY statement with what we were going to do anyway. Then, if an error occurs, the program will jump to the ELSE of the TRY. Ex:



I'll just tell you that setfold(#F) will change the present folder to the folder with the name that is the value of F. You'll find out why in section IX. So this program tries to set the folder to what you enter, buit if no such folder exists, errors and tells you it can't. If this TRY statement weren't being used, the program would error and quit. When the error occurs the system variable errornum is, so you can customize your error messages using IF statements to check is errornum equal to whatever type of error you are checking for. This is usually more trouble than it's worth, however. TIP: Use TRY statements when linking between calculators to avoid a program crash if the link comes out.