Controlling Program Flow in Scripts

If...Then Conditions In the Listener

If you are entering these commands into the Listener window, you may notice that the commands are not executed line by line, as is typical of the Listener. This is because the Listener recognizes that the "if" statement requires a "then" statement to be complete. In fact, the Listener does not execute the statement after the "then" statement, because it does not know if you intend to include an "else" statement. For this reason, the Listener does not execute an "if...then..." statement until you have either entered a "then" statement or after the next line you type, following the "if...then..." statement.

To avoid the waiting for an "else" statement when typing in the Listener, you can use "if...do..." instead. You cannot use "else" with "if...do...", so the Listener will not wait and will execute the expression after "do" as long as the condition expression after the "if" is true.

Comparison Operators

The double equal sign in the last statement, "==" tells MAXScript to compare two values. The single "=" sign always signifies value assignment.

This is common in many standard programming languages, such as C++.

There are several different conditional operators used in MAXScript. They are listed here:

• == equal to

• != not equal to

• > greater than

• >= greater than or equal to

• < less than

• <= less than or equal to

For Loops

The for loop iterates through a range of numbers, time values, or a sequenced collection of values, such as an array or object set.

You can use a for loop to copy the box you created.

Let's expand the script we started in the beginning of this lesson:

resetMaxFile #noprompt --reset the scene
mybox = box length:10 width:10 height:10 wirecolor:blue --new box
for i = 1 to 5 do --repeat five times, for each iteration do:
(
box_copy = copy mybox --create a copy of the original box
box_copy.pos = [i*20, 0, 0] --place it i*20 units along x
box_copy.wirecolor = [i*25,i*50,(5-i)*50] --blue-green gradient
)--end of the for loop 

Everything inside of the parentheses makes up a "block". All loop statements must be created as a block. This statement executes five times. Each time, a new box copy is created, placed to a new position and its color is changed.

• A for statement starts with the word 'for'.

• Next, you must define how many times the statement will repeat. In this case, we have used the variable "i" to control the loop. The line, "for i = 1 to 5" tells MAXScript to set the variable "i" to the value 1, execute the contents of the loop body, then set the variable "i" to 2, execute the loop body, and so on.

• The variable "i" is called the index for the loop. Each time the loop executes, the index is incremented by 1 automatically, without requiring any user input. Each repetition of the loop is called an iteration.

• To make MAXScript increment the index by a value other than 1, insert 'by x' after the index statement, where x is the amount to increment the index by.

The following statement will set the index to 1, then 3, then 5,

resetMaxFile #noprompt --reset the scene
mybox = box length:10 width:10 height:10 wirecolor:blue --new box
for i = 1 to 5 by 2 do --repeat with step of 2:
(
box_copy = copy mybox --create a copy of the original box
box_copy.pos = [i*20, 0, 0] --place it i*20 units along x
box_copy.wirecolor = [i*25,i*50,(5-i)*50] --blue-green gradient
)--end of the for loop 

• The "do" keyword tells MAXScript to execute the following block or statement each time the loop repeats. Note that in some loops you might see the word "in" in place of the equal sign, "=". These both mean the same thing in for loops, and can be used interchangeably.

• Note how the index was used within the loop statement to set the position of each new box by multiplying by an offset value. This is a useful way to work with multiple object creation. The same approach was used for the color, changing the R, G and B components based on the index.

For loops can also be used to build arrays and/or access values within an array. In the following example, you use a for loop to build an array called 'arr':

Remember that because MAXScript is an expression-based language, you are able to use expressions in variable assignment, as seen above in the assignment of the for loop expression to the variable, arr.

Note the 'collect' statement in the for loop above. This tells MAXScript to collect the results of each iteration in an array, rather than just calculate them. In this case, you only asked MAXScript to collect the value of the index; however, you can collect iterations of any expression.

Finally, for loops can also perform iterations with the contents of an array. In the following example, you use a for loop to access the values of the array you created in the previous example:

The 'print' command is a MAXScript function that displays the specified variable in the Listener window. In this example you used 'in' instead of an equal sign; however, they are still both acceptable, even when accessing values within an array.

for i = 1 to arr.count do print arr[i]

While and Do Loops

While and Do loops are used to repeatedly execute an expression until a test expression evaluates as false. The while and do loop are simple variants of one another.

The syntax is:

do <expr> while <expr> -- do loop while <expr> do <expr> -- while loop

The following example will return the value of a variable, incrementing the variable by -1 with each iteration.

x=10
while x>0 do print (x-=1)

9
8
7
6
5
4
3
2
1
0
0

MAXScript automatically returns the final value of the loop to the Listener. In this example, MAXScript is also instructed to return the value at each iteration of the loop; therefore, the final value is returned twice.

Both loops repeat the do <expr> as long as the while <expr> evaluates to the value true. The do form evaluates the do <expr> at least at least once, before evaluating the text expression at the end of the loop. The while form evaluates the test expression at the start of the loop, and might not loop at all.

Both forms return as their value the result of the do <expr> from the last successful loop iteration. The while form returns the value undefined if the test expression immediately returns false.

x=10
do print (x-=1) while x>10

9
undefined

x=10
while x>10 do print (x-=1)

undefined

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Local and Global Variables