Random '?'

The Random operator is a special, immediate operator that uses a variable's value as a seed to create a pseudo random number and assigns that number to the same variable. It can only be used in run-time variable expressions. Random has two forms, forward and reverse, depending on which side of the variable it appears on. The forward form appears to the left of a variable and the reverse form appears to the right of a variable.

Random generates pseudo-random numbers ranging from -2,147,483,648 to +2,147,483,647. It's called "pseudo-random" because the numbers appear random, but are really generated by a logic operation that uses a "seed" value as a tap into a sequence of over 4 billion essentially random numbers. If the same seed value is used again, the same sequence of numbers is generated. The Propeller chip's Random output is reversible; in fact, specifically it is a 32-bit maximum-length, four-tap LFSR (Linear Feedback Shift Register) with taps in both the LSB (Least Significant Bit, rightmost bit) and the MSB (Most Significant Bit, leftmost bit) allowing for bi-directional operation.

Think of the pseudo-random sequence it generates as simply a static list of over 4 billion numbers. Starting with a particular seed value and moving forward results in a list of a specific set of numbers. If, however, you took that last number generated and used it as the first seed value moving backward, you would end up with a list of the same numbers as before, but in the reverse order. This is handy in many applications.

Here's an example:

?X

The above shows the Random forward form; it uses X's current value to retrieve the next pseudo-random number in the forward direction and stores that number back in X. Executing ?X again results in yet a different number, again stored back into X.

X?

The above shows the Random reverse form; it uses X's current value to retrieve the next pseudo-random number in the reverse direction and stores that number back in X. Executing X? again results in yet a different number, again stored back into X.

Since Random is always an assignment operator, the rules of Intermediate Assignments apply to it (see Intermediate Assignments).