Editing PILOT (section)

===Command letters===

The following commands are used in "core PILOT".  Lines beginning with "R:" indicate a remark (or a comment) explaining the code that follows.

; A: Accept [[Input/output|input]] into "accept [[Buffer (computer science)|buffer]]". Examples:

  R:Next line of input replaces current contents of accept buffer
  A:
  R:Next line of input replaces accept buffer, and [[string (computer science)|string]] [[variable (programming)|variable]] 'FREE'
  A:$FREE
  R:Next 3 lines of input assigned to string variables 'X', 'Y' and 'Z'
  A:$X,$Y,$Z
  R:Numeric input assigned to numeric variable "Q"
  A:#Q

; C: Compute and assign numeric value. Most PILOT implementations have only [[integer (computer science)|integer]] [[arithmetic]], and no [[Array data structure|arrays]].  Example:

  R:Assign [[arithmetic mean]] of #X and #Y to #AM
  C:#AM=(#X+#Y)/2

; D: Dimension an array, on some implementations.

; E: End (return from) [[subroutine]] or (if outside of a subroutine) abort program.  Always used without any operand.

; J: Jump to a label. Example:
<pre>
  J:*RESTART
</pre>

; M: Match the accept buffer against string variables or [[string literal]]s. Example:
<pre>
  R:Search accept buffer for "TRUTH", the value of MEXICO and "YOUTH", in that order
  M:TRUTH,$MEXICO,YOUTH
</pre>

The first match string (if any) that is a substring of the accept buffer is assigned to the special variable $MATCH.  The buffer [[character (computing)|character]]s left of the first match are assigned to $LEFT, and the characters on the right are assigned to $RIGHT.

The match flag is set to 'yes' or 'no', depending on whether a match is made.  Any statement that has a Y following the command letter is processed only if the match flag is set.  Statements with N are processed only if the flag is not set.

; N: Equivalent to TN:  (type if last match unsuccessful)

; R: The operand of R: is a [[comment (computer programming)|comment]], and therefore has no effect.

; T: 'Type' operand as output. Examples:
<pre>
  R:The next line prints a literal string
  T:Thank you for your support.
  R:The next line combines a literal string with a variable expression
  T:Thank you, $NAME.
</pre>

; U: Use (call) a subroutine.  A subroutine starts with a label and ends with E: Example:
<pre>
  R:Call subroutine starting at label *INITIALIZE
  U:*INITIALIZE
</pre>

; Y: Equivalent to TY: (type if last match successful)

; Parentheses: If there is a parenthesized expression in a statement, it is a [[conditional (programming)|conditional]] expression, and the statement is processed only if the test has a value of 'true'. Example:
<pre>
  R:Type message if x>y+z
  T(#X>#Y+#Z):Condition met
</pre>