User guide

22 CHAPTER 2. THE BCPL LANGUAGE

Notice that these precedence values imply that

! f x means ! (f x)

! @ x means ! (@ x)

! v ! i ! j means ! ((v!i)!j)

@ v ! i ! j means @ ((v!i)!j)

x << 1+y >> 1 means (x<<(1+y))>>1)

~ x!y means ~ (x!y)

~ x=y means ~ (x=y)

NOT x=y means NOT (x=y)

b1-> x, b2 -> y,z means b1 -> x, (b2 -> y, z)

2.2.10 Manifest Constant Expressio ns

Manifest constant expressions can be evaluated at compile time. They may

only consist of manifest const ant names, numbers and character constants, TRUE,

FALSE, BITSPERBCPLWORD, ?, the operators REM, MOD, SLCT, *, /, +, -, ABS, the

relational operators, <<, >>, NOT, ~, &, |, EQV, NEQV, XOR, and conditional ex-

pressions. Manifest expressions are used in MANIFEST, GLOBAL and STATIC dec-

larations, the upper bound in vector declaration s and the step length in FOR

commands, an d as the l eft hand operand of OF.

2.3 Commands

The primary purpose of commands is for updating variables, for input/output

operations, and for controlling the ﬂow of control.

2.3.1 Assignments

A command of the form L:=E updates the lo ca ti o n speciﬁed by the expressi on

L with the value of expression E. The following are some examples:

cg_x := 1000

v!i := x+1

!ptr := mk3(op, a, b)

str%k := ch

%strp := ’A’

SLCT 8:10:1 OF p := 5

Syntactically, L must be either a variable name or an expression whose leading

operator is !, % or OF. If it is a name, it must have been declared as a static or

dynamic variable. If the name denotes a function or label, it is only updateable

if it has been declared to reside in the global vector. If L has leading opera t or !,

then the location it refers to is updated by the assignment. If the % operator is