.AU
Brian Kernighan
Rob Pike
.AB
.I Hoc
is a simple programmable interpreter
for floating point expressions.
It has C-style control flow,
function definition and the usual
numerical built-in functions such as cosine and logarithm.
.AE
.NH
Expressions
.PP
.I Hoc
is an expression language,
much like C:
although there are several control-flow statements,
most statements such as assignments
are expressions whose value is disregarded.
For example, the assignment operator
= assigns the value of its right operand
to its left operand, and yields the value,
so multiple assignments work.
The expression grammar is:
.DS
.I
expr: number
| variable
| ( expr )
| expr binop expr
| unop expr
| function ( arguments )
.R
.DE
Numbers are floating point.
The input format is
that recognized by @scanf@(3):
.ix [scanf]
digits, decimal point, digits,
.ix [hoc] manual
.ix assignment expression
.ix multiple assignment
@e@ or @E@, signed exponent.
At least one digit or a decimal point
must be present;
the other components are optional.
.PP
Variable names are formed from a letter
followed by a string of letters and numbers,
@binop@ refers to binary operators such
as addition or logical comparison;
@unop@ refers to the two negation operators,
'!' (logical negation, 'not')
and '\-' (arithmetic negation, sign change).
Table 1 lists the operators.
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\fBTable 1:\fP Operators, in decreasing order of precedence
.sp .5
^ exponentiation (\s-1FORTRAN\s0 **), right associative
! \- (unary) logical and arithmetic negation
* / multiplication, division
+ \- addition, subtraction
> >= relational operators: greater, greater or equal,
< <= less, less or equal,
\&== != equal, not equal (all same precedence)
&& logical AND (both operands always evaluated)
|| logical OR (both operands always evaluated)
\&= assignment, right associative
.ТЕ
.ix table~of [hoc] operators
.PP
Functions, as described later, may be defined by the user.
Function arguments are expressions separated by commas.
There are also a number of built-in functions,
all of which take a single argument,
described in Table 2.
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\fBTable 2:\fP Built-in Functions
.sp .5
abs(x) @| x |@, absolute value of @x@
atan(x) arc tangent of @x@
cos(x) @cos (x)@, cosine of @x@
exp(x) @e sup x@, exponential of @x@
int(x) integer part of @x@, truncated towards zero
log(x) @log (x)@, logarithm base @e@ of @x@
log10(x) @log sub 10 (x)@, logarithm base 10 of @x@
sin(x) @sin (x)@, sine of @x@
sqrt(x) @sqrt x@, @x sup half@
.ТЕ
.ix table~of [hoc] functions
.PP
Logical expressions have value 1.0 (true) and 0.0 (false).
As in C,
any non-zero value is taken to be true.
As is always the case with floating point numbers,
equality comparisons are inherently suspect. .PP
.I Hoc
also has a few built-in constants, shown in Table 3.
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