UNIX — универсальная среда программирования

Керниган Брайан Уилсон

} Symbol;

Symbol *install, *lookup;

typedef union Datum { /* interpreter stack type */

 double val;

 Symbol *sym;

} Datum;

extern Datum pop;

extern eval, add, sub, mul, div, negate, power;

typedef int (*Inst);

#define STOP (Inst)0

extern Inst *progp, *progbase, prog[], *code;

extern assign, bltin, varpush, constpush, print, varread;

extern prexpr, prstr;

extern gt, lt, eq, ge, le, ne, and, or, not;

extern ifcode, whilecode, call, arg, argassign;

extern funcret, procret;

3.7.13

hoc.ms

.EQ

delim @@

.EN

.TL

Hoc - An Interactive Language For Floating Point Arithmetic

.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.

.TS

center, box;

с s

lfCW l.

\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.

.TS

center, box;

с s

lfCW l.

\fBTable 2:\fP Built-in Functions

.sp .5

abs(x) @| x |@, absolute value of @x@