UNIX — универсальная среда программирования - Керниган Брайан Уилсон

.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 @[email protected](3):

.ix [scanf]

digits, decimal point, digits,

.ix [hoc] manual

.ix assignment expression

.ix multiple assignment

@[email protected] or @[email protected], 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,

@[email protected] refers to binary operators such

as addition or logical comparison;

@[email protected] 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

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^       exponentiation (s-1FORTRANs0 **), 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

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abs(x)   @| x |@, absolute value of @[email protected]

atan(x)  arc tangent of @[email protected]

cos(x)   @cos (x)@, cosine of @[email protected]

exp(x)   @e sup [email protected], exponential of @[email protected]

int(x)   integer part of @[email protected], truncated towards zero

log(x)   @log (x)@, logarithm base @[email protected] of @[email protected]

log10(x) @log sub 10 (x)@, logarithm base 10 of @[email protected]

sin(x)   @sin (x)@, sine of @[email protected]

sqrt(x)  @sqrt [email protected], @x sup [email protected]

.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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