Брайан Керниган - UNIX — универсальная среда программирования

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

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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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\fBTable 3:\fP Built-in Constants

.sp .5

DEG 57.29577951308232087680 @180/ pi@, degrees per radian

E 2.71828182845904523536 @e@, base of natural logarithms

GAMMA 0.57721566490153286060 @gamma@, Euler-Mascheroni constant

PHI 1.61803398874989484820 @( sqrt 5 +1)/2@, the golden ratio

PI 3.14159265358979323846 @pi@, circular transcendental number

.ТЕ

.ix table~of [hoc] constants

.NH

Statements and Control Flow

.PP

.I Hoc

statements have the following grammar:

.DS

.I

stmt: expr

| variable = expr

| procedure ( arglist )

| while ( expr ) stmt

| if ( expr ) stmt

| if ( expr ) stmt else stmt

| { stmtlist }

| print expr-list

| return optional-expr

stmtlist: \fR(nothing)\fI

| stmlist stmt

.R

.DE

An assignment is parsed by default as a statement rather than

an expression, so assignments typed interactively

do not print their value.

.PP

Note that semicolons are not special to

.ix [hoc] input~format

@hoc@: statements are terminated by newlines.

This causes some peculiar behavior.

The following are legal

.IT if

statements:

.DS

.ft CW

if (x < 0) print(y) else print(z)

if (x < 0) {

print(y)

} else {

print(z)

}

.ft

.DE

In the second example, the braces are mandatory:

the newline after the

.I if

would terminate the statement and produce a syntax error were

the brace omitted.

.PP

The syntax and semantics of @hoc@

control flow facilities are basically the same as in C.

The

.I while

and

.I if

statements are just as in C, except there are no @break@ or

@continue@ statements.

.NH

Input and Output: @read@ and @print@

.PP

.ix [hoc] [read]~statement

.ix [hoc] [print]~statement

The input function @read@, like the other built-ins,

takes a single argument. Unlike the built-ins, though, the argument

is not ал expression: it is the name of a variable.

The next number (as defined above) is read from the standard input

and assigned to the named variable.

The return value of @read@ is 1 (true) if a value was read, and 0 (false)

if @read@ encountered end of file or an error.

.PP

Output is generated with the ©print© statement.

The arguments to @print@ are a comma-separated list of expressions

and strings in double quotes, as in C.

Newlines must be supplied;

they are never provided automatically by @print@.

.PP

Note that @read@ is a special built-in function, and therefore takes

a single parenthesized argument, while @print@ is a statement that takes

a comma-separated, unparenthesized list:

.DS

.ft CW

while (read(x)) {