UNIX — универсальная среда программирования

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)) {

 print "value is ", x, "\n"

}

.ft

.DE

.NH

Functions and Procedures

.PP

Functions and procedures are distinct in @hoc@,

although they are defined by the same mechanism.

This distinction is simply for run-time error checking:

it is an error for a procedure to return a value,

and for a function @not@ to return one.

.PP

The definition syntax is:

.ix [hoc] function~definition

.ix [hoc] procedure~definition

.DS

.I

.ta 1i

function: func name() stmt