Matrix Elements VB-800 Instrukcja Użytkownika Pobierz pdf (Strona 143)

A6.6 External declarations 135

a2 = 1; /* a2 may be changed */

/* ... */

retp(a2,b1);

endp;

}

{x1, x2} = test2(2,3);

The example shows that external functions need not be declared before they are called

(although it would be good programming practice): if test1 is not found at the link

stage, an error will be reported.

All functions may have a return value, but this return value need not be used by the

caller. If a function does not return a value, its actual return value is undeﬁned. Use

call to call a function and discard the return values. A function has only one return

value when the number of returns is left unspeciﬁed.

If a function is redeﬁned, it automatically replaces the function which existed earlier

(without printing a warning).

The local statement allocates a local variable. If the local variable has the same

name as a global variable, the local will hide the global variable. Multiple declarations

result in a warning, unless it is a type change (such as from a matrix to a function, see

the genfunc example below).

The retp statement returns one or more values from the function, and also exits the

function. So, when the program ﬂow reaches a retp statement, control returns to the

caller, without executing the remainder of the function. If a function fa returns p values,

and is in a call function fb, then the return from fa counts for p arguments in the call to

fb.

A fn function is a function with one return value. So the following two are equiva-

lent:

fn func(arg) = expr;

proc(1) func(arg); retp(expr); endp;

A keyword function differs from a proc in two ways: there is no return value, and

only one argument which is always a string. When a keyword is called, the argument

text up to the semicolon is passed as one string to the keyword function, unless the

argument starts with a ^, in which case it is interpreted as a variable name.

Functions can be passed as arguments, and an array of functions can be created. As

an example of the ﬁrst:

proc(0)= func(a);

print("\nfunc:", a);

endp;

proc(0)= func3(&fnc); /* takes a function as argument */

local fnc:proc; /* tell compiler about this */

print("\nin func3:");

call fnc(100); /* and call the passed function */

endp;

call func3(&func); /* call func3 with func as argument */

And an example of an array of functions:

1 2 ... 138 139 140 141 142 143 144 145 146 147 148 ... 171 172

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