View source for Ada Programming/Subprograms

<noinclude>{{Ada/Navigation 1|Subprograms}}
__TOC__</noinclude>

In Ada the subprograms are classified into two categories: [[#Procedures|procedures]] and [[#Functions|functions]]. A procedures call is a statement and does not return any value, whereas a function returns a value and must therefore be a part of an expression.

Subprogram parameters may have four modes.

; {{Ada/kw|in}}: The actual parameter value goes into the call and is not changed there. The formal parameter is a constant and allows only reading. This is the default when no mode is given. The actual parameter is an expression.
; {{Ada/kw|in}} {{Ada/kw|out}}: The actual parameter goes into the call and may be redefined. The formal parameter is a variable and can be read and written.
; {{Ada/kw|out}}: The actual parameter's value before the call is irrelevant, it will get a value in the call. The formal parameter can be read and written. (In Ada 83 {{Ada/kw|out}} parameters were write-only.)
; {{Ada/kw|access}}: The formal parameter is an access (a pointer) to some variable.

Note that parameter modes do not specify the parameter passing method. Their purpose is to document the data flow.

The parameter passing method depends on the type of the parameter. A rule of thumb is that parameters fitting into a register are passed by copy, others are passed by reference. For certain types, there are special rules, for others the parameter passing mode is left to the compiler.

Unlike other programming languages, Ada does not need an empty set of <code>{{Ada/delimiter|(}} {{Ada/delimiter|)}}</code> when a subprogram has no parameters.

== Procedures ==

A procedure call in Ada constitutes a statement by itself.

For example:

 {{Ada/kw|procedure}} A_Test {{Ada/delimiter|(}}A, B: {{Ada/kw|in}} Integer; C: {{Ada/kw|out}} Integer{{Ada/delimiter|)}} {{Ada/kw|is}}
 {{Ada/kw|begin}}
    C := A + B;
 {{Ada/kw|end}} A_Test;

When the procedure is called with the statement
 A_Test (5 + P, 48, Q);
the expressions 5 + P and 48 are evaluated (expressions are only allowed for in parameters), and then assigned to the formal parameters A and B, which behave like constants. Then, the value A + B is assigned to formal variable C, whose value will be assigned to the actual parameter Q when the procedure finishes.

C, being an {{Ada/kw|out}} parameter, is an unitialized variable before the first assignment. (Therefore in Ada 83, there existed the restriction that {{Ada/kw|out}} parameters are write-only. If you wanted to read the value written, you had to declare a local variable, do all calculations with it, and finally assign it to C before return. This was awkward and error prone so the restriction was removed in Ada 95.)

Within a procedure, the return statement can be used without arguments to exit the procedure and return the control to the caller.

For example, to solve an equation of the kind <math>ax^2 + bx + c = 0</math>:

 {{Ada/kw|with}} {{Ada/package 3|Ada|Numerics|Elementary_Functions}};
 {{Ada/kw|use}}  {{Ada/package 3|Ada|Numerics|Elementary_Functions}};
 
 {{Ada/kw|procedure}} Quadratic_Equation
    {{Ada/delimiter|(}}A, B, C :     Float;   {{Ada/--|By default it is "in".}}
     R1, R2  : {{Ada/kw|out}} Float;
     Valid   : {{Ada/kw|out}} Boolean{{Ada/delimiter|)}}
 {{Ada/kw|is}}
    Z : Float;
 {{Ada/kw|begin}}
    Z := B**2 - 4.0 * A * C;
    {{Ada/kw|if}} Z < 0.0 {{Ada/kw|or}} A = 0.0 {{Ada/kw|then}}
       Valid := False;  {{Ada/--|Being out parameter, it must be modified at least once.}}
       R1    := 0.0;
       R2    := 0.0;
    {{Ada/kw|else}}
       Valid := True;
       R1    := (-B + Sqrt (Z)) / (2.0 * A);
       R2    := (-B - Sqrt (Z)) / (2.0 * A);
    {{Ada/kw|end}} {{Ada/kw|if}};
 {{Ada/kw|end}} Quadratic_Equation;

The function SQRT calculates the square root of non-negative values.  If the roots are real, they are given back in R1 and R2, but if they are complex or the equation degenerates (A = 0), the execution of the procedure finishes after assigning to the Valid variable the False value, so that it is controlled after the call to the procedure.  Notice that the {{Ada/kw|out}} parameters should be modified at least once, and that if a mode is not specified, it is implied {{Ada/kw|in}}.

== Functions ==

A function is a subprogram that can be invoked as part of an expression.  Functions can only take {{Ada/kw|in}} (the default) or {{Ada/kw|access}} parameters. The latter can be used as a work-around for the restriction that functions may not have {{Ada/kw|out}} parameters. In this sense, Ada functions behave more like mathematical functions than in other languages. The specification of the function is necessary to show to the clients all the information needed to invoke it.

A function body example can be:

 {{Ada/kw|function}} Minimum (A, B : Integer) {{Ada/kw|return}} Integer {{Ada/kw|is}}
 {{Ada/kw|begin}}
    {{Ada/kw|if}} A <= B {{Ada/kw|then}}
       {{Ada/kw|return}} A;
    {{Ada/kw|else}}
       {{Ada/kw|return}} B;
    {{Ada/kw|end}} {{Ada/kw|if}};
 {{Ada/kw|end}} Minimum;

The formal parameters of a function behave as local constants whose values are provided by the corresponding actual parameters.  The statement {{Ada/kw|return}} is used to indicate the value returned by the function call and to give back the control to the expression that called the function.  The expression of the {{Ada/kw|return}} statement may be of arbitrary complexity and must be of the same type declared in the specification.  If an incompatible type is used, the compiler gives an error.  If the restrictions of a subtype are not fulfilled, e.g. a range, it raises a Constraint_Error exception.

The body of the function can contain several {{Ada/kw|return}} statements and the execution of any of them will finish the function, returning control to the caller.  If the flow of control within the function branches in several ways, it is necessary to make sure that each one of them is finished with a {{Ada/kw|return}}
statement.  If at run time the end of a function is reached without encountering a {{Ada/kw|return}} statement, the exception Program_Error is raised.  Therefore, the body of a function must have at least one such {{Ada/kw|return}}
statement.

Every call to a function produces a new copy of any object declared within it.  When the function finalizes, its objects disappear.  Therefore, it is possible to call the function recursively.  For example, consider this implementation of the factorial function:

 {{Ada/kw|function}} Factorial (N : Positive) {{Ada/kw|return}} Positive {{Ada/kw|is}}
 {{Ada/kw|begin}}
    {{Ada/kw|if}} N = 1 {{Ada/kw|then}}
       {{Ada/kw|return}} 1;
    {{Ada/kw|else}}
       {{Ada/kw|return}} (N * Factorial (N - 1));
    {{Ada/kw|end}} {{Ada/kw|if}};
 {{Ada/kw|end}} Factorial;

When evaluating the expression <code>Factorial (4);</code> the function
will be called with parameter 4 and within the function it will
try to evaluate the expression <code>Factorial (3)</code>, calling itself as a function, but in this case parameter N would be 3 (each call copies the parameters) and so on until N = 1 is evaluated which will finalize the recursion and then the expression will begin to be completed in the reverse order.

A formal parameter of a function can be of any type, including vectors or
records.  Nevertheless, it cannot be an anonymous type, that is, its
type must be declared before, for example:

 {{Ada/kw|type}} Float_Vector {{Ada/kw|is}} {{Ada/kw|array}} (Positive {{Ada/kw|range}} <>) {{Ada/kw|of}} Float;
 
 {{Ada/kw|function}} Add_Components (V: Float_Vector) {{Ada/kw|return}} Float {{Ada/kw|is}}
    Result : Float := 0.0;
 {{Ada/kw|begin}}
    {{Ada/kw|for}} I {{Ada/kw|in}} V'{{Ada/attribute|Range}} {{Ada/kw|loop}}
       Result := Result + V(I);
    {{Ada/kw|end}} {{Ada/kw|loop}};
    {{Ada/kw|return}} Result;
 {{Ada/kw|end}} Add_Components;

In this example, the function can be used on a vector of arbitrary dimension.
Therefore, there are no static bounds in the parameters passed to the
functions. For example, it is possible to be used in the following
way:

 V4  : Float_Vector (1 .. 4) := (1.2, 3.4, 5.6, 7.8);
 Sum : Float;
 
 Sum := Add_Components (V4);

In the same way, a function can also return a type whose bounds are not known a priori. For example:

 {{Ada/kw|function}} Invert_Components (V : Float_Vector) {{Ada/kw|return}} Float_Vector {{Ada/kw|is}}
    Result : Float_Vector(V'{{Ada/attribute|Range}});   {{Ada/--|Fix the bounds of the vector to be returned.}}
 {{Ada/kw|begin}}
    {{Ada/kw|for}} I {{Ada/kw|in}} V'{{Ada/attribute|Range}} {{Ada/kw|loop}}
       Result(I) := V (V'{{Ada/attribute|First}} + V'{{Ada/attribute|Last}} - I);
    {{Ada/kw|end}} {{Ada/kw|loop}};
    {{Ada/kw|return}} Result;
 {{Ada/kw|end}} Invert_Components; 

The variable Result has the same bounds as V, so the returned vector will always have the same dimension as the one passed as parameter.

A value returned by a function can be used without assigning it to a variable, it can be referenced as an expression.  For example, <code>Invert_Components (V4) (1)</code>, where the first element of the vector returned by the function would be obtained (in this case, the last element of V4, i.e. 7.8).

== Named parameters ==

In subprogram calls, named parameter notation (i.e. the name of the formal
parameter followed of the symbol {{Ada/special_2|arrow|<nowiki>=></nowiki>}} and then the actual parameter) allows the rearrangement of the parameters in the call. For example:

 Quadratic_Equation (Valid => OK, A => 1.0, B => 2.0, C => 3.0, R1 => P, R2 => Q);
 F := Factorial (N => (3 + I));

This is expecially useful to make clear which parameter is which.

 Phi := Arctan (A, B);
 Phi := Arctan (Y => A, X => B);

The first call (from Ada.Numerics.Elementary_Functions) is not very clear. One might easily confuse the parameters. The second call makes the meaning clear without any ambiguity.

Another use is for calls with numeric literals:

 Ada.Float_Text_IO.Put_Line (X, 3, 2, 0);  -- ?
 Ada.Float_Text_IO.Put_Line (X, Fore => 3, Aft => 2, Exp => 0);  -- OK

== Default parameters ==

On the other hand, formal parameters may have default values. They can, therefore, be omitted in the subprogram call. For example:

 {{Ada/kw|procedure}} By_Default_Example (A, B: {{Ada/kw|in}} Integer := 0);

can be called in these ways:

 By_Default_Example (5, 7);      {{Ada/--|<nowiki>A = 5, B = 7</nowiki>}}
 By_Default_Example (5);         {{Ada/--|<nowiki>A = 5, B = 0</nowiki>}}
 By_Default_Example;             {{Ada/--|<nowiki>A = 0, B = 0</nowiki>}}
 By_Default_Example (B => 3);    {{Ada/--|<nowiki>A = 0, B = 3</nowiki>}}
 By_Default_Example (1, B => 2); {{Ada/--|<nowiki>A = 1, B = 2</nowiki>}}

In the first statement, a "regular call" is used (with positional association); the second also uses positional association but omits the second parameter to use the default; in the third statement, all parameters are by default; the fourth statement uses named association to omit the first paramer; finally, the fifth statement uses mixed association, here the positional parameters have to precede the named ones.

Note that the default expression is evaluated once for each formal parameter that has no actual parameter. Thus, if in the above example a function would be used as defaults for A and B, the function would be evaluated once in case 2 and 4; twice in case 3, so A and B could have different values; in the others cases, it would not be evaluated.

== See also ==
 
=== Wikibook ===

* [[Ada Programming]]
* [[Ada Programming/Operators]]

=== Ada 95 Reference Manual ===

* {{Ada/RM1|6|Subprograms}}
* {{Ada/RM2|4|4|Expressions}}

=== Ada 2005 Reference Manual ===

* {{Ada/2005/RM1|6|Subprograms}}
* {{Ada/2005/RM2|4|4|Expressions}}

=== Ada Quality and Style Guide ===

* {{Ada/SG3|4|1|3|Subprograms}}

<noinclude>
[[Category:Ada Programming|Subprograms]]
[[es:Programación en Ada / Subprogramas]]
</noinclude>