Chapter Nine: Subprograms

Lesson 09

What are they

Modularized code Might return a value

Functions Or not

Procedures Subroutines

In object orientation tend to be known as Methods

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Subprograms are the fundamental building blocks of programs and are, therefore, among the most important concepts in programming language design

Anatomy of a subprogram Whole thing is the subprogram definition When use or invoke it is known as a

subprogram “call” A subprogram header is the first part

Includes name, the kind of subprogram, and the formal parameters

The parameter profile (aka signature) The number, order, and types of its parameters If include return type then known as the protocol

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int sum(int a, int b){ return a + b;}

int sum(int a, int b){ return a + b;}

Definition

Header Parameter profile

Protocol

Prototypes

Function declarations in C and C++ are often called prototypes

Useful to the compiler—allows visibility of subprogram form without “seeing” body

Allows independent compilation subprograms Object files that become linked

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Actual/Formal Parameter Correspondence

Positional: Java, C/C++ Keyword

Advantage: Parameters can appear in any order, thereby avoiding parameter correspondence errors

Disadvantage: User must know the formal parameter’s names

Python, R, Matlab Can mix and match

If know order, can use order, if don’t can use names in any order

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sumer(length = myLength, list = myArray, sum = mySum)

Formal Parameter Default Values

In certain languages (e.g., C++, Python, Ruby, Ada, PHP), formal parameters can have default values (if no actual parameter is passed)

Variable numbers of parameters C# (must be same type) Ruby (employs a hash) Python (passes a list) Lua (syntax is …, and loop through them with a for loop)

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void calc_y(double m = 1, double x = 0, double b = 0);

Ruby Blocks

def fibonacci(last) first, second = 1, 1 while first <= last yield first first, second = second, first + second end end

puts "Fibonacci numbers less than 100 are:" fibonacci(100) {|num| print num, " "} puts

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Block

Models of Parameter Passing

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In, out, inout

Pass Modes

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Pass-by-Value (In Mode) Usually pass by copy

Pass-by-Reference (Inout Mode) Pass an access path

Modes continued

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Pass-by-Result (Out Mode) Empty variable location placed on stack at call (because

OUT) Compiler must assign value on stack back into the

original variable upon return Pass-by-Value-Result (inout Mode)

No empty start enforcement Pass by copy, then compiler assigns back into original

variable upon return

Default method: pass-by-valuePass-by-reference is specified by preceding both a formal parameter and its actual parameter with ref

void doIT(out int x, int index){ x = 17; index = 14;}

void doIT(out int x, int index){ x = 17; index = 14;}

Multidimensional Arrays as Parameters

No way to know the size, should pass

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Multidimensional Arrays as Parameters

Ada – not a problem Size is part of the array’s type

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Multidimensional Arrays as Parameters

Similar to Ada Arrays are objects; they are all single-

dimensioned, but the elements can be arrays Each array inherits a named constant (length

in Java, Length in C#) that is set to the length of the array when the array object is created

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Subprograms as parameters When might one wish to do this?

Action listener callback functions Could pass a sort function along with some data to

be sorted Sort it differently depending upon the situation

Example: the LISP map function takes as its arguments a function and a list, and

returns the list formed by applying the function to each member of the list.

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Passing functions in C/C++

How do you suppose its done in C and C++? Why, pointers of course

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int func(int a, int b){ return a + b;}

int (*f)(int a, int b);f = func;

Pointers

Example: function pointers Passing a function as an argument

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#include <stdio.h>

int adder(int a, int b){ return a+b;}

int multiplier(int a, int b){ return a*b;}

int binary(int (*passedFunc)(int a, int b), int c, int d){ return (*passedFunc)(c,d);}

int main(int argc,char *argv[]){ printf("adder result for 3 and 4: %d\n",binary(adder,3,4)); printf("multiplier result for 3 and 4: %d\n",binary(multiplier,3,4)); return(0);}

Results

adder result for 3 and 4: 7multiplier result for 3 and 4: 12

Results

adder result for 3 and 4: 7multiplier result for 3 and 4: 12

Overloaded Subprograms Same name

Every version of an overloaded subprogram has a unique protocol (signature)

Ada, Java, C++, and C# allow

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Generic Subprograms

A generic or polymorphic subprogram takes parameters of different types on different activations Common in object oriented programming languages

Subprograms are expected to take any child objects Project 6: will convert your stack class into a

template: can declare a stack of any type Overloaded subprograms: a form of polymorphism In Java and C#, called a generic method

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User-Defined Overloaded Operators Operators can be overloaded in Ada, C++,

Python, and Ruby An Ada example: turn the * operator into a dot

product operator for two vectorsfunction "*" (A,B: in Vec_Type): return Integer isSum: Integer := 0;beginfor Index in A'range loop

Sum := Sum + A(Index) * B(Index)end loopreturn sum;

end "*";…c = a * b; -- a, b, and c are of type Vec_Type

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The end

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