Procedural Abstraction and Functions That Return a Value

Slide 4- 5
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Top Down Design

To write a program

Develop the algorithm that the program will use

Translate the algorithm into the programming
language

Top Down Design
(also called stepwise refinement)

Break the algorithm into subtasks

Break each subtask into smaller subtasks

Eventually the smaller subtasks are trivial to
implement in the programming language
Slide 4- 6
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Benefits of Top Down Design

Subtasks, or functions in C++, make programs

Easier to understand

Easier to change

Easier to write

Easier to test

Easier to debug

Easier for teams to develop
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
4.2
Predefined Functions
Slide 4- 8
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Predefined Functions

C++ comes with libraries of predefined
functions

Example: sqrt function

the_root = sqrt(9.0);

returns, or computes, the square root
of a number

The number, 9, is called the argument

the_root will contain 3.0
Slide 4- 9
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Display 4.1
Function Calls

sqrt(9.0) is a function call

It invokes, or sets in action, the sqrt function

The argument (9), can also be a variable or an
expression

A function call can be used like any expression

bonus = sqrt(sales) / 10;

Cout << “The side of a square with area “ << area
<< “ is “
<< sqrt(area);
Slide 4- 10
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Function Call Syntax

Function_name (Argument_List)

Argument_List is a comma separated list:
(Argument_1, Argument_2, … ,
Argument_Last)

Example:

side = sqrt(area);

cout << “2.5 to the power 3.0 is “
<< pow(2.5, 3.0);
Slide 4- 11
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Function Libraries

Predefined functions are found in libraries

The library must be “included” in a program
to make the functions available

An include directive tells the compiler which
library header file to include.

To include the math library containing sqrt():
#include <cmath>

Newer standard libraries, such as cmath, also require
the directive
using namespace std;
Slide 4- 12
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Display 4.2
Other Predefined Functions

abs(x) int value = abs(-8);

Returns absolute value of argument x

Return value is of type int

Argument is of type x

Found in the library cstdlib

fabs(x) double value = fabs(-8.0);

Returns the absolute value of argument x

Return value is of type double

Argument is of type double

Found in the library cmath
Slide 4- 13
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Type Casting

Recall the problem with integer division:
int total_candy = 9, number_of_people = 4;
double candy_per_person;
candy_per_person = total_candy / number_of_people;

candy_per_person = 2, not 2.25!

A Type Cast produces a value of one type
from another type

static_cast<double>(total_candy) produces a double
representing the integer value of total_candy
Slide 4- 14
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Integer division occurs before type cast
Type Cast Example

int total_candy = 9, number_of_people = 4;
double candy_per_person;
candy_per_person = static_cast<double>(total_candy)
/ number_of_people;

candy_per_person now is 2.25!

This would also work:
candy_per_person = total_candy /
static_cast<double>( number_of_people);

This would not!
candy_per_person = static_cast<double>( total_candy /
number_of_people);

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