CS118 –

Programming
Fundamentals
Functions
Lecture # 19
Wednesday, December 16, 2020
FALL 2020
FAST – NUCES, Faisalabad Campus

Rizwan Ul Haq
 Objectives
2

In this week/classes, you will:

 Learn about standard (predefined) functions and
discover how to use them in a program
 Learn about user-defined functions
 Examine value-returning functions, including actual
and formal parameters
 Explore how to construct and use a value-returning,
user-defined function in a program

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 Modular Programming
3

 Modular programming: Breaking a program up into

smaller, manageable functions or modules
 Function: A collection of statements to perform a
task
 Motivation for modular programming
 Improves maintainability of programs
 Simplifies the process of writing programs

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 Introduction
4

 Boss to worker analogy

 A boss (the calling function or caller) asks a worker (the
called function) to perform a task and return (i.e., report
back) the results when the task is done

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 Introduction
5

 Functions are like building blocks

 They allow complicated programs to be divided into
manageable pieces
 Some advantages of functions:
 A programmer can focus on just that part of the program
and construct it, debug it, and perfect it
 Different people can work on different functions
simultaneously
 Can be re-used (even in different programs)
 Enhance program readability

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 Introduction (cont'd.)
6

 Functions
 Called modules
 Like miniature programs
 Can be put together to form a larger program

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 Predefined Functions
7

 In algebra, a function is defined as a rule or

correspondence between values, called the
function’s arguments, and the unique value of the
function associated with the arguments
 If f(x) = 2x + 5, then f(1) = 7, f(2) = 9, and f(3) = 11
 1, 2, and 3 are arguments
 7, 9, and 11 are the corresponding values

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 Predefined Functions (cont'd.)
8

 Some of the predefined mathematical functions

are:
 sqrt(x);
 sqrt( 4 );
 sqrt( 3 - 6x );
 pow(x, y);
 floor(x);
 Predefined functions are organized into separate
libraries
 I/O functions are in iostream header
 Math functions are in cmath/math header

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 Predefined Functions (cont'd.)
9

 pow(x,y) calculates xy
 pow(2, 3) = 8.0
 Returns a value of type double
 x and y are the parameters (or arguments)
 This function has two parameters
 sqrt(x) calculates the nonnegative square root of x,
for x >= 0.0
 sqrt(2.25) is 1.5
 Type double

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 Predefined Functions (cont'd.)
10

 The floor function floor(x) calculates largest whole

number not greater than x
 floor(48.79) is 48.0
 Type double
 Has only one parameter

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 Predefined Functions (cont'd.)
11

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 Predefined Functions (cont'd.)
12

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 Predefined Functions (cont'd.)
13 Example 6-1
//How to use predefined function
#include <iostream>
#include <cmath>
#include <cctype>
#include <cstdlib>
using namespace std;
int main(){
int x;
double u, v;
cout << "Line 1: Uppercase a is "
<< static_cast<char> (toupper('a’)) << endl; //Line 1
u = 4.2 ; //Line 2
v = 3.0 ; //Line 3
cout << "Line 4: " << u << " to the power of "
<< v << " = " << pow(u,v) << endl; //Line 4
cout << "Line 5 : 5.0 to the power of 4 = "
<< pow(5.0,4) << endl; //Line 5

u = u + pow(3.0, 3); //Line 6

cout << "Line 7: u = " << u << endl; //Line 7

x = -15 ; //Line 8
cout << "Line 9: Absolute value of " << x
<< " = " << abs(x) << endl; //Line 9
return 0;
}
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 User-Defined Functions
14

 Value-returning functions: have a return type

 Return a value of a specific data type using the return
statement function, called the type of the function
 You need to add the following items :
 The name of the function
 The number of parameters, if any
 The data type of each parameter
 The data type of the value computed (that is, the value
returned) by the Function
 Void functions: do not have a return type
 Do not use a return statement to return a value

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 Syntax: Value-Returning function
15

functionType functionName(formal parameter list)

{
statements
}

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 Function Header
16

 The function header consists of

 the function return type
 the function name
 the function parameter list
 Example:
int main()

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 Function Return Type
17

 If a function returns a value, the type of the value

must be indicated
int main()
 If a function does not return a value, its return type is
void
void printHeading()
{
cout << "\tMonthly Sales\n";
}

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 Syntax: Formal Parameter List
18

Function Return Type Function name

Function parameter
Formal Heading
int abs(int number) {

if (number<0) Formal parameter list

number = -number;
return number;
}
Formal Body

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 Defining and Calling Functions
19

 Function call: Statement that causes a function to

execute
 Function definition: Statements that make up a function

int abs(int number);

Similarly the function abs might have the following definition:

int abs(int number)

{
if(number < 0)
number = -number;

return number;
}

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 Function Definition
20

 Definition includes
 return type: Data type of the value the function returns to
the part of the program that called it
 name: Name of the function. Function names follow same
rules as variable names
 parameter list: Variables that hold the values passed to the
function
 body: Statements that perform the function’s task

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 Calling a Function
21

 To call a function, use the function name followed

by () and ;
printHeading();
 When a function is called, the program executes
the body of the function
 After the function terminates, execution resumes in
the calling function at the point of call

 main() is automatically called when the program

starts
 main() can call any number of functions
 Functions can call other functions

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 Function Call
22

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 Syntax: Actual Parameter List
23

Can be zero
parameter

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 Actual Parameter Vs Formal
24 Parameter
Suppose that the heading of the function pow is:

double pow(double base, double exponent);

From the heading of the function pow, it follows that the formal
parameters of pow are base and exponent. Consider the following
statements:

double u = 2.5;
double v = 3.0;
double x, y;
x = pow(u, v); //Line1
y = pow(2.0, 3.2) + 5.1; //Line2
cout << u << " to the power of 7 = " << pow(u, 7) << endl; //Line3

Formal Parameter: A variable declared in the function heading.

Actual Parameter: A variable or expression listed in a call to function.

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25

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 Function Call Notes
26

 Value of argument is copied into parameter when

the function is called
 Function can have >= 0 parameter(s)
 There must be a data type listed in the prototype ()
and an argument declaration in the function
heading () for each parameter
 Arguments will be promoted/demoted as necessary
to match parameters

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 Calling Functions with Multiple
27 Arguments
 When calling a function with multiple arguments:
 The number of arguments in the call must match the
function prototype and definition
 The first argument will be copied into the first parameter, the
second argument into the second parameter, etc.

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 Calling Functions with Multiple
28 Arguments – an Illustration
void displayData(int h, int w) //heading
{
cout << "Height = " << h << endl;
cout << "Weight = " << w << endl;
}

displayData(height, weight); //Call

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 Value-Returning Functions --
29 Example
int abs(int number)
{
if(number < 0)
number = -number;

return number;
}

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 return Statement
30

 Once a value-returning function computes the

value, the function returns this value via the return
statement
 It passes this value outside the function via the return
statement

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 Syntax: return Statement
31

 The return statement has the following syntax:

 In C++, return is a reserved word

 When a return statement executes
 Function immediately terminates
 Control goes back to the caller
 When a return statement executes in the function
main, the program terminates
e.g return 0;

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 Returning a Value From a Function
32

 return statement can be used to return a value from

the function to the module that made the function
call
 Prototype and definition must indicate data type of
return value (not void)
 Calling function should use return value
 assign it to a variable
 send it to cout
 use it in an arithmetic computation
 use it in a relational expression
 Pass it as a parameter to another function

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 Syntax: return Statement (cont’d.)
33

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 Example
34

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 Function to compare three numbers
35

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 Returning a Boolean Value
36

 Function can return true or false

 Declare return type in function prototype and
heading as bool
 Function body must contain return statement(s) that
return true or false
 Calling function can use return value in a relational
expression

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 Boolean return Example
37
bool isValid(int); // prototype

bool isValid(int val) // heading

{
int min = 0, max = 100;
if (val >= min && val <= max)
return true;
else
return false;
}

if (isValid(score)) // call
…
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 Function Prototypes
38

 The compiler must know the following about a

function before it is called
 name
 return type
 number of parameters
 data type of each parameter

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 Function Prototype
39

Function prototype: Function heading

without the body of the function
Syntax:

It is not necessary to specify the variable

name in the parameter list
The data type of each parameter must be
specified

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 Function Prototype
40

 The function heading without the body of the

function.

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 Prototype Notes
41

 Place prototypes near top of program

 Program must include either prototype or full
function definition before any call to the function,
otherwise a compiler error occurs
 When using prototypes, function definitions can be
placed in any order in the source file. Traditionally,
main is placed first.
 Use a function prototype (similar to the heading of
the function
 Header: void printHeading()
 Prototype: void printHeading();

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 Function Prototype (Illustration)
42

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 Function Prototype (Illustration
43 cont'd.)

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 Example Program I
44 1
2
// C++ Program
// Creating and using a programmer-defined function.
3 #include <iostream>
4 Function prototype: specifies
5 using std::cout; data types of arguments and
6 using std::endl; return values. square expects
7
and int, and returns an int.
8 int square(int); // function prototype
9
10 int main()
11 {
12 // loop 10 times and calculate and output
13 // square of x each time
14 for (int x = 1; x <= 10; x++) Parentheses () cause function
15 cout << square(x) << " "; // function call
16
to be called. When done, it
17 cout << endl; returns the result.
18
19 return 0; // indicates successful termination
20
21 } // end main
22
23 // square function definition returns square of an integer
24 int square(int y) // y is a copy of argument to function
25 {
26 return y * y; // returns square of y as an int
27 Definition of square. y is a copy
28 } // end function square of the argument passed.
CS118 - FALL 2020 Returns y * y, or y squared.
 Output
45

1 4 9 16 25 36 49 64 81 100

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 Example Program II
46 1
2
// C ++ Program
// Finding the maximum of three floating-point numbers.
3 #include <iostream>
4
5 using std::cout;
6 using std::cin;
7 using std::endl;
8
9 double maximum(double, double, double); // function prototype
10
11 int main()
12 {
13 double number1;
14 double number2; Function maximum takes 3
15 double number3;
16
arguments (all double) and
17 cout << "Enter three floating-point numbers: "; returns a double.
18 cin >> number1 >> number2 >> number3;
19
20 // number1, number2 and number3 are arguments to
21 // the maximum function call
22 cout << "Maximum is: "
23 << maximum(number1, number2, number3) << endl;
24
25 return 0; // indicates successful termination
26
27 } // end main
28

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 Example Program II
47
Comma separated list
29 // function maximum definition; for multiple parameters.
30 // x, y and z are parameters
31 double maximum(double x, double y, double z)
32 {
33 double max = x; // assume x is largest
34
35 if (y > max) // if y is larger,
36 max = y; // assign y to max
37
38 if (z > max) // if z is larger,
39 max = z; // assign z to max
40
41 return max; // max is largest value
42
43 } // end function maximum

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 Sample run
48

Enter three floating-point numbers: 99.32 37.3 27.1928

Maximum is: 99.32

Enter three floating-point numbers: 1.1 3.333 2.22

Maximum is: 3.333

Enter three floating-point numbers: 27.9 14.31 88.99

Maximum is: 88.99

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 Value-Returning Functions: Some
49 Peculiarity

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 Value-Returning Functions: Some
50 Peculiarity (cont'd.)

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 Value-Returning Functions: Some
51 Peculiarity (cont'd.)

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 Questions
52

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