Math2220 Lecture 11

Default Argument

a default value is the value given by the program when the user doesn't specify a value
in C++, you can specify a default argument for one or more call-by-value parameters in a function.
if the corresponding argument is omitted, then it is replaced by the default argument.

#include <iostream>
using namespace std;

void showVolume(int length, int width=1, int height=1);

void main() {
    showVolume(4,6,2);
    showVolume(4,6); // same as showVolume(4,6,1)
    showVolume(4); // same as showVolume(4,6,1)

    char line[100];
    cin.getline(line,100);
}

void showVolume(int length, int width, int height) {
    cout << "Volume of the box with \n"
         << "lenght = " << length
         << ", width = " << width << endl
         << "and height = " << height
         << " is " << length*width*height
         << endl << endl;
}

Output

Volume of the box with
lenght = 4, width = 6
and height = 2 is 48

Volume of the box with
lenght = 4, width = 6
and height = 1 is 24

Volume of the box with
lenght = 4, width = 1
and height = 1 is 4

Syntax

function declaration

similar to ordinary function declarations
assign the values of the default arguments by variableType variableName = defaultValue.
e.g. double tax(double income, bool married = false, int dependent = 0, bool resident = true);.
you can't specific default argument to call-by-reference parameters.
This is incorrect : double tax(double income, bool& married = false);
You can have more than one default arguments but all the default argument positions must be in the rightmost positions.
Correct examples:
- double calculateGPA(char grade = 'A', int unit = 4);
- double calculateInterest(double balance, double interestRate = 0.05);
Incorrect examples:
- void showVolume(int length = 1, int width, int height);
- double double tax(double income, bool married = false, int dependent, bool resident = true)

Function header

exactly same as the function headers of ordinary functions
you should not assign the default value in the function header because the assignments are already done in the function declaration. e.g.
Suppose the function declaration is

double tax(double income, bool married = false, int dependent = 0);

incorrect

double tax(double income, bool married = false, int dependent = 0) {

     // function body

}

The correct version should be

double tax(double income, bool married, int dependent) {

     // function body

}

Function body

Exactly same as the function bodies of ordinary functions.

Function call

you can omit the default arguments
if you omit arguments, you must omit arguments starting from the right.
the program substitutes the missing arguments by the default values

example:
Suppose the fucntion declaration is

double tax(double income, bool married = false, int
dependent = 0, bool resident = true);

Function call	Meaning
tax(100000,true,2,false)	tax(50000,true,2,false)
tax(200000,true,3)	tax(200000,true,3,true)
tax(75555,false,2)	tax(75555,false,2,true)
tax(22333.75,true)	tax(22333.75,true,0,true)
tax(35678.25)	tax(35678.25,false,0,true)

If you provide value for one default argument, you must provide all the values before it. In the above example, if you pvoivde value for resident, you must provide the value for married and dependent.

We can rewrite the first example as follows

#include <iostream>

using namespace std;


void showVolume(int length, int width, int height);
void showVolume(int length, int width);
void showVolume(int length);

void main() {
    showVolume(4,6,2);
    showVolume(4,6);
    showVolume(4);

    char line[100];
    cin.getline(line,100);


}

void showVolume(int length, int width, int height) {
    cout << "Volume of the box with \n"
         << "lenght = " << length 
         << ", width = " << width << endl
         << "and height = " << height
         << " is " << length*width*height 
         << endl << endl;
}

void showVolume(int length, int width) {
     showVolume(length, width, 1);
}

void showVolume(int length) {
    showVolume(length, 1, 1);
}

Summary

Now you should know:

how to make a function call
the flow of the program when you made a function call
what is void
functions can have no argument
how to use return
how to define your own functions
- function declaration
- function definition
what is local variables
what is global varaibles, how to define them.
call-by-value vs call-by-reference
function overload
default arguments

Array

An array is a sequence of numbers.

#include <iostream>
using namespace std;

// the program asks user to input 5 numbers
// then output the average
void main() {
    // declare an array with 5 numbers
    // sum is the total sum of all the scores
    double score[5], sum = 0;
    cout << "Please input 5 numbers. "
         << "Seperate the numbers by spaces: \n";
   
    int i;
    for(i = 0; i < 5; i++) {
        cin >> score[i]; //ask the user to input the i-th score
        sum+=score[i];
    }

    // magic formula
    cout.setf(ios::fixed);
    cout.setf(ios::showpoint);
    cout.precision(2);
    cout << "The scores are : \n";
   
    for(i = 0; i < 5; i++) {
        cout << score[i] << " ";
    }

    cout << endl;
    cout << "The average is " << sum/5 <<endl;

    char line[100];
    cin.getline(line,100);
    cin.getline(line,100);
}

Output

Please input 5 numbers. Seperate the numbers by spaces:
92.25 33 45.75 88.8 77.77
The scores are :
92.25 33.00 45.75 88.80 77.77
The average is 67.51

`score[0]`	`score[1]`	`score[2]`	`score[3]`	`score[4]`
92.25	33	45.75	88.8	77.77

Array consists of 2 parts

name of the array
size of the array

Syntax

Declare an array
Type arrayName[size];

size should be a number or a const variable. It shouldn't be other type of variables.

Example:

int year[50];
int is called the base type of the array, 50 is the size. year is the name.
You can declare arrays and regular variables together. example:
```
double taxRate, income[99];
```
The order doesn't matter.

You can use constant to declare an array size.

#include <iostream>
using namespace std;

// the program asks user to input 5 numbers
// then output the average
void main() {
    const int NUM_OF_STUDENTS = 5;
    // declare an array with NUM_OF_STUDENTS numbers
    // sum is the total sum of all the scores
    double score[NUM_OF_STUDENTS], sum = 0;
    cout << "Please input "<< NUM_OF_STUDENTS << " numbers. "
         << "Seperate the numbers by spaces: \n";
   
    int i;
    for(i = 0; i < NUM_OF_STUDENTS; i++) {
        cin >> score[i]; //ask the user to input the i-th score
        sum+=score[i];
    }

    // magic formula
    cout.setf(ios::fixed);
    cout.setf(ios::showpoint);
    cout.precision(2);
    cout << "The scores are : \n";
   
    for(i = 0; i < NUM_OF_STUDENTS; i++) {
        cout << score[i] << " ";
    }

    cout << endl;
    cout << "The average is " << sum/NUM_OF_STUDENTS <<endl;

    char line[100];
    cin.getline(line,100);
    cin.getline(line,100);
}

You can't use variables to declare an array size.
Compiliing error

#include <iostream>
using namespace std;

void main() {
    int numOfStudents;
    cout << "Please enter the number of students: ";
    cin >> numOfStudents;
    double score[numOfStudents], sum = 0; // Incorrect! 
  
    char line[100];
    cin.getline(line,100);
}

Dereference

score[0],score[1],score[2],score[3],score[4] represent the first, second, third, forth and fifth element in the array score
Array indexes always start with zero and ends with the size-1
you can use score[i] as variables. i.e. you can use it in assignments, arithmetic expression etc etc
You can put any arithmetic expression inside the bracket, like score[i], or income[n+1].