Lecture 20

Reading

p176-178, p417-418, p422-427

Pointer and array

An array variable is actually the address of the first element of the array.
An array variable is actually a pointer.
Refer to p178, display 5.2

#include<iostream>
using namespace std;

int main() {
    int a[] = {1,2,3};
    cout << "The address of a[0]: " << &a[0] <<endl;
    cout << "a: " << a << endl;

    return 0;
}

Output on my computer

The address of a[0]: 0012FF74
a: 0012FF74

What is the output of the following program and why?

#include<iostream>
using namespace std;

int main() {
    int a[] = {11,22,33};
    cout << *a << endl; //dereference
    return 0;
}

Answer:

Reason: : a holds the address of a[0].

Can you guess what is the output of the following program?

#include<iostream>
using namespace std;

int main() {
    char ch[] = {'a','b','c'};
    cout << *ch << endl
         << *(ch+1) << endl
         << *(ch+2) << endl;
    return 0;
}

Answer

a
b
c

Reason: The value of a is the address of a[0]. The value of a+1 is the address of a[1]. The value of a+2 is the address of a[2].

An array variable is actually a pointer.
What is the output of the following program?

#include<iostream>
using namespace std;
    
int main() {
    double x[2];
    x[0] = 3.14;
    x[1] = 2.71;
    double *pt;
    pt = x; // assign x to pt
    cout << pt[0] << endl
         << pt[1] << endl;
    pt[0]++;
    cout << x[0] << endl
         << x[1] << endl;

    return 0;
}

Answer:

3.14
2.71
4.14
2.71

One more example, what is the output of the following program?

#include<iostream>
using namespace std;
    
int main() {
    int *p;
    int a[10];
    int b[10] = {2,4,6,8,10,12,14,16,18,20};
    int index;

    // a[index] is index
    for(index = 0; index < 10; index++) 
        a[index] = index;

    p = a;
    
    // p[index] is same as a[index]
    cout << "p array is :\n";
    for(index = 0; index < 10; index++) {   
        cout << p[index] << " ";
    }
    cout << endl;

    // change p[index] will change a[index]
    for(index = 0; index < 10; index++) {
        p[index] = p[index] + 1;
    }

    cout << "p array is :\n";
    for(index = 0; index < 10; index++) {       
        cout << p[index] << " ";
    }
    cout << endl;

    cout << "a array is: \n";
    for(index = 0; index < 10; index++) {       
        cout << a[index] << " ";
    }
    cout << endl;

    p = b;
    // p[index] is same as b[index] now
    cout << "p array is :\n";
    for(index = 0; index < 10; index++) {
        cout << p[index] << " ";
    }
    cout << endl;

    return 0;
}

Answer:

p array is :
0 1 2 3 4 5 6 7 8 9
p array is :
1 2 3 4 5 6 7 8 9 10
a array is:
1 2 3 4 5 6 7 8 9 10
p array is :
2 4 6 8 10 12 14 16 18 20

Trap: The following program can't be compiled, why?

#include<iostream>
using namespace std;
    
int main() {
    double x[2];
    x[0] = 3.14;
    x[1] = 2.71;
    double *pt;
    x = pt;
}

Error Message:

Cannot convert from double * to double[2]

x is actually of type const double*. A constant can't be changed.

typedef

typedef is used to define an alias for any type name or definition.
mainly used in defining type for pointer variables.

Example

#include<iostream>
using namespace std;
    
typedef int* IntPtr;

int main() {
    IntPtr pt1, pt2;
    int x = 1, y = 2;
    pt1 = &x;
    pt2 = &y;
    cout << *pt1 << " " << *pt2 << endl;

    return 0;
}

It is same as

#include<iostream>
using namespace std;
    
typedef int* IntPtr;

int main() {
    int *pt1, *pt2;
    int x = 1, y = 2;
    pt1 = &x;
    pt2 = &y;
    cout << *pt1 << " " << *pt2 << endl;

    return 0;
}

What is the advantage of using typedef?
Refer to the book p418
Avoid confusion. Example

#include<iostream>
using namespace std;
    
int main() {
    int *p1, p2;
    return 0;
}

What is the type of p2 (an integer or a pointer) ?

Dynamic arrays

What is wrong with the following program?

#include <iostream>
using namespace std;

int main() {
    int numOfStudents, index;

    cout << "Please enter the number of students: ";
    cin >> numOfStudents;
    double scores[numOfStudents];
    
    cout << "Please enter the scores:\n";
    for(index = 0; index < numOfStudents; index++) {
        cin >> scores[index];
    }

    cout << "The scores are:\n";
    for(index = 0; index < numOfStudents; index++) {
        cout << scores[index] << " ";
    }
   
    return 0;

}

When you declare an array, you must estimate the largest possible size you may need for the array.
What is the disadvantage of this?

You underestimate the size. Your array is not big enough to hold all the data.
You overestimate the size, as a result, you waste some computer memory.

Dynamically allocated array can be used to solve this problem.
The above program can be rewritten as

#include <iostream>
using namespace std;

int main() {
    int numOfStudents, index;
    double* scores;

    cout << "Please enter the number of students: ";
    cin >> numOfStudents;
    
    scores = new double[numOfStudents]; // dynamically allocated array
    
    cout << "Please enter the scores:\n";
    for(index = 0; index < numOfStudents; index++) {
        cin >> scores[index];
    }

    cout << "The scores are:\n";
    for(index = 0; index < numOfStudents; index++) {
        cout << scores[index] << " ";
    }
    cout << endl;
   
    return 0;
}

Output

Please enter the number of students: 5
Please enter the scores:
90.5 91.25 55 25.9 73
The scores are:
90.5 91.25 55 25.9 73

scores = new double[numOfStudents]: The size of the dynamic array (numOfStudents) is given in square bracket.
The size can be given using an int variable or other expression.
The size can be a variable of type int whose value is determined while the program is running.
It is used like ordinary array.

Return an array

Recall that you can't return an array in a function
instead you should return a pointer

int[] someFunction(); // illegal!!!!
int* someFunction(); // legal!

Example:

#include <iostream>
using namespace std;

int* doubler(int a[], int size);

int main() {
    int a[] = {1,2,3,4,5};
    int *b;

    b = doubler(a,5);

    int i;
    // display a
    cout << "Array a:\n";
    for(i = 0; i < 5; i++) {
        cout << a[i] << " ";
    }
    cout << endl;

    // display b
    cout << "Array b:\n";
    for(i = 0; i < 5; i++) {
        cout << b[i] << " ";
    }
    cout << endl;

    return 0;
}

int* doubler(int a[], int size) {
    int* temp = new int[size];

    for(int i = 0; i < size; i++) {
        temp[i] = 2*a[i];
    }

    return temp;
}

Output

Array a:
1 2 3 4 5
Array b:
2 4 6 8 10