Lecture 22

Reading

p416, 429-430, 434

C-string and pointer

A c-string is an array of characters and thus is a pointer of char type.

#include<iostream>
#include<cstring>
using namespace std;

struct Employee {
    char *name;
    char *address;
    char gender;
    double income;
};

void display(Employee employee);

int main() {
    Employee charles;

    
    char myname[] = "Charles";
    charles.name = new char[strlen(myname)+1];
    strcpy(charles.name, "Charles");
    
    char myaddress[] = "1111 Westwood Blvd.\nLos Angeles\nCA 90024";
    charles.address = new char[strlen(myaddress)+1];
    strcpy(charles.address, "1111 Westwood Blvd.\nLos Angeles\nCA 90024");
    
	
    charles.gender = 'm';
    charles.income = 123123.25;
    
    display(charles);

    return 0;
}

void display(Employee employee) {
    cout << "name: " << employee.name << endl
         << "address: "<< endl
         << employee.address << endl
         << "gender: " << employee.gender << endl
         << "income:  $" << employee.income << endl;
}

Output

name: Charles
address:
1111 Westwood Blvd.
Los Angeles
CA 90024
gender: m
income:  $123123

Compare the above program to this

struct Employee {
   char name[50];
   char address[100];
   char gender;
   double income;
}

Careful You should allocate memory to the pointers first before you use strlen or other functions.

Pointer arithmetic

you can perform arithmetic on pointers
it is the arithmetic on address (not the arithmeic on numbers, or on what it points to)
You can do addition or subtraction (multiplication and division are meaningless)

Example

#include<iostream>
using namespace std;

int main() {
    double *pt = new double;
    cout << pt << endl;
    cout << pt+1 << endl;
    cout << pt+2 << endl;

    return 0;
}

Output

002F10F0
002F10F8
002F1100

the address is represented by a hexadecimal number
pt+1 does not really add 1 to pt. It actually add 8 to pt. The increment is related to the size of the type. Try to replace the double by int.
pt+1, pt+2 are actually pointing to something meaningless.

#include<iostream>
using namespace std;

int main() {
    int a[] = {2,3,5,7,11};
    int i;
    for(i = 0; i < 5; i++) {
        cout << a[i] << " ";
    }
    cout << endl;
    for(i = 0; i < 5; i++) {
        cout << *(a+i) << " ";
    }
    cout << endl;

    return 0;
}

Most of the time, we use pointer arithmetic on array
a + i points to a[i]. i.e. *(a+i) is a[i]

`delete` operator

free the memory that used by a pointer
may not be useful in PIC10A but will be useful in PIC10B and PIC10C

a very simple(and useless) example

#include<iostream>
using namespace std;

int main() {
    
    int *temp, *temp2;
    temp = new int[10];
    temp2 = new int; 
    for(int i = 0; i < 10; i++) {
        *(temp + i) = i;
    }

    delete [] temp; // release the memory occpied by temp
    delete temp2; 
    return 0;
}

a better example

#include<iostream>
using namespace std;

// returns sum of a[0], a[1], ....
int sumOfArray(int a[], int size);

// returns sum of 1 + 2 + 3 + .... n
int sum(int n);

int main() {
    cout << sum(100) << endl; // 5050
    return 0;
}

int sumOfArray(int a[], int size) {
    int sum = 0;
    for(int i = 0; i < size; i++) {
        sum+=a[i];
    }
    return sum;
}

int sum(int n) {
    int* temp = new int[n];
    for(int i = 0; i < n; i++) {
        temp[i] = i+1;
    }
    int result = sumOfArray(temp, n);
    delete [] temp; // don't need temp any more
    return result;
}

Struct and pointer

Most of the examples in my handout use int, double, char pointers
you can use pointer for structure.

#include<iostream>
using namespace std;
struct Stock {
    char symbol[5]; // symbol, 4 characters
    char name[26];
    double price;
};

int main() {
    Stock yahoo = {"yhoo", "Yahoo Inc", 10.5};
    Stock *pt = &yahoo;
    cout << "The symbol of " << (*pt).name 
         << " is " << (*pt).symbol << endl;

    return 0;
}

It is same as

#include<iostream>
using namespace std;
struct Stock {
    char symbol[5]; // symbol, 4 characters
    char name[26];
    double price;
};

int main() {
    Stock yahoo = {"yhoo", "Yahoo Inc", 10.5};
    cout << "The symbol of " << yahoo.name 
         << " is " << yahoo.symbol << endl;

    return 0;
}

Here is the output

The symbol of Yahoo Inc is yhoo

-> operator
Another way to write the program is

#include<iostream>
using namespace std;
struct Stock {
    char symbol[5]; // symbol, 4 characters
    char name[26];
    double price;
};

int main() {
    Stock yahoo = {"yhoo", "Yahoo Inc", 10.5};
    Stock *pt = &yahoo;
    cout << "The symbol of " << pt->name
         << " is " << pt->symbol << endl;

    return 0;
}

pt->name is same as (*pt).name
the first notation is more convenient and is almost always that notation used.

copy

What is the output of the program and why?

#include<iostream>
using namespace std;

int main() {
    char gfname[] = "Mary Smith";
    char* wifename;
  
    wifename = gfname; // copy girlfriend's name
    // change my wife's last name to Li
    wifename[5] = 'L';
    wifename[6] = 'i';
    wifename[7] = '\0';

    cout << gfname << endl;
    cout << wifename << endl;

    return 0;
}