6.2 arrays and strings

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6.2 Arrays and Strings

Computer Programming and Basic Software Engineering6. Pointers and Arrays

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What Is an Array?

• An array consists of a collection of data storage locations, each holds the same type of data

• An array can be easily declared as follows:

short shortArray[25]; It states that there is a sequence of 25 short integer data. The whole

sequence is named as shortArray

It states that there is a sequence of 25 short integer data. The whole

sequence is named as shortArray

shortArray[0] 000B 000C ... ... 0037 0038 0039 003A

Memory

Variables short shortArray[25]; //declaration

10 0A 21 3A ... ... 20 2A 4B 40


shortArray[1] shortArray[24]

In consecutive memory

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Array Element

• In an array, an array element is referred to by indicating its index

• The first element has index 0, and then 1, …

• Hence shortArray[0] is the first element

shortArray[1] is the second element

:

shortArray[24] is the last element

• No shortArray[25] !!!

• Do not try to use shortArray[25], result unpredictable.

25 short integers

25 short integers


4 Computer Programming and Basic Software Engineering6. Pointers and Arrays

short shortArray[25];

0009 000A 000B 000C ... ... 0037 0038 0039 003AAddress

(Hex)

Memory

Variables short shortArray[25];

10 0A 21 3A ... ... 20 2A 4B 40

• So shortArray[0] = 0x100A;// 4106 in deciaml

shortArray[1] = 0x213A;// 8506 in decimal

:

shortArray[23] = 0x202A;

shortArray[24] = 0x4B40;

Declaration

Assignment statements

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#include <iostream>using namespace std;int main(){ int myArray[5],i;

for (i=0; i<=5; i++)myArray[i] = 20;

for (i=0; i<5; i++)cout << myArray[i] << endl;

return 0;}

The kind of mistake people often make

The kind of mistake people often make


No myArray[5] !!! Do not try to use myArray[5], result unpredictable

No myArray[5] !!! Do not try to use myArray[5], result unpredictable

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Initializing Arrays• An array can be initialized during declaration

int IntegerArray[5] = {10,20,30,40,50};

int AnotherArray[] = {50,40,30,20,10};

int BiggerArray[5] = {10,20};

IntegerArray declares itself to have 5 integers

IntegerArray declares itself to have 5 integers

AnotherArray requests the memory space in stack just enough to hold the

data defined in the list

AnotherArray requests the memory space in stack just enough to hold the

data defined in the listBiggerArray declares itself to have 5 integers but only the first 2 of them

are initialized. The others are 0.It is different from :

BiggerArray declares itself to have 5 integers but only the first 2 of them

are initialized. The others are 0.It is different from :

int IncorrectArray[2] = {10,20,30};


int a[5]; NOT the same as int a[5]={};

int BiggerArray[] = {10,20};

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Array of Objects• Any object can be stored in an array

• Accessing member data in an array of objects is a two-step process

• Identify the member of array by []

• Access the member by .

CAT Litter[5]; //Litter[0] - Litter[4] are 5 objectsint i;for (i=0; i<5; i++) cout << Litter[i].GetAge() << endl;

To find out which CATTo find out which CAT Call GetAge() of that CATCall GetAge() of that CAT


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Multidimensional Array• It is possible to have an array of more than 1 dimension

Two-dimensional arrayA Chess Board

Two-dimensional arrayA Chess Board

int Board[8][8];

• A 2-dimensional array can be declared as

• Each element can be written or read as

Board[5][3] = 0;int number = Board[5][3];// number = 0

0 1 2 3 4 5 6 7

76543210

0


64 integers

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Initialize Multidimensional Arrays• Multidimensional Arrays can also be initialized during

declaration

int SomeArray[5][2] = { {0,0},{1,2},{4,6},{7,2},{4,4}};int AnotherArray[5][2] = {0,0,1,2,4,6,7,2,4,4};

0 0

1 2

4 6

7 2

4 4

0 1

4

3

2

1

0


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Array and Pointer• C++ allows the flexibility for user to use Array and Pointer

interchangeably

• The name of an array is a constant pointer pointing to the first element of the array

int a,b;int SomeArray[5] = {10,20,30,40,50};a = *SomeArray; // a = 10b = *(SomeArray+1); // b = 20, pointer arithmetic

• Compiler does all the calculation

• SomeArray+1 does not add 1 to SomeArray but add 4 (1 integer needs 4 bytes for storage) and point to the next element in the array


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0100 0104 0108 010c 0110 0114 0118 011c 0120 0124Address

The Stack

SomeArray[5]

10 20 30 40


50

x = *SomeArray; will be internally converted to

x = SomeArray[0]; // x = 10

px = SomeArray; will be internally converted to

px = &(SomeArray[0]); // px = 0104

x = *SomeArray; will be internally converted to

x = SomeArray[0]; // x = 10

px = SomeArray; will be internally converted to

px = &(SomeArray[0]); // px = 0104

• There is NOT a memory location to store the pointer SomeArray. Everything is done by an internal conversion


• A variable declared as a pointer can also be used as an arrayint SomeArray[5] = {10,11,12,13,14};int *pSomePointer = SomeArray; // It is a pointer but will

// later be used as arraycout << pSomePointer[0] << endl; // number 10 will be showncout << pSomePointer[1] << endl; // number 11 will be showncout << pSomePointer[2] << endl; // number 12 will be showncout << pSomePointer[3] << endl; // number 13 will be showncout << pSomePointer[4] << endl; // number 14 will be shown

0100 0104 0108 010c 0110 0114 0118 011c 0120 0124Address

The stack

pSomePointer = 0104

10 11 12 13 14

SomeArray[5]

SomeArray[1] and pSomePointer[1] are exactly the same.

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Array of Pointers

• So far the arrays are declared to store data in stack

• Usually the memory space of stack is very small

• If a large array is required, it is better to store the elements of arrays in Free Store

• In this case, for every element in an array, a pointer is assigned to indicate its location in Free Store

• This becomes an array of pointers.


The array itself is still in the stack.

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6. Pointers and Arrays#include <iostream>using namespace std;class CAT{public:

CAT() {itsAge = 1;}~CAT() {}int GetAge() const {return itsAge;}void SetAge(int age) {itsAge =

age;}private:

int itsAge;};int main(){ CAT *Family[500];

int i;for (i=0; i<500; i++){ Family[i] = new CAT; }Family[255]->SetAge(1);for (i=0; i<500; i++){ delete Family[i]; }return 0;

}

Creating 500 CATs may use up all

memory in the stack

Creating 500 CATs may use up all

memory in the stack

Hence only an array of 500 pointers of CAT are created in the stack

Hence only an array of 500 pointers of CAT are created in the stack

Each CAT object is located in the Free Store by the keyword new

Each CAT object is located in the Free Store by the keyword new

To access member of a particular CAT, use the corresponding pointer

in the array

To access member of a particular CAT, use the corresponding pointer

in the array

Computer Programming and Basic Software Engineering

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Free Storeor

the heap

Global Name SpaceCode SpaceThe Stack

CAT 0 CAT 1

CAT 2

CAT 499

...

Array of Pointers

An array of pointers called Family is kept to point to

different elements in Free Store

An array of pointers called Family is kept to point to

different elements in Free Store


(*Family[0])

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Pointer of Array • If one feels that 500 pointers in the previous example are

still too many, we can put the whole array into Free Store

• Hence one pointer is enough to point to the Array itself but not an individual element

• This becomes a pointer of array.

CAT *Family = new CAT[500];

int a = 0, b=0;(Family+255)->SetAge(10);a = (Family+255)->GetAge();// a = 10b = Family[255].GetAge(); // b = 10

Family is the pointer of array and points to Family[0] in

Free Store

Family is the pointer of array and points to Family[0] in

Free Store

Individual element of the array of the CAT objects

can be accessed by pointer arithmetic

Individual element of the array of the CAT objects

can be accessed by pointer arithmetic


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Pointer of Array

Free Storeor

the heap

Global Name SpaceCode SpaceThe Stack

CAT 0 CAT 1 ... CAT 499

A pointer Family is kept to point to the beginning memory location of an array

of CAT objects in Free Store

A pointer Family is kept to point to the beginning memory location of an array

of CAT objects in Free Store


Family[0] Family[499]

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6. Pointers and Arrays#include <iostream>using namespace std;class CAT{public:

CAT() {itsAge = 1;}~CAT(){;}int GetAge() const {return itsAge;}void SetAge(int age) {itsAge = age;}

private:int itsAge;

};int main(){ CAT *Family = new CAT[10];

for (int i=0; i<10; i++){ Family[i].SetAge(2*i+1);

cout <<' '<< Family[i].GetAge();}delete [] Family;return 0;

}

Arrays created in Free Store can also be deleted

Arrays created in Free Store can also be deleted

The [] symbol after delete lets the system know the whole array is to be deleted

The [] symbol after delete lets the system know the whole array is to be deleted

Computer Programming and Basic Software Engineering

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Exercise 6.2

• Based on the program in the last page, add a destructor to CAT such that when it is called, the age of the cat will be shown on the screen.

• How many times the destructor will be called when the “delete [] Family;” statement is executed? Why?

• What will happen if we use the statement “delete Family” instead? Can it be executed in Visual C++?



Exercise 6.2b

• Modify the program in Ex.6.2 such that the “Array of Pointers” approach is used to define the 10 objects of CAT in the heap. Make sure your program will not introduce memory leaks.

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String - char Arrays • A string is an array of characters

char Greeting[] = {'H','e','l','l','o',' ','W','o','r','l','d','\0'};

• A string can be simply initialized as follows:

The null character represents the end of string; it must be added.

The null character represents the end of string; it must be added.

• However, this method can easily introduce error

• C++ provides a shorthand that makes use of the double quote " ".

char Greeting[] = {"Hello World"};Totally 12 bytes are

allocated for Greeting.Null character is

automatically generated

Totally 12 bytes are allocated for Greeting.

Null character is automatically generated


Size of array is 12

The braces can be removed


Copying String • We often need to copy string from one character array to

another character array

char Greeting[12] = {"Hello World"};char Greeting2[12];

• Common errors:

Greeting2 = Greeting;

Wrong. Greeting2 is a constant pointer; we cannot assign anything to it - See explanation in the next page.

Wrong. Greeting2 is a constant pointer; we cannot assign anything to it - See explanation in the next page.


0100 0101 0102 0103 010a 010b 010c 010dAddress

The Stack

Greeting[12]

'H' 'e' 'l' 'd' '\0''l'…

0200 0201 0202 0203 020a 020b 020c 010dAddress

The Stack ? ? ? ? ??…

Greeting2[12]

Greeting2 = Greeting; Wrong. We try to make Greeting2 = 0101. However, Greeting2 must = 0201 as it is assigned by the OS, and is constant

Wrong. We try to make Greeting2 = 0101. However, Greeting2 must = 0201 as it is assigned by the OS, and is constant


Very wrong. Greeting2[12] means only the 13th element of Greeting2, not the whole string. Besides, there is no 13th element in Greeting or Greeting2

Very wrong. Greeting2[12] means only the 13th element of Greeting2, not the whole string. Besides, there is no 13th element in Greeting or Greeting2

Greeting2[12] = Greeting[12];

Note: Greeting2[11] = Greeting[11]; is legal, which means assigning the 12th element of Greeting2 with the value of the 12th element of Greeting.

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Strcpy() and strncpy() • C++ inherits from C a library of functions for tackling strings

• The two most common ones are strcpy() and strncpy()

#include <iostream>#include <string.h>using namespace std;int main(){ char String1[] = {"Copy String1 to String2"}; char String2[80]; strcpy(String2,String1); cout << "String1: " << String1 << endl; cout << "String2: " << String2 << endl; return 0;}

Copy String1 to String2Copy String1 to String2

State the definition of strcpy()You may omit this line because it is already included by iostream

State the definition of strcpy()You may omit this line because it is already included by iostream


ResultResult

Warning: unsafe

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• Strcpy() will overwrite past the end of the destination if the source were larger than the destination, damaging other data

• To solve the problem, strncpy() can be used

#include <iostream>#include <string.h>using namespace std;int main(){ const int MaxLength = 80; //MaxLength > strlen(String1) = 23 char String1[] = "Copy String1 to String2"; char String2[MaxLength+1]; //Initialize String2 if MaxLength<=23 strncpy(String2,String1,MaxLength); cout << "String1: " << String1 << endl; cout << "String2: " << String2 << endl; return 0;}

Strncpy() needs another parameter MaxLength which specifies the maximum

number of data (not including null) to be copied

Strncpy() needs another parameter MaxLength which specifies the maximum

number of data (not including null) to be copied

State the definition of strncpy()State the definition of strncpy()

Computer Programming and Basic Software Engineering6. Pointers and Arrays Warning: unsafe

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Exercise 6.2c

a. Write a program that creates an array of 3 objects of the class ACCOUNT in the free store.

b.Ask the user to input three names and save to each element of the array

c. Read the content of each element in the array and display on the screen.

class ACCOUNT{public:

void writename(char nm[]);char * readname();

private:char name[80];

};

void ACCOUNT::writename(char nm[]){

strncpy(name,nm,79);}

char * ACCOUNT::readname(){

return name;}


Pointer of array


A few more library functions• To measure the

length of a string, the function strlen() is useful

• To combine two strings into one, we can use the function strcat().

#include <iostream>#include <string.h>using namespace std;int main(){ char String1[100]; cout << "Please enter a word: "; cin >> String1; char String2[] = " has "; char String3[5]; char String4[] = " characters."; itoa(strlen(String1),String3,10); strcat(String1,String2);//ret String1 strcat(String1,String3); strcat(String1,String4); cout << String1 << endl; return 0;}

Convert an integer into a string

Convert an integer into a string

Set decimal

Append a string

5 warnings


• We can compare if two strings are the same using the function strcmp().

#include <iostream>#include <string.h>using namespace std;int main(){ char String1[100]; cout << "Please enter your name: "; cin.getline(String1,100); char String2[] = "Dr F Leung"; if (strcmp(String1, String2)==0)

cout << "Welcome Dr Leung.\n"; else

cout << "Login incorrect.\n"; return 0;}

Syntax:int strcmp(string1, string2)Return 0 if string1 is the same as string2; otherwise returns a +ve or ve number.

Syntax:int strcmp(string1, string2)Return 0 if string1 is the same as string2; otherwise returns a +ve or ve number.

Get one line of text (including the spaces in between)

No warning

6.2 arrays and strings

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