Lecture 20 Arrays and Strings Introduction to Computer

Lecture 20: Arrays and Strings Introduction to Computer Science Spring 2006 1

Data Types n n A data type is called simple if variables of that type can store only one value at a time A structured data type is one in which each data item is a collection of other data items 2

Arrays n Array - a collection of a fixed number of elements n n n All the elements of an array must be the same data type; The elements of an array are stored sequentially in memory. One-dimensional array - an array in which the components are arranged in a list form 3

Declaring an array n The general form of declaring a one-dimensional array is: data. Type array. Name[int. Exp]; where int. Exp is any expression that evaluates to a positive integer n Example: int num[5]; n n Layout: The base address of an array is the address, or memory location of the first array component n In the upper example, base address of num is the address of the component num[0] 4

Accessing Array Components n n Array component can be accessed by using index. The general form (syntax) of accessing an array component is: array. Name[index. Exp] where index. Exp, called index, is any expression whose value is a nonnegative integer n n Index value specifies the position of the component in the array The [] operator is called the array subscripting operator The array index always starts at 0 Example: After we declare array num: int num[5]; We can access the first element by using num[0] the second element by using num[1] … the last element by using num[4] 5

Array Index Out of Bounds n If we have the statements: double num[10]; int i; n The component num[i] is a valid index if i = 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9 n The index of an array is in bounds if the index >=0 and the index <= array. Size-1 n If either the index < 0 or the index > array. Size-1 n n then we say that the index is out of bounds There is no guard against indices that are out of bounds n C++ does not check if the index value is with in range 6

Array Initialization n As with simple variables n n When initializing arrays while declaring them n n n Arrays can be initialized while they are being declared Not necessary to specify the size of the array Size of array is determined by the number of initial values in the braces For example: double sales[] = {12. 25, 32. 50, 16. 90, 23, 45. 68}; 7

Partial Initialization n The statement int list[10] = {0}; declares list to be an array of 10 components and initializes all components to zero n The statement int list[10] = {8, 5, 12}; declares list to be an array of 10 components, initializes list[0] to 8, list[1] to 5, list[2] to 12 and all other components are initialized to 0 n The statement int list[] = {5, 6, 3}; declares list to be an array of 3 components and initializes list[0] to 5, list[1] to 6, and list[2] to 3 8

Processing One-Dimensional Arrays n Some basic operations performed on a one-dimensional array are: n n n Initialize Input data Output data stored in an array Find the largest and/or smallest element Each operation requires ability to step through the elements of the array Easily accomplished by a loop 9

Accessing Array Components (continued) n Consider the declaration int list[100]; int i; n //list is an array of the size 100 This for loop steps-through each element of the array list starting at the first element for(i = 0; i < 100; i++) process list[i] //Line 1 //Line 2 10

Initializing an array: the statement in Line 2 takes the form of an assignment statement, for example: for(i = 0; i < 100; i++) //Line 1 list[i]=0; Input data: the statement in Line 2 takes the form of an input statement, such as the cin statement for(i = 0; i < 100; i++) //Line 1 cin>>list[i]; Output data: the statement in Line 2 takes the form of an output statement, such as the cout statement for(i = 0; i < 100; i++) //Line 1 cout<<list[i]<<“ ”; Finding the sum: sum=0; for(i = 0; i < 100; i++) sum=sum+list[i]; //Line 1 Find the largest element in the array: int max. Index=0; for(i = 0; i < 100; i++) //Line 1 if (list[max. Index] < list[i]) max. Index=i; Largest. Num = num[max. Index]; 11

Restrictions on Array Processing n Assignment does not work with arrays n If x and y are two arrays of the same type and size then the following statement is illegal: y = x; // illegal n n In order to copy one array into another array we must copy component-wise This can be accomplished by a loop: for(j = 0; j < 25; j++) y[j] = x[j]; 12

Restrictions on Array Processing (continued) n Comparison of arrays, reading data into an array and printing the contents of an array must be done component-wise cin>>x; //illegal cout<<y; //illegal n n C++ does not allow aggregate operations on an array An aggregate operation on an array is any operation that manipulates the entire array as a single unit 13

Arrays as Parameters to Functions n n n Arrays are passed by reference only Do not use the symbol & when declaring an array as a formal parameter The size of the array is usually omitted n n Example: void func. Array(int list. One[], double list. Two[]) If the size of one-dimensional array is specified when it is declared as a formal parameter n It is ignored by the compiler 14

Arrays as Parameters to Functions (continued) n The reserved word const in the declaration of the formal parameter can prevent the function from changing the actual parameter n n When we pass an array as a parameter n n Example: void func. Array(int list. One[], const double list. Two[]) base address of the actual array is passed to the formal parameter Functions cannot return a value of the type array 15

C Strings (Character Arrays) n C++ has two methods for representing strings: n the string class n n n String - a sequence of zero or more characters enclosed in double quote marks Example: string sentence; C-style character arrays (C-String) n low-level, primitive representation of string data n used in the C programming language n Character array - an array whose components are of type char n Example: char name[16]; char sentence[] = "Hello World"; 16

C Strings (Character Arrays) (continued) n C-stings are null terminated (‘’) n n The last character in a string is the null character There is a difference between 'A' and “A” n ‘A’ is the character A n “A” is the string A Because C strings are null terminated, C-string "A" represents two characters, 'A' and '‘ Similarly, C-string "Hello" contains six characters, 'H', 'e', 'l', 'o', and '' 17

C Strings (Character Arrays) (continued) n Consider the statement char name[16]; n Because C-strings are null terminated and name has sixteen components n n The largest string that can be stored in name is 15 If you store a string of length, say 10 in name n The first 11 components of name are used and the last 5 are left unused 18

C Strings (Character Arrays) (continued) n The statement char name[16] = "John"; declares a string variable name of length 16 and stores "John" in it n The statement char name[] = "John"; declares a string variable name of length 5 and stores "John" in it 19

Functions for C-string 20

Functions for C-string : example 21

Reading and Writing C-Strings n C-String Input n Aggregate operations are allowed for string input n Example: char name[16]; cin>>name; stores the next input string in name n C-Strings that contain blanks cannot be read using the extraction operator >> 22

Reading and Writing C-Strings (continued) n To read C-strings with blanks, use the get function with an input stream variable, such as cin: cin. get(str, m+1); n n Stores the next m characters into str but the newline character is not stored in str If the input string has fewer than m characters, the reading stops at the newline character 23

Reading and Writing C-Strings (continued) n C-String Output n n n For the declaration of C-string name: char name[16] = "John"; The statement cout<<name; outputs the content of name on the screen The insertion operator << continues to write the contents of name until it finds the null character If name does not contain the null character, then we will see strange output: << continues to output data from memory adjacent to name until '' is found 24

Input/Output Files n The argument to the function open must be a null terminated string (a C-string) n Example: ifstream infile; char filename[51]; cin>>filename; infile. open(filename); n n n Variables of type string can be used to read and store the names of input/output files C++ strings are not null terminated If we use a variable of type string to read the name of an input/output file and then use this variable to open a file n The value of the variable must first be converted to a C-string (nullterminated string) 25

Input/Output Files (continued) n The header file string contains the function c_str n n Converts a value of the type string to a null-terminated character array The syntax to use the function c_str is: str. Var. c_str() where str. Var is a variable of the type string n Example: ifstream infile; string filename; cin>>filename; infile. open(filename. c_str()); 26

Parallel Arrays n n Two (or more) arrays are called parallel if their corresponding components hold related information For example: int student. Id[50]; char course. Grade[50]; 27

End of lecture 20 Thank you! 28