1

Arrays and Strings

2

Declaring arrays

scores : 85 79 92 57 68 80 . . . scores : 85 79 92 57 68 80 . . . 0 1 2 3 4 5 98 99

Inspecting arrays

Passing arrays as parameters

3

Design Problem

Consider a program to calculate class average

Why??Why??

?

4

Add to Design Problem

Now your client says, I need to ALSO calculate and display “deviations” from the average

Describe why this will or will NOT workDescribe why this will or will NOT work

5

Enter in the scores again Use 100 separate variables

» and cout and cin commands Read (then re-read) from a file The real answer …

Possible Solutions

Use arrays!!

6Simple vs Structured Data Types

Simple data type => data element contains a single value

Structured data type => a data element contains a collection of data values

x : 15x : 15 avg : 84.35avg : 84.35 ch : ‘A’ch : ‘A’

scores : 85 79 92 57 68 80scores : 85 79 92 57 68 80

name : ‘C’ ‘L’ ‘Y’ ‘D’ ‘E’name : ‘C’ ‘L’ ‘Y’ ‘D’ ‘E’

7

Arrays

Arrays are Structured Data Types They have a means of accessing

individual components Values can be retrieved from and stored

in the structure

scores : 85 79 92 57 68 80scores : 85 79 92 57 68 800 1 2 3 4 5

cout << scores[2];scores[0] = 100;cout << scores[2];scores[0] = 100;

8

One Dimensional Array

Structured collection of components» All of the same type

Structure given a single name Individual elements accessed by index

indicating relative position in collection Type of elements stored in an array can be

“just about” anything Index of an array must be an integer

9Use of Array for Our Problem

Store elements in array as read in Go back and access for deviations

Note declarationNote declaration

10

Declaring Arrays

Syntax: Data_type Array_name [constant];

Note declaration from our example

Tells how many elements set asideTells how many elements set aside

11

Declaring Arrays

Example specifies an array…» each element is an integer» there is space for 100 elements» the are numbered 0 through 99

scores : 85 79 92 57 68 80 . . . scores : 85 79 92 57 68 80 . . . 0 1 2 3 4 5 98 99

12Accessing Individual Components

Use the name of the array Followed by an integer expression

inside the square brackets [ ]

scores : 85 79 92 57 68 80 . . . scores : 85 79 92 57 68 80 . . . 0 1 2 3 4 5 98 99

max = scores[0];for (x = 0; x < 100; x++) if (scores[x] > max) max = scores[x];

max = scores[0];for (x = 0; x < 100; x++) if (scores[x] > max) max = scores[x];

Index can be:- constant- variable- expressionMUST be an integer

Index can be:- constant- variable- expressionMUST be an integer

13

Out of Bounds Index What happens if …

C++ does NOT check for index out of range Possible to walk off into “far reaches” of

memory -- clobbers ...» other variable locations» .exe code » the operating system (??)

float f_list [50];

f_list [100] = 123.456;

float f_list [50];

f_list [100] = 123.456;

14Initializing Arrays in Declarations

Possible to declare the size & initialize

Possible to omit size at declaration» Compiler figures out size of array

int results [5] = {14, 6, 23, 8, 12 }int results [5] = {14, 6, 23, 8, 12 }

float prices [ ] = { 2.41, 85.06, 19.95, 3.91 }float prices [ ] = { 2.41, 85.06, 19.95, 3.91 }

15

Aggregate Operations

Defn => an operation on the data structure as a whole» as opposed to operation on a SINGLE

element within the structure Example

» would be nice to read in a WHOLE array

16Lack of Aggregate Operations

Would be nice but . . . C++ does NOT have . . .

Assignment operator for whole array Arithmetic operations for whole array

(think matrix) Comparisons for arrays (not even = =) Return of an array type by a function

17How to Accomplish Aggregate Operations?

Most such tasks (assignment, read, write) can be performed some other way» CS II course will write “classes” to

provide these functions Otherwise

» these operations must be performed by the programmer

» element by element in a loop

18

Arrays as Parameters

This is one task that CAN be done to the WHOLE array

C++ always passes arrays by reference

19

Arrays as Parameters

The name of the array is a pointer constant

The address of the array is passed to the function

Size of thearray alsopassed tocontrol loop

20

Arrays as Parameters

Note the empty brackets in parameter list » A number can be placed here but it

will beignored

21

Sub-array Processing

Note we specified an array size of 100» but we don’t anticipate that many scores

Array always declared larger than needed Must keep track of how many have been

used» this is our limit when doing other things to

the array

22C-Strings or Character Arrays

We have learned that the elements of an array can be just about anything

Consider an array whose elements are all characters» Called a C-String» Has a collection of special routines» Treated differently for I/O than other

types of arrays

23

Declaration of C-Strings

Similar to declaration of any arraychar name[30];

// no initializationchar title [20] = "Le Grande Fromage"; // initialized at declaration

// with a stringchar chList [10] = {'a', 'b', 'c', 'd'}; // initialized with list of char

// values

24Working with Character Strings

String => a collection of characters interpreted as a single item» a structured data item» in C++ a null-terminated sequence of characters

stored in a char array All strings in C++ are terminated by the null character

» character 0,‘\0’

25

Initializing Strings

When a character array is declared, it is legal to use the assignment operator to initialize

Note : use of the = operator only legal for char array initialization

But : aggregate array assignment is NOT

greeting = “don’t do it;

26

String Output

Strings (character arrays) are handled differently than other types of arrays

This would NOT be allowed

This is legal:

int num_list [100];. . .cout << num_list;

int num_list [100];. . .cout << num_list;

char name [30] = “Snidly Q. Fizbane”; . . .

cout << name;

char name [30] = “Snidly Q. Fizbane”; . . .

cout << name;

27

String Input

Declare strings 1 element bigger than planned size to allow for ‘\0’

When input takes place, C++ automatically places the ‘\0’ in memory at the end of the characters typed in

28Problems with >> for String Input

Cannot be used to input a string with imbedded blanks

>> stops reading as soon as it encounters first whitespace character

29Problems with >> for String Input

Solve problem by using getline ( … )

Quits reading after 15 charactersor when it hits a newline,

whichever comes first.

Quits reading after 15 charactersor when it hits a newline,

whichever comes first.

Includes all charactersincluding spaces, tabs, etc

(whitespace characters)

Includes all charactersincluding spaces, tabs, etc

(whitespace characters)

30Problems with >> for String Input

If declared string is too small >> keeps putting characters in memory PAST that area in memory

s2 contents extendinto the memory

area of s3

s2 contents extendinto the memory

area of s3

31

Using Strings

Instead of “hard coding” file name for the open ( … ) command, » use a string variable, » use keyboard entry with cin.getline(…)» program more flexible, good for

different filesifstream inFile;char fname[31];cout << “Enter file name -> “;cin.getline (fname, 30, ‘\n’);inFile.open (fname);

ifstream inFile;char fname[31];cout << “Enter file name -> “;cin.getline (fname, 30, ‘\n’);inFile.open (fname);

32

String Library Routines Recall that we could not use the aggregate

assignment of one string to another C++ provides some string handling functions to

do this (and other similar tasks) Found in

<string.h>or <cstring>

33

Contrast/Compare Strings and C-Strings

Assignment is OKstring s;s = "hi mom";

Comparison OKif (s < "geek") …

I/O allowedcin >> s;cin.getline(s,'\n');cout << s;

Assignment is illegalchar cs[30];cs = "don't do it";

Comparisons not allowed I/O allowed much the

same way

34

Working with C-Strings

Functions provided in #include <cstring>

Used instead of assignment

Used for comparisons

35

Another Problem

Some functions require C-strings as parameters» The .open() command for files

C-strings are terminated by the null character (character 0)» Such functions are looking for that

String objects are built differently

36Solving the File Open Problem

One of the functions available for a string object will convert it to a C-String

The function is c_str() Remember that string functions are called by

using» The variable» The member operator var.c_str()» The name of the function

View example

37

Design Problem

Consider the task of keeping track of data about parts for manufacture» part number,

description, qty needed, unit price

38

Design Problem

Use “Parallel” arrays One array each for

part num, descrip, qty, price

nth item in any one of the arrays associated with same nth item of all the arrays

part # descrip qty price

A100 xxx 5 1.23

B25 yyy 23 8.95

0

1

2

39Testing and Debugging Hints

Range of legal index values is 0 to array_size - 1 Individual elements of the array are of the

component type No aggregate operations in arrays

» you must write the code to do this If array parameter is incoming, specify formal

parameter as const » prevents function from modifying

40Testing and Debugging Hints

Omitting array size in declaration» when array declared formal parameter» when array initialized at declaration

Don’t pass component when function expects entire array

Declare array size as max ever needed» process only part of array which is used

Pass array name and length to functions which process array or sub array

41

Testing and Debugging

Be sure to account for null character when you manipulate characters individually in a string

Remember proper use of the = » correct for initialization at declarationtime» INCORRECT for aggregate assignment

Aggregate input/output allowed for strings but NOT for other array types

42

Testing and Debugging If you use the >> for string input, make sure

» string is declared large enough» string will have no white spaces

The >> operator stops at, but does not consume the first trailing white space» such as ‘\n’ or a space

The cin.getline (whatever, 30, ‘\n’ ) function » stops when reading the ‘\n’» consumes the ‘\n’» has problems when ‘\n’ is still in the input stream

43

Testing and Debugging

When using the strcpy ( ), make sure that the destination array is declared long enough

Choose test data carefully for string handling programs» include strings that are too large» include strings with whitespace