1. Arrays and Strings
Chapter 9

2. "All students to receive arrays!" reports Dr. Austin.
Declaring arrays
Passing arrays as parameters
Stepping through arrays
Inspecting arrays
scores :

3. Design Problem
Consider a program to calculate class average
Why?? ?

4. Describe why this will or will NOT work
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 work

5. Possible Solutions    Enter in the scores again
Use 100 separate variables
and cout and cin commands
Read (then re-read) from a file
The real answer …  
Use arrays!!

6. Simple 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 : 15
avg : 84.35
ch : ‘A’
scores :
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
cout << scores[2]; scores[0] = 100;
scores :

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

9. Use of Array for Our Problem
Store elements in array as read in
Go back and access for deviations
Note declaration

10. Tells how many elements set aside
Declaring Arrays
Syntax: Data_type Array_name [constant];
Note declaration from our example
Tells 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 :

12. Accessing Individual Components
Use the name of the array
Followed by an integer expression inside the square brackets [ ]
scores :
Index can be: - constant - variable - expression MUST be an integer
max = scores[0]; for (x = 0; x < 100; x++) if (scores[x] > max) max = scores[x];

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] = ;

14. Initializing 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 }
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

16. Lack 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

17. How 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 the array also passed to control loop

20. Arrays as Parameters Note the empty brackets in parameter list
A number can be placed here but it will be ignored

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

22. C-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 array char name[30]; // no initialization char title [20] = "Le Grande Fromage"; // initialized at declaration // with a string char chList [10] = {'a', 'b', 'c', 'd'}; // initialized with list of char // values

24. Working 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. greeting = “don’t do it;
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;
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

28. Problems with >> for String Input
Cannot be used to input a string with imbedded blanks
>> stops reading as soon as it encounters first whitespace character

29. Problems with >> for String Input
Solve problem by using getline ( … )
Quits reading after 15 characters or when it hits a newline, whichever comes first.
Includes all characters including spaces, tabs, etc (whitespace characters)

30. Problems with >> for String Input
If declared string is too small >> keeps putting characters in memory PAST that area in memory
s2 contents extend into 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 files
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 OK string s; s = "hi mom";
Comparison OK if (s < "geek") …
I/O allowed cin >> s; cin.getline(s,'\n'); cout << s;
Assignment is illegal char 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

36. Solving 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 1 2

39. Testing 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

40. Testing 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