Arrays

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Arrays

• Hanly - Chapter 7
• Friedman-Koffman - Chapter 9

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3/13/02

• Program 4 questions?
• revised test1 and my version of the program
• program should always process at least one
  customer
• Program 6 (and HW2) may be done by two people
  working together

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9.1 The Array Data Type

• Array collection of elements with a common name
• Entire array is referenced through the name
• Array elements are of the same type the base
  type
• Base type can be any fundamental,
  library-defined, or programmer -defined type
• For now arrays are static - they have a fixed
  number of elements

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Array Elements

• Individual elements of the array are referenced
  by sub_scripting the group name
• Subscripts are denoted as expressions within
  brackets
• The index type is integer and the index range
  must be 0 ... n-1
• where n is the number of elements in the array

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Array Declaration
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Sample Declarations

• Suppose
• const int N 20
• const int M 40
• const int MaxStringSize 80
• const int MaxListSize 1000

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Sample Declarations

• Then the following are all correct array
  declarations.
• int A10
• char BMaxStringSize
• float CMN
• int ValuesMaxListSize
• Rational DN-15

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Array Initialization

• An array can be initialized at the time it is
  declared
• char grades5 'A', 'B', 'C', 'D', 'F'
• int primes 1, 2, 3, 5, 7, 11, 13, 17, 19,
  23
• if values provided do not fill the array,
  remainder of elements will be 0
• double x10 1.5, 2.5, 3.5, 4.5, 5.5

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Subscripting

• Suppose
• int A10 // array of 10 ints
• To access an individual element we must apply a
  subscript to array name A
• A subscript is a bracketed expression
• The expression in the brackets is known as the
  index
• First element of A has index 0
• A0

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Subscripting

• Second element of A has index 1, and so on
• A1
• Last element has an index one less than the size
  of the array
• A9
• Incorrect indexing is a common error
• be careful not to index past the end of the array

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Array Elements

• Suppose
• int A10 // array of 10 uninitialized ints
• To access an individual element we must apply a
  subscript to array name A

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Array Element Manipulation

• Given the following
• int i 7, j 2, k 4
• A0 1
• Ai 5
• Aj Ai 3
• Aj1 Ai A0
• AAj 12

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Array Element Manipulation

• cin gtgt Ak // where the next input value is 3

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Array Processing

• None of the operators are defined for an array
• ltlt gtgt arithmetic comparison
• To do the same thing to every element in an
  array, use a loop

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Inputting Into An Array

• int AMaxListSize
• int n 0
• int CurrentInput
• while((n lt MaxListSize) (cin gtgt
  CurrentInput))
• An CurrentInput
• n

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Displaying An Array

• // List A of n elements has
• // already been set
• for (int i 0 i lt n i)
• cout ltlt Ai ltlt " "
• cout ltlt endl

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Partially filled arrays

• You don't have to use all the elements of an
  array
• don't always know how many elements you will need
• declare array big enough to handle all cases
• it is your job to keep track of how many elements
  are valid

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3/15/01

• HW 2 Planning for program 6
• you may do this in pairs
• look at P6 handout for ideas
• this is a planning exercise - you are not writing
  code
• P5 - start thinking about how to organize the
  main playing loop of a Yahtzee game

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Cheating

• making copies of other students programs is NOT
  appropriate behavior
• I will be comparing all Program 4 submissions to
  check for copying
• Acknowledge anyone that you get help from in your
  readme

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Access to Array Elements

• Sequential Access
• Process elements in sequential order starting
  with the first
• Random Access
• you can access elements in random order

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9.3 Array Elements

• Use lt, , gt, , - to test and modify array
  elements
• At times it might benefit you to pass an entire
  array to a function
• Can pass array elements to functions
• actual function call
• swap (s3, s5)

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swap.cpp

• // FILE Exchange.cpp
• // Exchanges two type float values
• void exchange (float a1, float a2)
• float temp
• temp a1
• a1 a2
• a2 temp

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Arrays as Function Arguments

• Arrays are always passed by reference
• Passing the array address
• contents can be changed
• Points to remember
• function definition needs only to indicate
  that the actual argument is an array
• You can use the reserved word const to prevent
  the contents of an array from being changed

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9.4 Reading Part of an Array

• Sometimes it is difficult to know how many
  elements will be in an array
• Scores example
• 150 students
• 200 students
• Always allocate enough space at compile time
• Remember to start with index 0

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9.5 Searching and Sorting Arrays

• Look at 2 common array problems
• Searching
• Sorting
• How do we go about finding the smallest number in
  an array?
• Assume 1st is smallest and save its position
• Look for one smaller
• If you locate one smaller save its position

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Linear Search

• The idea of a linear search is to walk through
  the entire until a target value is located
• If the target is not located some type of
  indicator needs to be returned

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Sorting in Ascending OrderSelection Sort

• Idea of the selection sort is to locate the
  smallest value in the array
• Then switch positions of this value and that in
  position 0
• We then increment the index and look again for
  the next smallest value and swap
• Continue until sorted

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Section 1 3/18/02
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character arrays

• arrays can have any base type
• char arrays are useful for storing text
• C used a char array for strings
• use null character to designate the end of the
  string
• C has a string class

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literal strings

• A literal string constant is a sequence of zero
  or more characters enclosed in double quotes
• "Are you aware?\n"
• Individual characters of string are stored in
  consecutive memory locations
• The null character ('\0') is appended to strings
  so that the compiler knows where in memory
  strings ends

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C-style strings

• array of char with a null character as the last
  element
• null character has ASCII code 0
• '\0'

operators and not defined for C-style strings
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Declaring and initializing

• Declaring uninitialized
• char cstr15
• Declaration with initialization
• char letters "abcde"
• char grades 'A', 'B', 'C', 'D', 'F', '\0'
• char notString 'A', 'B', 'C', 'D',
• In first case above, the null character is
  appended for you

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string.h
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length vs number of elements

• char word "word"
• word has 5 elements
• BUT strlen( word) 4

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assigning values

• NOT ALLOWED
• char a "stuff", b
• b a
• How do you do this?
• strcpy( b, a)
• Now b also contains "stuff"

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comparing C-style strings

• not defined for strings
• use strcmp
• strcmp( a, b)
• returns 0 if the strings are the same
• returns a number gt0 if a comes after b
  lexicographically
• returns a number lt 0 if a comes before b

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reading strings

• char a10
• cin gtgt a
• whitespace delimited
• to read multiple words, use getln
• cin.getline( a, 10)
• the number is the number of elements in the array

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concatenating C-style strings

• char str1"cat", str2"top"
• strcat(str2, str1)
• after this code has executed, str2 will contain
  "topcat"

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C string class

• library is string (no .h)
• declaration and initialization

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string Class

• include ltstringgt
• Various operations on strings are defined
• assignment with
• comparison with
• concatenation with ,
• length() function returns the string length
• c_str() returns a C-style string as const

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Remember

• Arrays are always passed by reference
• Can use const if array elements are not to be
  modified
• You do not need to include the array size within
  the brackets when defining an array parameter
• Initialize array with 0 or some other known value

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9.9 Common Programming Errors

• Watch non int subscripts (ASCII value)
• Enumerated types can be used
• Out of range errors
• C no range error checking
• Lack of subscript to gain access
• Subscript reference to non-array variable
• Type mixing when using with functions
• Initialization of arrays

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Common Programming Errors

• No prefix to reference a struct member
• Incorrect prefix reference to a struct member
• Missing following definition of struct
• Initialization of struct members

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