Arrays

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Chapter 4

• Arrays

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1. Introduction

• Arrays are data structures consisting of related
  items of the same type.
• This chapter covers C-style pointer-based arrays.
• Arrays remain the same size throughout program
  execution.

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2. Arrays

• An array is a consecutive group of memory
  locations that all have the same name and the
  same type.
• To refer to a particular location or element in
  the array, we specify the name of the array and
  the position number of the element.
• The first element in every array is the zeroth
  element. (c0) The second is referred to as c1

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2. Arrays (cont.)

• The position number contained within square
  brackets is more formally called a subscript.
• A subscript must be an integer or an integer
  expression.
• If a program uses an expression as a subscript,
  then the expression is evaluated to determine the
  subscript.
• x c6/2

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3. Declaring Arrays

• To tell the compiler to reserve 12 elements for
  an integer array named c, use the declaration
  int c12
• Memory may be reserved for several arrays with a
  single declaration. char b100, x27

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4. Examples Using Arrays

• Using a for loop to initialize an array int
  i, n10 for(i0 ilt10 i)
  ni0
• for output in tabular format for(i0 ilt10
  i) cout ltlt setw(7) ltlt i ltlt
  setw(13) ltlt nI ltlt endl

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4. Examples Using Arrays (cont.)

• Using initializer lists.
• int n10 0
• if there are fewer initializers than elements,
  the rest are automatically initialized to zero.
• int n 1, 2, 3, 4, 5
• If the array size is omitted from the
  declaration, the number of elements in the array
  will be the number in the initializer list.

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4. Examples Using Arrays (cont.)

• Using initializer lists. (cont.)
• int n5 32, 27, 64, 18, 95, 14
• providing more initializers than there are
  elements in an array is a syntax error.

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4. Examples Using Arrays (cont.)

• Bounds Checking
• Referring to an element outside of the array
  bounds is an execution-time logic error. It is
  not a syntax error.
• The (normally serious) effects of referencing
  elements outside the array bounds are system
  dependent.

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4. Examples Using Arrays (cont.)

• Character Arrays
• A character array can be initialized using a
  string literal
• char string1 first
• Note first contains 5 characters plus the
  special NULL terminator.
• Input
• char string220
• cin gtgt string2

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5. Passing Arrays to Functions

• To pass an array argument to a function, specify
  the name of the array without any brackets.
• int hourlyTemperature24 modifyArray(hourlyTempe
  rature, 24)
• Arrays are not passed by value.

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5. Passing Arrays to Functions (cont.)

• For a function to receive an array, the
  functions parameters must specify that an array
  will be received.
• The function header would be void
  modifyArray(int b , int arraysize)
• The function prototype would be void
  modifyArray(int , int )
• Note variable names are ignored in prototypes.

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5. Passing Arrays to Functions (cont.)

• The const qualifier.
• There may be situations in your program in which
  a function should not be allowed to modify array
  elements.
• C provides the type qualifier const that can be
  used to prevent modification of array values.
• void tryToModify( const int )

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6. Sorting Arrays

• Sorting data is one of the most important
  computing applications.
• In this chapter we discuss a simple sorting
  scheme. In Chapter 15, we investigate more
  complex schemes that yield superior performance.

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6. Sorting Arrays (cont.)

• Swapping two array elements.
• Declare a temporary variable. int hold
• hold ai ai ai1 ai1
  hold

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6. Sorting Arrays (cont.)

• The code for(pass0 pass lt arraySize -1
  pass) for(i0 iltarraySize-1 i)
  if(aigtai1) hold ai
  ai ai1 ai1 hold

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7. Computing Mean, Median, and Mode Using Arrays

• The mean is the arithmetic average.
• add up the numbers in the array and divide by the
  number of elements.
• The median is the middle value.
• sort the array a.
• return asize/2
• The mode is the value that occurs most frequently.

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8. Linear Search and Binary Search.

• Often, a programmer will be working with large
  amounts of data stored in arrays. It may be
  necessary to determine whether an array contains
  a value that matches a certain key.
• The process of finding a particular element in an
  array is called searching.
• In this section we discuss two searching
  techniques linear search and binary search.

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8. Linear Search and Binary Search. (cont.)

• Linear Search
• works well for small arrays or unsorted arrays.
• Compares each element of the array with the
  search key.
• Since the array is not in any particular order,
  it is just as likely that the value will be found
  in the first element as the last.

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• Linear Search (cont.) int linSearch(const int
  array , int key, int size)
  int i for(i0 i lt size
  i) if(arrayi key) return i
  return -1

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8. Linear Search and Binary Search. (cont.)

• Binary Search
• If the array is sorted the Binary Search
  technique can be used.
• Locate the middle element and compare it to the
  key.
• if the key is less than the middle, search the
  first half.
• if the key is greater than the middle, search the
  second half.
• If the two are equal you are done.

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• Binary Search (cont.) int binSearch(int b ,
  int key, int low, int high, int size) int
  middle while(low lt high) middle
  (low high)/2 if (key bmiddle)
  return middle else if (key lt
  bmiddle) high middle -1 else low
  middle 1 return -1
  

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8. Linear Search and Binary Search. (cont.)

• Binary Search Performance
• In a worst case scenario, searching an array of
  1024 elements will take only 10 comparisons.
• Repeatedly dividing 1024 by 2 (because after each
  comparison we are able to eliminate half of the
  array.) yields 512, 256, 128, 64, 32, 16, 8, 4,
  2, and 1

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9. Multiple-Subscripted Arrays

• A common use of multiple-subscripted arrays is to
  represent tables of values consisting of
  information arranged in rows and columns.
• C compilers support at least 12 array
  subscripts.

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9. Multiple-Subscripted Arrays (cont.)

• Declaration
• int a23
• Declaration and Initialization.
• int b22 1,2, 3,4
• int array123 1,2,3, 4,5,6
• int array2231,2,3,4,5
• int array323 1,2, 4