pld7e_ch06

Report
Arrays
Objectives
In this chapter, you will learn about:
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Arrays and how they occupy computer memory
Using an array to replace nested decisions
Using constants with arrays
Searching an array
Using parallel arrays
Searching an array for a range match
Remaining within array bounds (bounds checking)
Using a for loop to process arrays
Understanding Arrays and How They Occupy
Computer Memory
• Array
– Conceptually – a collection of variables in computer memory
• an array is an aggregate data type
• the term element is used instead of variable
• all elements share the same name (the array name)
• You access an element in the array using a subscript
• each element has the same data type
num weeklyTotals[6]
valid subscripts for weekly Totals is 0 .. 5
• Subscript (aka index and offset)
– Position number of an item in an array
– May be a named constant, literal, variable, or in general, an expression that evaluates to
an integer.
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How Arrays Occupy Computer
Memory
• Array elements are stored in contiguous memory
• The size of an array is the number of elements it will
hold
• For most languages:
– the value of the smallest subscript is 0
– the value of the largest subscript is array size – 1
(0 based indexing)
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How Arrays Occupy Computer
Memory (continued)
Figure 6-1 Appearance of a three-element array in computer
memory
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Declaring Arrays
• Array Declaration
–
<data_type> <array_name> [ <number_of_elements> ]
–
num someVals[3]
//pseudocode
• data type is num
• name is someVals
• size of the array is 3 elements
• valid subscripts are: 0, 1, 2
• Languages have differences in syntax for arrays
– some languages use ( ) instead of [ ]
– style of declaration can be quite different
• Java array declaration:
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int someVals[] = new int[3];
Arrays in RAPTOR
The index in a RAPTOR array starts at 1!
Arrays in RAPTOR are easily expanded
SET counter[5]
TO 0
indices are: 1, 2, 3, 4, 5
SET counter[7]
TO 0
indices are: 1, 2, 3, 4, 5, 6, 7
SET counter[100] TO 0
Programming Logic and Design, Seventh Edition
indices are: 1, 2, 3, … , 100
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Using Arrays
• Given this pseudocode declaration: int someVals[3]
– valid references are:
someVals[0], someVals[1], someVals[2]
– invalid references:
someVals[-1], someVals[3]
• would usually generate an error message such as:
– Java: ArrayIndexOutOfBoundsException
– A subscript used in an array reference can be any integer
expression
• variable, literal, named constant, calculation
An array element can be used in any statement in which a variable
of the same type can be used.
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Using an Array to
Replace Nested If Decisions
• Example:
Human Resources - Department Dependents report
– Count employees who have claimed zero through five
dependents ( 0 – 5 ) to get count for each: count[0] ..
count[5]
• Assume no employee has more than five dependents
• Application produces counts for dependent
categories by using a series of decisions.
• Application does not scale easily to a different range
of dependents
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Variable dep may
have the values
0 – 5.
Test dep to
determine which
counter to
increment.
6 variables had to
be declared to
implement this
solution.
Figure 6-3
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Flowchart and pseudocode of decision-making process using a series of decisions
Using an Array to
Replace Nested Decisions (continued)
• Replacing the nested-if logic with array logic reduces
the number of statements needed
• Six dependent count accumulators can be redefined as
a single array: int count[6] //valid subscripts are 0 - 5
• The array elements should be initialized to zero since they will be used as
counters.
• Use the variable dep as a subscript for the array:
count[dep]
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The comma separated list of values
here is called an "initializer list".
This declaration could be written in Java
as:
int count[] = new int[6];
All of the elements in the array would
automatically be initialized to zero in
Java.
This approach has not simplified the
logic because nested-if's are still being
used.
All of this nested-if logic can be replaced
by a single assignment statement.
Figure 6-4 Flowchart and pseudocode of decision-making Version 2
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Still not there yet…
Can you see the
pattern
developing?
Replace the
integer literal
subscript with the
variable dep
Figure 6-5
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Flowchart and pseudocode of decision-making process
Version 3
Using an Array to Replace Nested Decisions
(continued)
Aha!
One statement!
Figure 6-6
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Flowchart and pseudocode of efficient decision-making process using an array
Named constant
What would this
loop look like in
RAPTOR?
Figure 6-7
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Flowchart and pseudocode for Dependents Report program
Final Version
Using an Array to Replace Nested Decisions
(continued)
“mainline” logic
main()
Sentinelcontrolled loop.
Notice the module
names:
main()
getReady()
countDependents()
finishUp()
Counter-controlled
loop.
Should have used a
for loop.
Figure 6-7 Flowchart and pseudocode for Dependents Final Version
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Using Constants with Arrays
• Remember, constants are
– named constants
– literals
• Use the constants in several ways
– array size
– array value
– array subscript
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Using a Constant as the Size of an Array
•
Avoid “magic numbers” (unnamed constants)
– magic numbers are literals…
•
Declare a numeric named constant to be used:
– in the declaration of the array
– to determine array size in other parts of your code
•
Make sure any subscript remains less than the constant value
•
Many languages provide a mechanism for determining the size of an array
without needing to refer to a named constant.
– Java has a public field called length
– The expression myArray.length would return the number of elements in the array myArray
– RAPTOR provides the Length_of procedure
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Length_of(counters)
Using Constants as Array Element Values
• Sometimes the values stored in arrays should be
constants
• int ARRAY_SIZE = 12
string MONTH[ARRAY_SIZE] = //initializer list
"January",
"February",
"March",
"April",
"May",
"June",
"July",
"August",
"September",
"October",
"November",
"December"
• Valid subscripts are 0 – 11
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[ RAPTOR 1 – 12 ]
Using Constants as Array Element Values
•
Here is a modified version of the previous example which makes it possible for
use to use the array more naturally.
•
num ARRAY_SIZE = 13 //this allows us to ignore element 0
//instead of 0 – 11, now we can use 1 – 12
string MONTH[ARRAY_SIZE] =
"",
"January",
"April",
"July",
"October",
"February",
"May",
"August",
"November",
"March",
"June",
"September",
"December"
•
Valid subscripts are 0 – 12. Subscripts actually used are 1 – 12. Ignore element 0
•
Element 0 is unused, however, note that we still need to give it a value!
– Small price to pay for the simplified functionality
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Using a Constant as an Array Subscript
• Use a named constant as a subscript in an array
• Example
– Declare a named constant as
• int INDIANA = 5
– Display value with:
output salesArray[INDIANA]
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Searching an Array
• Sometimes you must search through an array to find a
value or determine if the value is even in the array
• Example: mail-order business
– Item numbers are three-digit, non-consecutive numbers
– Customer orders an item
– Program needs to check if item number is valid
• Solution:
– Create an array that holds valid item numbers
– Search array for exact match
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The list of values
to be assigned to
the array
elements is called
an initializer list.
String foundIt is
being used as a
flag (boolean
variable).
"Constant Array"
Most languages won't let
you create a constant
array.
Declaration for array
VALID_ITEM in Java
would look more like this:
int validItem[ ] =
{ 106, 108, 307,
405, 457, 688 };
Figure 6-8
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Flowchart and pseudocode for program that verifies item availability
Rest of program is on the next page
String foundIt is being used as a flag
better choice is to use a boolean variable
boolean foundIt = false;
foundIt = true;
Figure 6-8 Flowchart and pseudocode for program that verifies item availability
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foundIt = "N"
for sub = 0 to SIZE-1
if item = VALID_ITEM[sub] then
foundIt = "Y"
endif
endfor
Using boolean variable foundIt in
Java
foundIt = false;
for ( sub = 0; sub < SIZE; sub++ )
if ( item == validItem[sub] )
foundIt = true;
Figure 6-8 Flowchart and pseudocode for program that verifies item availability
Programming Logic & Design, Sixth Edition
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Searching an Array (continued)
• Flag
– variable that indicates whether an event occurred
– declared as a string in the book:
String foundIt = "N";
– usually declared as a boolean or integer variable in most languages
• int
found_it = 0;
//0 for false, 1 for true
• boolean
found_it = false;
//Java
• string found_it = “Y”
//PLD textbook
• SET
//RAPTOR
found_it
TO
True
• Technique for searching an array
– Set a subscript variable to 0 to start at the first element
– Initialize a flag variable to false to indicate the desired value has not been found
– Examine each element in the array
– If the value matches, set the flag to true
– If the value does not match, increment the subscript and examine the next array element
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An example is given later for searching an array
Using Parallel Arrays
•
Example: mail-order business
– Two arrays, each with the same number of elements
•
Valid item numbers
•
Valid item prices
– Each price in valid item price array in same position as corresponding item in valid item number
array
•
Parallel arrays
– Each element in one array associated with an element in the same relative position in other array
– Do a search on one array and find an associated value in another array
– all arrays should be the same size!
•
Look through valid item array for customer item
– When match is found, get price from item price array
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Figure 6-9
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Parallel arrays in memory
Using Parallel Arrays
• Parallel arrays
– Two or more arrays contain related data
– A subscript relates the arrays
• Elements at the same position in each array are logically related
– A similar approach is to have an array of structures or
objects ( advanced topic )
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Figure 6-10
Flowchart and pseudocode for a program that finds an item’s price using parallel arrays
Program continues on next slide
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This search logic
works correctly
but is inefficient
because it
doesn't stop
searching if a
match is found.
A more efficient
approach is
shown later.
Figure 6-10 Flowchart and pseudocode of program that finds an item’s price
using parallel arrays (continued)
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Figure 6-10
Flowchart and pseudocode of program that finds an item’s price using parallel arrays
Programming Logic & Design, Sixth Edition
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Improving Search Efficiency
• Program should stop searching the array when a
match is found
• Use a flag (boolean variable) as a second condition
for the search criteria ( AND logic )
• Improves efficiency
• The larger the array, the better the improvement by
doing an early exit
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Pseudocode to search an array for a value
boolean foundIt = false
int
sub = 0 // item and price are defined elsewhere
while foundIt = false and sub < SIZE
if valid_item[sub] = item then
foundIt = true
price
= valid_price[sub]
endif
endwhile
if foundIt = true
print "The price of the item is “ + price
else
print "Item was not found in the array valid_item."
badItemCount += 1 //defined elsewhere
endif
Figure 6-11
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Flowchart and pseudocode of the module that finds item
price, exiting the loop as soon as it is found
Improving Search Efficiency (continued)
Figure 6-11 Flowchart and pseudocode of the module that finds item price,
exiting the loop as soon as it is found (continued)
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Searching an Array for a Range
Match
• Sometimes programmers want to work with ranges
of values in arrays
• Example: mail-order business
– Read customer order data
– determine discount based on quantity ordered
• First approach
– Array with as many elements as each possible order quantity
– Store appropriate discount for each possible order quantity
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Searching an Array for a Range
Match (continued)
Max 76 items
Quantity
Rate
0–8
0%
9 – 12
10%
13 – 25
15%
>= 26
20%
Figure 6-13
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Usable—but inefficient—discount array
Searching an Array for a Range Match (continued)
• Drawbacks of first approach
– Requires very large array; uses a lot of memory
– Stores same value repeatedly
– How do you know you have enough elements?
• Customer can always order more
• Better approach
– Create four discount array elements for each discount rate
– Parallel array with discount range
• Use loop to make comparisons
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Searching an Array for a Range Match
(continued)
Figure 6-14
Parallel arrays to use for determining discount
For range comparisons, store
either the low- or high-end value
of each range.
quantity
-------0 - 8
9 - 12
13 - 25
26 or more
discount rate
------------no discount
10%
15%
20%
subscript
--------0
1
2
3
Set value of subscript initially to array size – 1 ( 3 )
Use a loop to determine what subscript to use to access the
discount rate in the discount array.
This logic is more challenging than the if-then-else or switch
logic but is very flexible
subscript = 3
This example is checking the low
end of each range.
loop:
while quantity < quan_limit[subscript]
//26, 13, 9, 0
subtract 1 from the subscript
end while
When you exit the loop, use the value of the
subscript to access the discount rate from the
discount array.
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range check
Checking from
higher to lower
rates
Figure 6-15
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Program that determines discount rate
Array Size in Memory
• Every array has a finite size
– There are a specific number of elements in the array
– Each element uses some number of bytes (1,2,4,8,etc)
• The number of bytes in an array is always a multiple of number of array
elements
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Remaining within Array Bounds
Figure 6-16
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Determining the month string from the user’s numeric entry
Remaining within Array Bounds
• Program logic assumes every number entered by the user is
valid
• If an invalid subscript is used:
– Some languages stop execution and issue an error
– Other languages access a memory location outside of the array
– Java generates an exception ArrayIndexOutOfBoundsException
• Attempting to use an invalid array subscript is a logic error
• Out of bounds
– using a subscript that is not within the acceptable range for the array
• The Program should prevent bounds errors from occuring.
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Using a for Loop to Process
Arrays
• for loop
–
–
–
–
single statement
Initializes loop control variable
Compares it to a limit
Alters it
• A for loop is especially convenient when working with arrays
– To process every element
• Must stay within array bounds
• Highest usable subscript is one less than array size.
• Java gives us a field we can use in an expression
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Using a for Loop to Process Arrays (continued)
Figure 6-17
Pseudocode that uses a for loop to display an array of department names
Java for loops to process array elements:
Standard for loop:
for ( int dep = 0; dep < depts.length; dep++ )
System.out.println( depts[dep] );
Enhanced for loop:
for ( int dep : depts )
System.out.println( dep );
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Summary
• Array
–
–
–
–
series or list of variables in memory
common name
common type
different subscripts
• Use a variable as a subscript to the array to replace
multiple nested decisions
• Some array values determined during program
execution
– Other arrays have hard-coded values (constant array)
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Summary (continued)
• Search an array
– Initialize the subscript
– Test each array element value in a loop
– Set a flag when a match is found
• Parallel arrays
– each element in one array is associated with the element in
second array
– Elements have same relative position
• For range comparisons, store either the low- or highend value of each range
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Summary (continued)
• Access data in an array
– Use subscript containing a value that accesses memory
occupied by the array
• Subscript is out of bounds if not within defined
range of acceptable subscripts
• for loop is a convenient tool for working with
arrays
– Process each element of an array from beginning to end
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Java Arrays
int evenNumbers[] = { 2, 4, 6, 8, 10 };
2
evenNumbers
4
6
8
length = 5
Object of type int[ ]
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10

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