Programming Logic and Design Seventh Edition презентация

Programming Logic and Design Seventh Edition Chapter 6 Arrays

Objectives In this chapter, you will learn about: Storing data in arrays How an array can replace nested decisions Using constants with arrays Searching an array for an exact match Using parallel arrays

Objectives (continued) Searching an array for a range match Remaining within array bounds Using a for loop to process arrays

How Arrays Occupy Computer Memory Array A series or list of variables in computer memory All variables share the same name Each variable has a different subscript Subscript (or index) Position number of an item in an array Subscripts are always a sequence of integers

Each item has the same name and the same data type Each item has the same name and the same data type Element: an item in the array Array elements are contiguous in memory Size of the array: the number of elements it will hold Subscripts or indexes: indicate the position of a particular element in the array Adding data values is called populating the array

How Arrays Occupy Computer Memory (continued)

How an Array Can Replace Nested Decisions Example: Human Resources Department Dependents report List employees who have claimed zero through five dependents Assume no employee has more than five dependents Application produces counts for dependent categories Uses a series of decisions Application does not scale up to more dependents

How an Array Can Replace Nested Decisions (continued) The array reduces the number of statements needed Six dependent count accumulators are redefined as a single array Variable as a subscript to the array Array subscript variable must be: Numeric with no decimal places Initialized to 0 Incremented by 1 each time the logic passes through the loop

How an Array Can Replace Nested Decisions (continued)

How an Array Can Replace Nested Decisions (continued) Figure 6-7 Flowchart and pseudocode for Dependents report program (continued)

Using Constants with Arrays Use constants in several ways To hold the size of an array As the array element values As an array subscript

Using a Constant as the Size of an Array Avoid “magic numbers” (unnamed constants) Declare a named numeric constant to be used every time the array is accessed Make sure any subscript remains less than the constant value Constants are created automatically in many languages

Using Constants as Array Element Values Sometimes the values stored in arrays should be constants Example string MONTH[12] = "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"

Using a Constant as an Array Subscript Use a numeric constant as a subscript to an array Example Declare a named constant as: num INDIANA = 5 Display value with: output salesArray[INDIANA]

Searching an Array for an Exact Match Sometimes you must search through an entire array to find a value Example: mail-order business Item numbers are three-digit, non-consecutive numbers Customer orders an item; check if item number is valid Create an array that holds valid item numbers Search the array for an exact match

Searching an Array for an Exact Match (continued) Flag: a variable that indicates whether an event occurred 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

Using Parallel Arrays Example: mail-order business Two arrays, each with six elements Valid item numbers Valid item prices Each price in the valid item price array is in the same position as the corresponding item in the valid item number array Parallel arrays Each element in one array is associated with an element in the same relative position in the other array Look through the valid item array for the customer’s item When a match is found, get the price from the item price array

Using Parallel Arrays (continued)

Using Parallel Arrays (continued) Use 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 Indirect relationship Relationship between an item’s number and its price Parallel arrays are very useful

Improving Search Efficiency The program should stop searching the array when a match is found Set a variable to a specific value instead of letting normal processing set it Improves efficiency The larger the array, the better the improvement by doing an early exit

Improving Search Efficiency (continued)

Searching an Array for a Range Match Programmers may want to work with ranges of values in arrays, 1 through 5 or 20 through 30 Example: mail-order business Read the customer order data; determine the discount based on the quantity ordered First approach An array with as many elements as each possible order quantity Store the appropriate discount for each possible order quantity

Searching an Array for a Range Match (continued)

Searching an Array for a Range Match (continued) Drawbacks of first approach Requires a very large array; uses a lot of memory Stores the same value repeatedly How do you know when you have enough elements? Customer can always order more Better approach Create four discount array elements for each discount rate A parallel array with a discount range Use a loop to make comparisons

Searching an Array for a Range Match (continued)

Remaining within Array Bounds Every array has a finite size Number of elements in the array Number of bytes in the array Arrays are composed of elements of the same data type Elements of the same data type occupy the same number of bytes in memory The number of bytes in an array is always a multiple of the number of array elements Access data using a subscript containing a value that accesses memory occupied by the array

Remaining within Array Bounds (continued)

Remaining within Array Bounds (continued) Program logic assumes every number entered by the user is valid When an invalid subscript is used: Some languages stop execution and issue an error Other languages access a memory location outside of the array An invalid array subscript is a logical error Out of bounds: using a subscript that is not within the acceptable range for the array The program should prevent bounds errors

Using a for Loop to Process Arrays for loop: a single statement Initializes the loop control variable Compares it to a limit Alters it The 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 the array size

Using a for Loop to Process Arrays (continued)

Using a for Loop to Process Arrays (continued)

Summary Array: a named series or list of values in memory Same data type Different subscript Use a variable as a subscript to the array to replace multiple nested decisions Constants can be used to hold an array’s size Searching through an array requires Initializing a subscript Using a loop to test each element Setting a flag when a match is found

Summary (continued) Parallel arrays: each element in one array is associated with the element in a second array Elements have the same relative position For range comparisons, store either the low- or high-end value of each range

Summary (continued) Access data in an array Use a subscript containing a value that accesses memory occupied by the array A subscript is out of bounds if it is not within the defined range of acceptable subscripts The for loop is a convenient tool for working with arrays Process each element of an array from beginning to end