Chapter 5 Arrays Lists and Tables Extended Prelude

Chapter 5: Arrays: Lists and Tables Extended Prelude to Programming Concepts & Design, 3/e by Stewart Venit and Elizabeth Drake Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

5. 1 One-Dimensional Arrays • A one-dimensional array is a list of related data of the same data type, referenced by the same name. • An array is composed of elements, each of which is referenced by the array name and the element’s position in the array. • Arrays are stored in consecutive memory locations. • The position of an element in an array is called a subscript or an index. – In the text example, Scores[3], Scores is the array name and 3 is the subscript. 5 -2

Declaring Arrays • When an array is declared, the computer sets aside memory to store the values of the elements of the array. • Declaring an array means telling the computer what is the data type of the array and how many elements will be stored in it. • Pseudocode to declare arrays: Declare Height[5] Of Integers declares an array with 5 elements, all of which are integers • Note: the first element of an array has subscipt 0 Address: Height[0] Height[1] Height[2] Height[3] Content: 5 8 13 22 Height[4] 39 5 -3

Filling (loading) an Array Here is how to load an array with 52 values. The values represent a salesperson’s sales for each week of the year. Declare Weekly. Sales[52] Of Reals Declare K As Integer For K = 0 Step 1 to 51 Write “Enter sales for week #”, K + 1 Input Weekly. Sales[K] End For Note that the sales for Week 1 correspond to the element Weekly. Sales[0] and the sales for Week 52 correspond to the element Weekly. Sales[51]. 5 -4

Parallel Arrays • Parallel arrays can be used when array variables are related to each other by position, or subscript. • Tip: It is necessary that the data in each array be loaded in the correct order. • If we have arrays Number. Grade and Letter. Grade (as below), each element of Number. Grade is related to an element in Letter. Grade by the position in the array. Number. Grade: Letter. Grade: 90 “A” 80 ”B” 70 ”C” 60 ”D” 50 ”F” 5 -5

Why Use Arrays? • Arrays reduce the number of variable names in the program. Sales[K] versus Sales 1, Sales 2, Sales. N, …. • Arrays improve efficiency by allowing data to be read into the program once but processed as many times as necessary. • Arrays increase the flexibility of the program. • Arrays reduce the number of If statements needed in selection processing. If If Sales[K] Then rather than Sales 1 Then … Sales 2 Then … Sales 3 Then … 5 -6

5. 2 Searching and Sorting Arrays • If we have multiple, related arrays, with the elements ordered correctly, we can find values in one array based on a search key value in another array. • A search key is a known value for one of the arrays. In the text example, a specific value of ‘Flight’ can be used to find the Origin, Time, and Gate of that flight. It can be used as a search key. • This is called a table lookup or serial search. 5 -7

The Serial Search • 3 steps: – Load, or populate, the arrays. – Search the array that contains the key to find a match. – Display the key and corresponding values in the non-key arrays with the same subscript as the one with the key value, if a match is found. – Display an ‘unsuccessful search’ message if no matches are found. 5 -8

The Serial Search (continued) • Use a flag, or switch, to decide which message to print. • A flag is a variable that is set to zero or one, depending on the results of an operation. • The value of the flag can be checked later in the program with an If statement to determine which action to take. 5 -9

The Serial Search (continued) Pseudocode for the serial search of an array named Key. Data containing N elements: Set the subscript (here named Index) of the current table key equal to 0 Set a flag (here named Found) equal to 0 While (Found = 0) AND (Index < N) If Key. Data[Index] = Key Then Set Found = 1 End If Set Index = Index + 1 End While If Found = 1 Then Display array element with subscript Index – 1 Else Display an “unsuccessful search” message End If 5 -10

The Bubble Sort Technique • To sort data means to arrange it in a particular numerical or alphabetical order, ascending or descending. • To sort small amounts of data, use the bubble sort technique. – Make several passes through the data. – Compare adjacent pairs of data. – If the pairs are not in order, swap them. – Continue until all data is processed. 5 -11

Swapping Values To swap two values, you must store one of them in a temporary storage location while you make the swap. To interchange array elements Array[K] and Array[K+1], the pseudocode for the swap looks like this: Set Temp = Array[K] Set Array[K] = Array[K+1] Set Array[K+1] = Temp Walk through the swap routine with a few values to see how it works. For example, if Array[K] = 6 and Array[K +1] = 18 at first, after the swap, Array[K] will contain the value 18 and Array[K+1] will contain the value 6. What will be the value in Temp? 5 -12

5. 3 Other Uses of Arrays • String Processing: – Some languages do not contain string data types, but implement strings as an array of character data type. – Declare the data type of an array in your program: • Declare Profit[50] Of Integers • Declare Month[10] Of Characters – Array size is the number of elements in an array. – String length is the number of characters of a particular element of a character array. 5 -13

String Length and the Length Function • The length of a string is the number of characters it contains. • The Length function returns the number of characters in a given string. • For example: Declare Str[10] Of Characters Set Str = “Hello!” Write Length(Str) • The output will be 6 because the string “Hello!” is made up of 6 characters. 5 -14

5. 4 Two-Dimensional Arrays • In a two-dimensional array, the value of an element depends on two factors instead of just one. • Here is how to declare a two-dimensional array: Declare Profit[12, 24] Of Integers • A two-dimensional array is like a matrix or an electronic spreadsheet. – Two factors can be, for example, Row and Column or Test Number and Student. – In the example in the text, Boynton’s (Student #2) scores can be referenced as: Score[1, 0], Score[1, 1], Score[1, 2], Score[1, 3], Score[1, 4]. – Notice the ‘ 1’ subscript for Boynton remains constant, while the subscript for Test changes. 5 -15

Using Two-Dimensional Arrays • Use with nested loops to read in multiple values, multiple times. • In the text example, for each student input, we load 5 tests scores. • This allows us to process multiple tests for multiple students. • Can use in parallel array processing • Some languages allow for processing arrays of more than two dimensions. These are called higher-dimensional arrays. 5 -16

Nested Loops Help Load Arrays To input data into a two-dimensional array, use a nested loop. A two-dimensional array named Working contains the names of 20 work-study students in one department and the hours worked in each of the 15 weeks of the semester. The array is declared as follows (below). It is assumed that the students are identified by number (1 through 20). The pseudocode to load this array is shown on the next slide. Declare Working[20, 15] 5 -17

Nested Loops Help Load Arrays (continued) Declare Working[20, 15] For Student = 0 Step 1 to 19 Write “Enter hours worked each week” Write “for student # “ , Student + 1 For Week = 0 Step 1 to 14 Input Working[Student, Week] End For(Week) End For(Student) 5 -18

Pseudocode Language (Ch 5) Declare a one-dimensional array: Declare This. Array[N] Of Integers Declare That. Array[N] Of Reals Declare String. Array[N] Of Characters Declare a two-dimensional array: Declare This. Array[X, Y] declares an array with X rows and Y columns 5 -19