Understanding Arrays and How They Occupy Computer Memory

• Array– A sequenced collection of elements of the same

data type is called array– Series or list of variables in computer memory– All variables share the same name but with different

index– Each variable has a different subscript

• Element– Each item in array is called element.

Array 1

How Arrays Occupy Computer Memory

• Subscript (or index)– Position number of an item in an array– Subscripts are always a sequence of integers

• Array elements are contiguous in memory

• Size of the array: number of elements it will hold

Array 2

How Arrays Occupy Computer Memory (continued)

Array 3

Figure 6-1 Appearance of a three-element array in computer memory

How Arrays Occupy Computer Memory (continued)

• All elements have same group name– Individual elements have unique subscript– Subscript indicates distance from first element– Subscripts are a sequence of integers

• Subscripts placed in parentheses or brackets following group name

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Declaration & Initialization of Array

• Syntax in C to declare an arraydatatype Name[size];

– Exampleint subjectMark[7];

• In C language index start with the value 0

– int count[3];• It will declare 6 variable named with count and can be

accessed by count[0],count[1],count[2]

• Example of declaration with initialization– int count[6]={0};

• It will initialize the each variable of the array with the value 0

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Declaration & Initialization of Array(continued)

– int count[6]={1,2};• It will initialize the first element with the value 1 which

have the index 0;

– Example: int num[3]; • First value is stored on index 0• scanf(“%d”,&num[0]);

– String is a series of characters treated as a unit.– char name[4]=“FOP”;

• Name is a string having size 4

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F O P \0

Manipulating an Array to 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 series of decisions

• Application does not scale up to more dependents

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Array 8

Figure 6-3 Flowchart and pseudocode of decision-making process using a series of decisions—the hard way

Manipulating an Array to Replace Nested Decisions (continued)

• Array reduces number of statements needed

• Six dependent count accumulators redefined as 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

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Array 10

Figure 6-4 Flowchart and pseudocode of decision-making process—but still the hard way

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Figure 6-5 Flowchart and pseudocode of decision-making process using an array—but still a hard way

Code Replacing the nested structureint main(){

int dep,i,count[6]={0};

printf(“Give the Number of dependent:”);

scanf(“%d”,&dep);//Prime read

while(dep>=0&&dep<6)

{

count[dep]=count[dep]+1;

printf(“Give the Number of dependent:”);

scanf(“%d”,&dep);

}

for(i=0;i<6;i++)

{

printf(“No of employee having %d child=%d”,i,count[i]);

}return 0;

}Array 12

Variable & Constant Arrays

• When the value of the array element is changed during the execution time is called variable array.

• When the array value is assigned at the coding time which is permanent and final is called constant array.

• Example:

– int FloorRent[4]={350,400,600,100};

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Floor Rent

0 350

1 400

2 600

3 1000

//code to make a list with rent. Single line comment

int main(){

char name[20];

int floor;

const intFloorRent[4]={350,400,600,100};

printf(“Give the floor number & name”);

scanf(“%d”,&floor);

scanf(“%s”,name);

while(floor>=0 && floor<4)

{

printf(“Name:%s,Rent=%d”,Name,FloorRent[floor]);

printf(“Give the floor number & name”);

scanf(“%d”,&floor);

scanf(“%s”,name);

}

return 0;

}Array 14

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]

Array 15

Searching an Array

• Sometimes must search through an array to find a value

• Example:– Class numbers are three-digit, non-consecutive

numbers– Faculty enters class number, check if class number is

valid– Create an array that holds valid class numbers– Search array for exact match

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int main(){

int class,i=0,flag=0,ClassCode[5]={102,203,101,203,104};printf(“Enter class number:”);scanf(“%d”,&class);while(i<5){

if(ClassCode[i]==class){

flag=1;}i++;

}if(flag==1)

printf(“Correct class”);else

printf(“Incorrect class”);} Array 17

Searching an Array (continued)

• Flag: 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

Array 18

Using Parallel Arrays

• Example: studentId-mark – Two arrays, each with sixty elements

• Valid student id [1,2,3,…,60]

• Valid marks [ 0-50 ]

– 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 element in

same relative position in other array

• Look through valid item array for customer item– When match is found, get price from item price array

Array 19

Array 20

Figure 6-9 Parallel arrays

studId 1 2 3 4 5

Marks 20 44 33 22 11

Using Parallel Arrays

• 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

Array 21

/*Program to store student data in parallel array. Here studId[5],marks[5] are parallel array.

Array using for loop.

*/

int main()

{

int studId[5],marks[5],i;

for(i=0;i<5;i++)

{

scanf(“%d”,&studId[i]);

scanf(“%d”,&marks[i]);

}

return 0;

}Array 22

Improving Search Efficiency

• Program should stop searching the array when a match is found

• The larger the array, the better the improvement by doing an early exit

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int main(){

int class,i=0,flag=0,ClassCode[5]={102,203,101,203,104};printf(“Enter class number:”);scanf(“%d”,&class);while(i<5 && flag==0){

if(ClassCode[i]==class){

flag=1;}i++;

}if(flag==1)

printf(“Correct class”);else

printf(“Incorrect class”);}

Array 24

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

Array 25

Searching an Array for a Range Match (continued)

Array 26

Figure 6-13 Usable—but inefficient—discount array

Searching an Array for a Range Match (continued)

• Drawbacks of first approach– Requires very large array with the use a lot of

memory– Stores same value repeatedly– Difficult to know the no of elements

• Customer can order more

• Better approach– Create four discount array elements for each

discount rate– Parallel array with discount range

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Searching an Array for a Range Match (continued)

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Figure 6-14 Parallel arrays to use for determining discount

Psuedo code determines discount ratestart

num quantity,x

num SIZE=4

num Discount[SIZE]=0,0.10,0.15,0.20

num Discount_Range[SIZE]=0,9,13,26

get quantity

x=SIZE-1

while quantity<Discount_Range[x]

x=x-1

endwhile

print “Discount rate:”,Discount[x]

stopArray 29

Remaining within Array Bounds

• Every array has finite size– Number of elements in the array– Number of bytes in the array

• Arrays composed of elements of same data type

• Elements of same data type occupy same number of bytes in memory

• Number of bytes in an array is always a multiple of number of array elements

• Access data using subscript containing a value that accesses memory occupied by the array

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Array 31

Figure : Determining the month string from user’s numeric entry

Remaining within Array Bounds (continued)

• Program logic assumes every number entered by the user is valid

• When invalid subscript is used:– Some languages stop execution and issue an error– Other languages access a memory location outside

of the array• Invalid array subscript is a logical error• Out of bounds: using a subscript that is not within

the acceptable range for the array• Program should prevent bounds errors

Array 32

startnum monthnum Max_Month=12String Month_Name[Max_Month]=“January”, February”, ”March”, ”April”, ”May”, ”June”, ”July”, ”August”, September”, ”October”, ”November”, ”December”get monthif month>=1 AND month<=Max_Month then

month=month-1print Month_Name[month]

elseprint “Invalid month”

endifstop

Array 33

startnum monthnum Max_Month=12String Month_Name[Max_Month]=“January”, February”, ”March”, ”April”, ”May”, ”June”, ”July”, ”August”, September”, ”October”, ”November”, ”December”get monthwhile month<1 OR month>Max_Month

print “Invalid month”get month

endwhilemonth=month-1print Month_Name[month]

stop

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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

• for loop especially convenient when working with arrays– To process every element

• Must stay within array bounds

• Highest usable subscript is one less than array size

Array 35

Summary

• Array: series or list of variables in memory– Same name and type– Different subscript

• 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

Array 36

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 high-end value of each range

Array 37

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

Array 38