arrays chapter 12. overview arrays and their properties creating arrays accessing array elements...

ArraysArrays

Chapter 12Chapter 12

OverviewOverview

• Arrays and their properties• Creating arrays• Accessing array elements• Modifying array elements• Loops and arrays

Arrays and Their PropertiesArrays and Their Properties

• Hold several values of the same type

• Based on a slot number (the index number)

• Instant access• Linear (one after the other)• Static – once their size is set, it’s

set…

Arrays Hold Multiple Arrays Hold Multiple ValuesValues

Regular variable

Array days stores 3 values of type int

Variable count stores one value of type int

int days[ 3 ];

int count;

Array variable

Allocates memory for 3 ints

days1st element 2nd element 3rd element

What arrays look likeWhat arrays look like

• Things to notice

• There are 7 slots, with index numbers 0 – 6

• The name of the array is myArray• Easy to be off by one

0 1 2 3 4 5 6myArray

Creating ArraysCreating Arrays

<data type> <name> [<size>];

• Notice that we can create an array of any data type, just by changing the data type!

ExamplesExamples

• An array of shorts:short someArray [50];

• An array of floats:float myArray [25];

• An array of booleans:bool list [65];

• An array of chars:char characters [255];

Modifying an ArrayModifying an Array

• You must specify which slot you are putting information in

• Example:int myArray [50];myArray [ 3 ] = 12;

• This won’t work:int myArray [50];myArray = 12;

• Data type on the left of = is an array• Data type on right of = is an int

0 1 2 3 4…

Accessing InformationAccessing Information

• Copying information out of a particular slot

int clientAge;clientAge = myArray [ 4 ];

• This copies information from the fifth slot (slot four) into the variable clientAge

Initializing ArraysInitializing Arrays(large arrays)(large arrays)

• For most arrays, you will use a loop to initialize

• Example: Create an array of 5 bytes and fill each slot with the number 42

byte myList [5];for (int counter = 0; counter < 5; counter++) {

myList [counter] = 42;}

Line by LineLine by Line

byte myList [5];for (int counter = 0; counter < 5;

counter++) {myList [counter] = 42;

}0 1 2 3 4

0 0 0 0 0

}0 1 2 3 4

0 0 0 0 0

0counter

}0 1 2 3 4

0 0 0 0 0

0counter

}0 1 2 3 4

0 0 0 0 0

0counter

}0 1 2 3 4

42 0 0 0 0

0counter

}0 1 2 3 4

42 0 0 0 0

1counter

}0 1 2 3 4

42 0 0 0 0

1counter

}0 1 2 3 4

42 0 0 0 0

1counter

}0 1 2 3 4

42 42 0 0 0

1counter

}0 1 2 3 4

42 42 0 0 0

2counter

}0 1 2 3 4

42 42 0 0 0

2counter

}0 1 2 3 4

42 42 0 0 0

2counter

}0 1 2 3 4

42 42 42 0 0

2counter

}0 1 2 3 4

42 42 42 0 0

3counter

}0 1 2 3 4

42 42 42 0 0

3counter

}0 1 2 3 4

42 42 42 0 0

3counter

}0 1 2 3 4

42 42 42 42 0

3counter

}0 1 2 3 4

42 42 42 42 0

4counter

}0 1 2 3 4

42 42 42 42 0

4counter

}0 1 2 3 4

42 42 42 42 0

4counter

}0 1 2 3 4

42 42 42 42 42

4counter

}0 1 2 3 4

42 42 42 42 42

5counter

}0 1 2 3 4

42 42 42 42 42

5counter

Finding the Smallest ElementFinding the Smallest Element

• If you were given an array of numbers, how would YOU do it?

• Computers can only compare two at a time

• Go through entire list• Keep track of smallest so far• Let’s assume

– We have an array of 5 ints– The array contains random unknown numbers– The name of the array is called randomArray

Another TraceAnother Trace

int smallestSoFar;smallestSoFar = randomArray[0];for (int counter = 1; counter < 5; counter++)

{if (smallestSoFar > randomArray[counter]) {

smallestSoFar = randomArray[counter];} // if

} // for0 1 2 3 4

42 17 42 -8 4smallestSoFar

} // for0 1 2 3 4

42 17 42 -8 4smallestSoFar

} // for0 1 2 3 4

42 17 42 -8 4 42smallestSoFar

} // for0 1 2 3 4

1counter

} // for0 1 2 3 4

1counter

} // for0 1 2 3 4

1counter

Is 42 > 17?

} // for0 1 2 3 4

1counter

} // for0 1 2 3 4

1counter

} // for0 1 2 3 4

2counter

} // for0 1 2 3 4

2counter

} // for0 1 2 3 4

2counter

Is 17 > 42?

} // for0 1 2 3 4

2counter

} // for0 1 2 3 4

3counter

} // for0 1 2 3 4

3counter

} // for0 1 2 3 4

3counter

Is 17 > -8?

} // for0 1 2 3 4

42 17 42 -8 4 -8smallestSoFar

3counter

} // for0 1 2 3 4

3counter

} // for0 1 2 3 4

4counter

} // for0 1 2 3 4

4counter

} // for0 1 2 3 4

4counter

Is -8 > 4?

} // for0 1 2 3 4

4counter

} // for0 1 2 3 4

5counter

} // for0 1 2 3 4

5counter

} // for0 1 2 3 4

5counter

Passing Arrays to FunctionsPassing Arrays to Functions

#include <iostream.h>

void modifyArray (int inArray[ ], int arraySize) {// Stuff that changes the array

void main ( ) {int myArray [15];modifyArray (myArray, 15);

The Overall ConceptThe Overall Concept

• Arrays hold values that are all the same type

• Arrays are static in size• Creating arrays always follows a

format• You can create an array of any type• To initialize or search arrays, you’ll

almost always use a loop

More on Initializing Arrays

Initialize all three indexed variables of the array “a”

If fewer values are listed than there are indexed variables, remaining indexed variables are initialized to zero of the array base type

Automatically sized

int a[ 3 ] = { 2, 12, 1 }; int a[ 3 ];a[ 0 ] = 2;a[ 1 ] = 12;a[ 2 ] = 1;

int b[ ] = { 5, 12, 11 }; int b[ 3 ] = { 5, 12, 11 };is equivalent to:

is equivalent to:

Only initializes first 3 elements of 5 element arrayint c[ 5 ] = { 2, 4, 8 };

2 4 8 0 0 Un-initialized elements set to zero

Initializing StringsIf string constant is used, null terminator is automatically included

char short_str [ ] = “abc”;

Character array also has indexed variables

Sized automatically to length of string + 1 for the ‘\0’

Change value in short_str to contain all ‘X’ characters

Is equivalent to:char short_str [ 4 ] = “abc”;

Is not equivalent to:char short_str [ ] = {‘a’, ‘b’, ‘c’};

short_str[ 0 ] short_str[ 1 ] short_str[ 2 ] . . .

int index = 0;while ( short_str [ index ] != ‘\0’ ){ short_str [ index ] = ‘X’; index++;}

‘a’ ‘b’ ‘c’ ‘\0’ ‘a’ ‘b’ ‘c’

Element data type

Loop ends when element is \0

More Array Processing

Processing array elements is the same as processing other variables

Increment value in score[ 2 ]++ score [ 2 ];int result = score [ 4 ] * 2;

if ( score[ 3 ] < score[ 4 ] )

Loop iterates as long as score[ count ] does not equal 0

int score[ 5 ] = { 7, 8, 9, 10, 0 };

Is the value in score[ 3 ] less than the value in score[ 4 ]?

Initializes result to value of score[ 4 ] times 2

while ( score[ count ] != 0 )

Parallel Arrays

Using two or more arrays to represent relationships involving different data types

Elements are integers

const int NUMEMPS;int hours [NUMEMPS];float payRate [NUMEMPS];. . .for ( int index = 0; index < NUMEMPS; index++ ){ cout << “Hours employee #” << ( index + 1 ); cin >> hours[ index ]; cout << “Pay rate employee #” << ( index + 1 ); cin >> payRate[ index ];}

Stores hours worked by each employee

Stores pay rate for each employee

Number of employees

Elements are floats

Same index used to access both arrays

Printing Array ContentsUse a loop to display the contents of each array element

int testArr [ 5 ] = { 10, 20, 30, 40, 50 };

Displays address of the array, not the contents

Declare and initialize array

Loop displays value of each element

cout << testArr << endl;

for ( int ct = 0; ct < 5; ct++ ) cout << testArr [ ct ] << endl;

Doesn’t work!

Works!

Exception: Displaying the contents of a char array containing a C-string

char name [ ] = “Ned Nerd”;cout << name << endl;

cout uses \0 when given a char array to determine end of the string

Displays string, not array address

Array Elements as Function Arguments

Array element, type int, as the argument

void showVal ( int num );void main ( ){ int testArr [ 5 ] = { 5, 10, 15, 20, 25 }; for ( int ct = 0; ct < 5; ct++ ) showVal ( testArr [ ct ] );}void showVal ( int num ){ cout << num << “ “;}

Array elements can be passed by value or by reference

Parameter is also type int

With each loop iteration, the value contained in testArr[ct] is passed to the function showVal

5 10 15 20 25

Program output:

Arrays as Function Arguments

Starting address of array is passed to function showVal

void showVal ( int nums [ ] );void main ( ){ int testArr [ 5 ] = { 5, 10, 15, 20, 25 }; showVal ( testArr );}void showVal ( int nums [ ] ){ for ( int ct = 0; ct < 5; ct++ ) cout << nums [ ct ] << “ “;}

Any changes to parameter nums, effect argument testArr

5 10 15 20 25

Program output:

5 10 15 20 25testArr

0 1 2 3 4

nums[0] nums[1] nums[2] nums[3] nums[4]

Arrays as Function ArgumentsUse two arguments: The address of the array

The size of the array

int testArr1 [ 2 ] = { 5, 10 };int testArr2 [ 5 ] = { 5, 10, 15, 20, 25 };int testArr3 [ 7 ] = { 5, 10, 15, 20, 25, 30, 35 };showVal ( testArr1, 2 );showVal ( testArr2, 5 );showVal ( testArr3, 7 );

Modify function showVal to display the contents of an int array of any size

Function calls to showVal

In main:

Address of arraySize of array

void showVal ( int nums [ ], int size ){ for ( int ct = 0; ct < size; ct++ ) cout << nums [ ct ] << “ “;}

New showVal:

Arrays as Function Arguments

Array parameters give direct access to the array argument

Changes values in parameter nums and argument testArr

void doubleArr ( int nums [ ], int size );void main ( ){ int testArr [ 5 ] = { 1, 2, 3, 4, 5 }; for ( int ct = 0; ct < 5; ct++ ) cout << testArr [ ct ] << “ “; doubleArr ( testArr, 5 ); for ( int ct = 0; ct < 5; ct++ ) cout << testArr [ ct ] << “ “;}void doubleArr ( int nums [ ], int size ){ for ( int i = 0; i < size; i++ ) nums [ i ] *= 2;}

1 2 3 4 52 4 6 8 10

Program output:

Two-dimensional ArraysTwo-dimensional array is several identical arrays put together in the form of a table

score[0][0] score[0][1] score[0][2]

column 0 column 1 column 2

Exam scores

Students

float score [ 3 ] [ 3 ];

score [ 1 ] [ 2 ] = 93.2;

cout << score [ 0 ] [ 2 ];

Number of rows Number of columns

Row index

Column index

Assign value to an element

Display value of an element

Two-dimensional ArraysNested loops are used to process each element of a two-dimensional array

float score [ 3 ] [ 3 ];for ( int std = 0; std < 3; std++ ){ for ( int exam = 0; exam < 3; exam++ ) { cout << “Student “ << std + 1 << “ , exam “ << exam + 1 << “: “; cin >> score [ std ] [ exam ]; } cout << endl;}

92.3 88.5 83.6

79.2 72.8 ?

Outer loop iterates over rows

Inner loop iterates over columns

Student 1, exam 1: 92.3Student 1, exam 2: 88.5Student 1, exam 3: 83.6Student 2, exam 1: 79.2Student 2, exam 2: 72.8. . .

Program output:

Read in an exam scoreColumn index

Row index

Two-dimensional Arrays as Function Arguments

Number of columns is specified in a two-dimensional array parameter

void showArr ( int Arr [ ] [ 2 ], int rows );void main ( ){ int table [ 3 ] [ 2 ] = { { 8, 5 }, { 7, 9 }, { 6, 3 } }; showArr ( table, 3 );}void showArr ( int Arr [ ] [ 2 ], int rows ){ for ( int r = 0; r < rows; r++ ) { for ( int c = 0; c < 2; c++ ) cout << Arr [ r ] [ c ] << “ “; cout << endl; }}

8 59 96 3

Program output:

Row 1 Row 2 Row 3

Extra braces that enclose each row’s values are optional

Empty Column indexNumber of rows

Iterates rows

Iterates columns

Arrays StringsA two-dimensional array of characters can be used as multiple arrays of strings

char team [ 4 ] [ 9 ] = { “Ned”, “Connie”, “Pat”, “Greg” };

cout << team [ 2 ];

for ( int ct = 0; ct < 4; ct++ ) cout << team [ ct ] << endl;

Name of array with only a row index is the address of that row

Loop displays all names in the array

N e d \0

C o n n i e \0

P a t \0

G r e g \0

NedConniePatGreg

Maximum length of string is 9 – 1 (for null terminator)

Four names, 8 characters long

Programoutput:

End of Lecture 4End of Lecture 4

• Next Time, more on Arrays

• Program – 2, Due Thurs. 7/7 (classes)

• Program – 3, Due Thurs. 7/21 (arrays)

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