Lecture 3. Arrays. Name of array. c[0] c[1] c[2] c[3] c[4] c[5] c[6] c[7] c[8] c[9] c[10] c[11] Position number of the element within array c

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Lecture 3

• Data structures

– structs

• C strings: array of chars

• Arrays as parameters to functions

• Multiple subscripted arrays

• Struct s as parameters to functions

• Default arguments

• Inline functions

• Redirection of stdin and stdout

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Arrays

• Consecutive group of memory locations of the same type.

• To refer to an element (e.g. 3^rd)

c[2]

• First element at position 0

• ith element at position i-1

c[6]

-45 6 0 72 1543 -89

0 62 -3

1 6453 78 Name of array

c[0]

c[1]

c[2]

c[3]

c[11]

c[10]

c[9]

c[8]

c[7]

c[5]

c[4]

Position number of the element within array c

int c[12];

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Arrays

• Array elements like other variables

– Assignment and printing for an integer array c

c[ 0 ] = 3;

cout << c[ 0 ] << endl;

• Can perform operations inside subscript

c[ 5 – 2 ] same as c[3]

Declaring Arrays

• When declaring arrays, specify

int c[10]; // array of 10 integers float d[3284]; // array of 3284 floats

– The array size can be defined as a constant

const int arraySize = 10; See Fig04_06.cpp int c[ arraySize ];

• Declaring multiple arrays of same type

variables

int b[ 100 ], x[ 27 ];

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Initializing Arrays

• With a loop

• Initializer list

– Specify each element when array declared

int n[ 5 ] = { 1, 2, 3, 4, 5 };

int v[ 100 ] = {0};

– If too many initializers then syntax error – If array size omitted, initializers determine

size

int n[] = { 1, 2, 3, 4, 5 };

• 5 initializers, therefore 5 element array

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Defining Array Size

• Can be specified with constant variable

int size = 20;

char v[size];

• Constants cannot be changed

• Constants must be initialized when declared

• Also called read-only variables

See Fig04_08.cpp Fig04_07.cpp

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Examples

• Histogram

– SeeFig04_10.cpp

• Throwing a dice and counting frequencies – See Fig04_11.cpp

• Listing student grades frequencies – See Fig04_09.cpp

• Compute mean, median, and mode – See Fig04_17.cpp

Arrays in C++

• No range checking

• The array does not ”know” its size

• Cannot input or output an array directly , except strings

• Comparison and assignment not

implemented

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Character Arrays

• C strings

– Arrays of characters

– All C strings end with null ('\0')

• char string1[] = { 'h', 'e', 'l', 'l', 'o', '\0’ };

• char string1[] = "hello";

– Nullcharacter implicitly added – string1has 6 elements

– Subscripting

• string1[0]is 'h‘

• string1[2]is 'l'

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Character Arrays

• Input from keyboard

char string2[ 10 ];

cin >> string2;

– Reads user input in string2

• Stops at first white space character

• Adds null character

– If too much text entered, data written beyond array

#include <iomanip>

char string3[ 5 ];

cin >> setw(5) >> string3;

See Fig04_12.cpp

Reads at most 4 chars and last array position is for the null

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Character Arrays

• Printing strings

cout << string2 << endl;

• Does not work for other array types

– Characters printed until null character found

Passing Arrays to Functions

• Function prototype

void modifyArray( int b[], int arraySize );

void modifyArray( int [], int);

• No need for array size between brackets

• Ignored by compiler

• Array parameter as const

• Array values cannot be modified (compiler error)

void doNotModify( const int [] );

• See Fig04_15.cpp

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Calling Functions with Array Arguments

• Call function modifyArray

– To pass array myArray to modifyArray

modifyArray(myArray, 24);

• Array size is usually an argument

– Not required

– Useful to iterate over all elements

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Passing Arrays to Functions

• Arrays passed-by-reference

– Functions can modify original array data – The array’s name is address of first element

• Function knows where the array is stored

• Can change original memory locations

• Individual array elements passed-by-value

square(myArray[3]);

– See Fig04_14.cpp

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Default Arguments

• Function prototype

void modifyArray( int b[], int arraySize = 100);

• Function call

modifyArray(v);

//modifyArray(v, 100);

modifyArray(myArray, 24);

Default value

Static Local Arrays

• If

, local variables are saved between function calls

– Visible only in function body – Used for performance reasons – Can declare local arrays to be static

• Initialized to zero by default

static int array[3];

• If not static

– Created (and destroyed) in every function call

See Fig04_13.cpp

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Multiple-Subscripted Arrays

• Multiple subscripts

– a[ i ][ j ] int a[ 3 ][ 4 ];

– Tables with rows and columns – Specify row, then column – “Array of arrays”

• a[0]is an array of 4 elements

• a[0][0] is the first element of that array

Row 0 Row 1 Row 2

Column 0 Column 1 Column 2 Column 3 a[ 0 ][ 0 ]

a[ 1 ][ 0 ] a[ 2 ][ 0 ]

a[ 0 ][ 1 ] a[ 1 ][ 1 ] a[ 2 ][ 1 ]

a[ 0 ][ 2 ] a[ 1 ][ 2 ] a[ 2 ][ 2 ]

a[ 0 ][ 3 ] a[ 1 ][ 3 ] a[ 2 ][ 3 ]

Row subscript Array name

Column subscript

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Multiple-Subscripted Arrays

• To initialize

– Initializers grouped by row in braces

int b[ 2 ][ 2 ] = { { 1, 2 }, { 3, 4 } };

int b[ 2 ][ 2 ] = { { 1 }, { 3, 4 } };

1 2 3 4

1 0 3 4 Row 0 Row 1

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Multiple-Subscripted Arrays

• Access to an element cout << b[0][1];

– Outputs 0

– Cannot access an element using commas cout << b[0, 1];

• Syntax error!!!

• Function prototypes

– Must specify sizes of subscripts

• First subscript not necessary, as with single- scripted arrays

void printArray( int [][2], int size );

1 0 3 4

See Fig04_22.cpp See Fig04_23.cpp

StructureType in C++

•

is used to contain data of different types

struct Card { int value;

char suit [10]; //field, member };

Card ace = {13, ”Hearts”}; // initialization

• Size (i.e. number of bytes)?

• sizeof(ace).

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Structure Type

• How do you write in C?

typedef struct { int value; //

char suit [10];

} Card;

Card ace = {13, ”Hearts”};

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Operations on structs

• Memberwise assignment ace.value = 1;

• Assignment is allowed Card ace2 = ace;

– Yes, array fields get copied without need to copy each field at a time !!!

– But surprise !!!,

//doesn’t compile ace2.suit = ace.suit;

//instead use a string copy function strcpy(ace2.suit, ace2.suit);

#include<cstring>

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Operations on structs

• Comparison, input, and output are not available

if (ace2 == ace)... //does not work cin >> ace2; //does not work

• Implement your own functions

– Overload operators ==, >> and <<

See Fig06_01.cpp

Using Structures as Function Arguments

• Pass the structure argument by value

cout << “Value: “ << c.value

<< “Suit: “ << c.suit << endl;

}

• Pass an array by value to a function

– Create a structure with the array as a member

• See struct1.cpp

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Using Structures as Function Arguments

• Pass the structure argument by reference

void printCard(const Card &c){

cout << “Value: “ << c.value

<< “Suit: “ << c.suit

<< endl;

}

– Pass-by-reference more efficient

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Inline Functions

inline double cube(const double side) { return side * side * side;

}

int main() { double val;

cin >> val;

cout << “Cube volume: ”

<< cube(val) << endl;

return 0;

}

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• Function call replaced by functions code

– Compiler is free to ignore the inline qualifier

– Advantage

• Efficiency: avoid function call overhead

– Disadvantages

• Changes in a inline function require that whole project to be recompiled

• Program size increases

Inline Functions

Reading a Sequence of Values

• cin >> k returns

when

– a value of the wrong type is given

int main(){

int k, sum = 0;

while (cin >> k) sum += k;

cout << sum << endl;

return 0;

} See sum.cpp

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Reading Variables

int k;

cin >> k;

cout << k;

int k;

if (cin >> k) cout << k;

Prints any trash in k, if user gives a non-integer value.

No problem!!!

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Redirection of input and output

• Redirection of input (DOS and Unix)

reads sequence of ints from file input.txt

• Redirection of output

writes the result to file output.txt

• Piping (DOS and Unix)

dice | sum

output of dice.exe is

redirected as input to sum.exe