[PDF] - COMP 1402 Winter 2008 Tutorial #5 Arrays Overview of Tutorial #5 PDF Document

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COMP 1402

Winter 2008 Tutorial #5 Arrays

Overview of Tutorial #5

What is an array?
Array syntax
Character arrays
Passing arrays to functions.
Processing/traversing arrays
Higher Dimension arrays (2d and 3d)

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Why do we need arrays?

Regular variables store a single value of the

specified type. eg.

But sometimes we need several copies of a

variable, such as the grades for 3 students. We could try:

int a; //stores a single variable of integer type double grade_student1; double grade_student2; double grade_student3; //etc.

This would get very tedious if we had 100

students!

What is an array?

An array is a special variable that can store many values.
In our student example instead of a separate variable for

each student we could use an array of grades that keeps the grades for each student.

87.0 student1_grade 75.0 student2_grade 91.0 student3_grade 3 Variables A Single Array with 3 Elements grade_array 87.0 75.0 91.0

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What are arrays ...

int a; An array reserves a contiguous block of memory addresses in which we can store values of the array type (int in this case). int a[5]; A regular variable reserves a single address in memory where its value is stored.

Regular Variable An Array

Array Syntax: Declaring

Arrays declarations have three parts, the data type, the

array name and the size (number of elements), eg.

int a[5]; a[0] = 7; a[1] = 13; a[2] = 11; a[3] = 5; a[4] = -8;

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13 7 11 5

int a[5]; // An array named ‘a’ which stores 5 // integer values.

On the left we see the

integer array ‘a’ which holds the five integer values 7, 13, 11, 5, -8.

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Array Elements

The elements of an array are referred to by both the

array name and an 'index' value.

Index values are numbered from 0 (for the arrays first

element).

The index for the last element is thus one less than the

array's size.

an_array 1 2 3 4

We would reference this element as: an_array[2]

Array Syntax: Accessing Elements

An array element is accessed by supplying the

array name, and the elements index inside square brackets [ ].

We can treat each element like we would any
ther variable.

int a[5]; // Create an array of 5 integers. a[0] = 15; // Set element 0 to the value 15 if( a[2] < 4 ) { ... } //Use elements anywhere we //would a regular variable.

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Syntax: Setting Elements (cont)

Each element corresponds to one address in

memory.

a[0] a[1] a[2] a[3] a[4]

In C we must be careful with arrays

because C does not stop us from attempting to access an element

utside the array, eg.

int a[5]; // Create an array of 5 integers. a[15] = 9; //But ‘a’ has only 5 elements //may crash program or cause //strange bugs.

Array Syntax: Initializing

As with variables we can initialize an array at the

moment we create it or afterwards.

int a = 15; //Creates a variable and sets its value. //Create and initialize an array in one step. int a[] = { 8, 7, 15, 9} //Note compiler can determine //array size. //Create, and then initialize the elements. int a[4]; //In this case you must provide # elements. a[0] = 8; a[1] = 7; a[2] = 15; a[3] = 9;

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Character Arrays

In C strings are stored as arrays of characters.
The last character in the string should be the special

string termination character '\0' (corresponds to 0).

You can pass your character arrays to C functions like

printf char my_string[] = { 'h', 'e', 'l', 'l', 'o', '\0'}; //You can still access elements putchar( my_string[1] ); //outputs 'e' //Or pass to C functions. printf(“%s there!”, my_string); //prints “hello there.”

Character Arrays

Note, the null terminator is ESSENTIAL for a
string. Omitting it will likely cause any calls to

printf() etc. to crash.

If you want to scan a ‘string’ you can use the null

terminator as a marker for the end of the string.

char my_string[] = { 'h', 'e', 'l', 'l', 'o', '\0'}; //count the length of my_string int pos = 0; while( my_string[pos] != '\0' ) { pos = pos + 1; } printf( "String: %s : is %d chars long\n", my_string, pos);

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Functions and Arrays

You can pass arrays to a function like any other variable

(or as a pointer).

For 1d arrays adding the size is optional.

#define N 10 int numbers[N]; void func1( int array[N]); //OK void func2( int array[] ); //Also OK void func3( int *array); //Also OK //When you invoke the function you don't use [ ] func1(numbers ); //correct func1(numbers[] ); //wrong - syntax error.

Accessing Array Elements

Quite often you want to step through each element an

array - use a for or while loop.

Use for most of the time, you must know the array

length.

//Count how many elements of array are greater than 100 int array[N]; array[0] = 237; //etc. int i; int count = 0; for( i = 0; i < N; i++) { if( array[i] > 100 ) count = count + 1; }

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Accessing Elements

#define N 5 int array[N]; array[0] = 237; //etc. int i; int c= 0; for( i = 0; i < N; i++) { if( (array[i] > 100 ) c = c + 1; } #define N 5 int array[N]; array[0] = 237; //etc. int i; int count= 0; if( array[0] > 100) count = count +1; if( array[1] > 100) count = count +1; if( array[2] > 100) count = count +1; if( array[3] > 100) count = count +1; if( array[4] > 100) count = count +1; Code is equivalent.

Using While Loops

In practice for loops are used more often.
Should only be used if the array has a

terminating symbol/flag (like '\0' in strings).

– This may be done with non-character arrays by specifying a special invalid element value. For example if the array recorded the weights

f people you could use a negative value.

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While Loops

#define END_OF_ARRAY -1.0 //Note that the terminator need not be in the final position. char astring[ ] = { 'h', 'e', 'l', 'l', 'o', '\0', 't', 'h', 'e', 'r', 'e', '\0' }; int i = 0; while( astring[i] != '\0' ) { // prints "hello" putchar( astring[i] ); i++; } double weights[ ] = { 123.0, 203.1, 176.0, 141.0, END_OF_ARRAY } i = 0; while( weights[i] != END_OF_ARRAY ) { //do something. //Note you must increment i or the loop runs forever! }

Higher Dimensional Arrays

An array can have more than 1 dimension, they can

have 2, 3, or more.

Syntax for creating is similar to 1-dimensional arrays.

// A 1-d array int a[10]; //A 2-d array - perhaps used to store a chess/checker board. int b[8][8]; //A 3-d array int c[3][4][5]; //Higher - you could do this if you wanted to ... why? int e[2][3][4][5][6];

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2d-Arrays

Easiest to think about the array as storing elements in

rows and columns.

First index is the row, second is the column.

// create an array with two // rows and three columns int matrix[2][3];

// To set row 0 column 1 to 7 matrix[0][1] = 7; matrix[0][0] matrix[0][1] matrix[0][2] matrix[1][0] matrix[1][1] matrix[1][2]

Functions

For 2d arrays the size of the 2nd dimension

must be supplied when declaring a function (compiler only checks the second dimension).

func1( int array[2][3] ); //OK func2( int array[ ][3] ); // Also OK func3( int array[ ][ ] ); //Not allowed. int a[2][3]; int b[3][3]; //Passing to functions is the same as for 1d arrays func1( a ); //OK same size. func1( b); //Will compile since b has 3 columns, but will likely //cause problems at run time. func2( a ); //OK same size. func2( b ); //OK because b has 3 columns.

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Accessing Elements in 2d Arrays

You can use 'nested' for loops. For

example the following code sets every element in array2d to 0:

#define ROWS 2 #define COLUMNS 2 int array2d[ROWS][COLUMNS]; int rw, cl; for( rw = 0; rw < ROWS; rw++ ) { for( cl = 0; cl < COLUMNS; cl++ ) { array2d[rw][cl] = 0; } }

3d-Arrays

A little trickier to think about, think of levels, rows, and

columns.

First index is the level, second is the row, third is the

column (not all elements shown in diagram)

// array with 3 levels, rows // and columns int cube[3][3][3];

// Set centre cell to 5 cube[1][1][1] = 5; [0][0][0] [0][0][1] [0][1][1] [0][1][2] [0][2][2] [0][1][0] [0][0][2] [0][2][0] [0][2][1]

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

Array cube

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Functions

For 3d arrays the size of the 2nd and 3rd

dimensions must be supplied when declaring a function (only checks these).

func1( int array[2][3][4] ); //OK func2( int array[ ][3][5] ); // Also OK func3( int array[ ][ ][4] ); //Not allowed. int a[2][3][4]; int b[3][3][5]; //Passing to functions is the same as for 1d arrays func1( a ); //OK same size. func2( b); //OK columns and rows are same. func2( a ); //Syntax error a has 3 rows, 4 columns.

Accessing Elements in 3d Arrays

Use 'nested' for loops again, gets a bit messy.

#define LEVELS 2 #define ROWS 2 #define COLUMNS 2 int array3d[LEVELS][ROWS][COLUMNS]; int lv, rw, cl; for( lv = 0; lv < LEVELS; lv++) { for( rw = 0; rw < ROWS; rw++ ) { for( cl = 0; cl < COLUMNS; cl++ ) { array2d[lv][rw][cl] = 0; } } }