Unit 8

Arrays and Strings

Introduction toC Programming

Unit 7 Review

Unit 8: Review of Past Material

Functions - ReviewFunction Header

Return TypeIdentifier - name of functionParameter List

Data type and Identifier for each parameterComma-separated list, enclosed in parentheses

Function BodyCompound Statement in bracesMust contain a return statement unless

return type is void

Variable ScopeGlobal variable

Declared at top of a fileAccessible throughout the compilation

Local variableDeclared inside a function at top of function bodyKnown only within the function

Block scopeDeclared inside a nested compound statement

(block of code)Known only within the block

Two or more variables with same name - the one at the smaller scope hides the one(s) at larger scope(s)

PointersPointer has a data type corresponding to the variable

pointed toPointers are declared using the "*" asterisk characterThe address of a variable is obtained using the "&"

characterTo use a pointer to access memory, precede it with "*"Used to create output parameters for functions

Each output parameter is declared as a pointerThe function call must use the address of a variable as

the argumentInside the function, the external variable is accessed by

pointer

Arrays

Unit 8: Arrays and Strings

Data StructuresGroup of related data items

Stored in one contiguous area of memoryCan be accessed and manipulated as a group

or unitRecord (C language "struct")

Combination of data items of various data types

Each data item called a "field" in the recordArray (C language "array")

Multiple data items of matching data typeEach data item called an "element" in the array

Arrays - DeclarationData type, variable identifier, size in square

bracketsSize must be a constant expression, not a run-time

expressionSize must be a positive integer

int numbers [ 5 ];

The example above declares an array variable of 5 elements

The elements are numbered starting from 0:numbers[0], numbers[1], numbers[2],

numbers[3], numbers[4]

Notice that the last element is "4", not "5" !

Arrays - Access to Array ElementsThe element is identified by its index

(numerical position)Name of array (identifier) followed by

subscriptSubscript is index in square bracketsIndex value must be integer expression

Examples using the numbers array:value = numbers[10];

value = numbers[i+2];

numbers[4] = value + 3;

numbers[0] =

Arrays - Caution!Array element numbering (index) starts at 0, not

1The elements are numbered [0], [1], [2], … [n-1]int numbers[5];

numbers[0], numbers[1], numbers[2],

numbers[3], numbers[4]

Common Programming Error:Accessing past end of array - logic errornumbers[5]

C Language does not check for this error

Array InitializersFollow array declaration with a list of

expressions in bracesint numbers[5] = {3, 12, -42, 0, 6};

If a size is specified, and list is too short, zeros are used at endIf a size is specified, and list is too long, this is an errorIf a size is not specified (empty square brackets)

The compiler counts the number of initializersThe count is used as the size

int numbers[] = {3, 12, -42, 0, 6};

Array Usage - Parallel ArraysParallel Arrays - Arrays of related data

itemsExample: A list of 100 rectangles, dimensions

length and widthUse two arrays of doubles, each size 100

double length[100], width[100];

length[i] = 40;

width[i] = 25;

Array Usage - Use with FunctionsArrays and Functions

An element can be passed as an argument to a functionarea = calcArea(length[3], width[3]);

An entire array can be passed as an argument to a functionSize not passed automatically, must be an explicit

separate parameter

f(numbers, 5);

An array cannot be returned from a function

Multi-Dimension ArraysArrays can have one to six dimensions

A two-dimensional array is like a gridThe first dimension is the row, second is the column

Specify each size in separate square bracketsdouble grid[50][25];

When accessing, provide separate subscriptsgrid[r][c] = 42.5;

Multi-Dimension ArraysWhen initializing, initialize one row at a time

Initializer is "list of lists"Use braces for the entire listUse braces for list of initializers for each row

double xy[4][3] = {{0, 12.2, 14.7},{2.5, 8.3, -6},{2.0}, /* "Short" list filled with zeros */{1E6, 3.2E-1, 47.9} };

Searching and Sorting

Unit 8: Arrays and Strings

SearchingOften-used algorithm

Goal: Find a value, discover it's indexMay have result of "not found"

Array is list of values, look through the listIs the list unsorted (random order)? Linear Search

pp. 411-413Look through each element from beginningFinds the first occurrence

Is the list sorted (sequential order) ? Binary SearchDivide list in halfFinds one occurrence, not necessarily the firstMore efficient than linear search (over the long run)

SortingCommon, often-used algorithmsMany sort algorithms have been developed

Simple, but less efficientComplex, faster performance

Example: Selection Sort pp. 413-415Find smallest element (first occurrence) in list [0]

… [n-1]Swap smallest element with first elementNow, use Selection Sort on remaining list [1] …

[n-1]Continue until the "remaining unsorted list" has

1 element

String Variables: Arrays of Characters

Unit 8: Arrays and Strings

StringsImportant, commonly-used data typeGroup of characters treated as a unitExamples of common uses

Names of people, places, devices, etcInstructions to userError messagesWeb addresses and URLs

C Language StringsC does not actually have a string data typeA C string uses a character array

The string begins with the 0th element, might not fill array

To indicate end of string, a sentinel 0-value character is used

The sentinel must always be presentchar s1[10] = "George";

The format specifier for printf and scanf is "%s"

G e o r g e \0