Structured Programming Instructor: Prof. K. T. Tsang

Lecture 6: Arrays

数据序列 ,数组

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Arrays (p.22 K&R)数据序列 ,数组

• An array is a collection of data elements that are of the same type (e.g., a collection of integers, characters, doubles).

• Array provide a convenient vehicle to hold a collection, and to reference its individual elements.

• We need to know the maximum number of elements the array can hold before declaring it.

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Declaration of Arrays

An array is a compound type that consists of a type, an identifier, and a dimension.

int test_scores[40]

TypeIdentifier

Dimension, must be a constant.

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How Array is stored in memory?

When an array is declared, a block of memory is reserved so that they are stored one after another.

Example: int A[5] (4-bytes each)

100100…… …… …… ……Memory address: 1024

……1020

A[1]

A[2]

A[3]

A[4]

A[0]

1028 10361032

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Array Applications

• Given a list of test scores, determine the maximum and minimum scores.

• Read in a list of student names and rearrange them in alphabetical order (sorting).

• Given the height measurements of students in a class, output the names of those students who are taller than average.

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Arrays - example

#define MAXLINE 1000 /*define for C preprocessor*/

const int MaxStudents = 100;

int number_of_students = 100;

const int array_size = 3;

char name[99]; /*ok*/

char line[MAXLINE]; /*ok*/

int student_ID[MaxStudents]; /*ok*/

int ia[array_size] = { 2, 8, 19 }; /*ok, explicitly initialized*/

int ib[ ] = { 3, 5, 8 }; /*ok, an array of dimension 3*/

int test_scores[number_of_students] /*error, dimension not a constant*/

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Subscript• Suppose

int A[10]; // array of 10 ints

• To access an individual element we must apply a subscript to list name A– A subscript is a bracketed expression

The expression in the brackets is known as the index

– First element of list has index 0A[0]

– Second element of list has index 1, and so onA[1]

– Last element has an index one less than the size of the listA[9]

• Incorrect indexing is a common error

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Accessing individual Array elements

int digit[8]; /*define an array with 8 integers*/digit[0] = 9; /*initialize individual array elements*/digit[1] = 8; /*array index starts at 0*/digit[2] = 6;digit[3] = 5;digit[4] = 4;digit[5] = 3;digit[6] = 2;digit[7] = 1;

int sum =0;for (int i=0; i<8; i++) { //i is a local variable

digit[i] = i + 2;sum = sum + digit[i];

}

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Example: loops & arrayconst int MaxSize = 10000;int A[MaxSize];int n = 0, CurrentInput, i;while (n< MaxSize && scanf(“%d\n”, &CurrentInput)==1) {

A[n] = CurrentInput;n++;

}

printf(“Total number of input data is\n”);for (i=0; i<n; i++) printf(“%d ”, A[i]);printf(“\n”);

int SmallestValueSoFar = A[0];for (i=1; i<n; i++)

if (A[i] < SmallestValueSoFar)SmallestValueSoFar = A[i];

printf(“The smallest is %d\n”, SmallestValueSoFar);

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Character arrays(string字串 ,字符串 ) (p.28

K&R) C-style string always terminated with the null character.

char name[7];name[0]=‘H’;name[1]=‘e’;name[2]=‘l’;name[3]=‘e’;name[4]=‘n’;name[5]=‘\n’;name[6]=‘\0’; /*always ends with null character*/

char ch1[ ] = { ‘H’, ‘e’, ‘l’, ‘l’, ‘o’, ‘\0’ }; /*ok*/char ch2[ ] = “ world”; /*ok*/ch1 = ch2 ; /*error, cannot assign one character array to another*/ 10

C-style stringchar ca[] = { ‘C’, ‘+’, ‘+’ }; //no null, not C-style string

char ca1[] = { ‘C’, ‘+’, ‘+’, ‘\0’ }; //with null, C-style string

char ca2[] = “C++”;//null terminator added automatically

To assign one character array to another, use library function strcpy

#include <string.h>

(void)strcpy(name, “Helen\n”);

printf(“Hi, my name is %s”, name);

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String copy: example code#include <stdio.h>

void str_copy(char s1[ ], char s2[ ]);

int main() { char s2[ ] = “This is a string!"; char s4[99]; str_copy(s4, s2); printf("s4 %s\n“, s4); return 0;}

void str_copy(char s1[], char s2[]){int i = 0;while (s2[i] != '\0') { s1[i] = s2[i]; ++i;}s1[i] = '\0';return;}

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String library functions contained in string.h

Copy string2 to string1: strcpy(string1, string2);

Concatenate s2 to s1: strcat(s1, s2)

Get the length of a string: len = strlen(s1)

Compare 2 strings, 0 if equal, otherwise nonzero: strcmp(s1, s2)

Important: The array used in the first argument of strcpy and strcat must be large enough to hold the generated string.

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Example: String library functions #include <string.h>#include <stdio.h>char first[100], last[100], full_name[200];int main(){

strcpy(first, “Mary”);strcpy(last, “Wang”);

strcat(full_name, first);strcat(full_name, “ “);strcat(full_name, last);

printf(“Hello, my full name is %s\n”, full_name);return 0;

}

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String I/O#include <stdio.h> /*for gets & puts*/

int main(){

char message[200]; /*set enough storage for message*/

printf (“Please enter a message\n”);gets (message); /*input string from keyboard*/

printf (“The string just entered is:\n”);puts (message); /*displaying string on screen*/

return 0;}

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Gets & puts

gets(str)Characters\n Characters\0

str[ ]

puts(str)Characters\0

str[ ]

Characters\n

(From keyboard)

(To screen)

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char str[200];

Passing array to function

float mean (int n, float a[ ]){

float sum = 0.0;for (int i=0; i<n; i++) sum += a[i];return (sum/(float)n);

}float std_dev (int n, float a[ ]){

float sum = 0.0, x;x = mean (n, a);for (int i=0; i<n; i++) sum += (x-a[i])*(x-a[i]);return ( sqrt(sum/(float)n) );

} 17

Passing array to function (1)

#include <stdio.h>#include <math.h>#define SIZE 6float mean (int n, float a[ ])float std_dev (int n, float a[ ])void main(){

float score[SIZE] = { 11., 9., 19., 15., 22., 7.};for (int i=0; i<SIZE; i++) printf(“%f ”, score[i]);printf(“\n”);float x = std_dev(SIZE, score);printf(“standard deviation = %f\n”, x);

}

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Array sorting– “Bubble sort”

void sort (int m, int x[ ]){

int i, j, temp;for (i=1; i<m; i++)

for (j=0; j<m-i; j++) {if (x[j] > x[j+1] ) {

temp = x[j]; x[j] = x[j+1] ; x[j+1] = temp ;

}}

} 19

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How does bubble sort works

• Starting from the top of the array/list, swapping the large number to the bottom one by one, so that the largest will sink to the bottom of the list.

• Repeat the procedure so that the next largest number will sink to on top of the largest number.

• And so on until the smallest number bubbles to the top, and that is what the name of the algorithm “Bubble sort” means.

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Using array sort#include <iostream>

void sort (int m, int x[ ]);void main(){

int score[ ] = { 11, 9, 19, 15, 22, 7};for (int i=0; i<6; i++) cout << score[i] << “ “;cout << endl;sort(6, score);for (int i=0; i<6; i++) cout << score[i] << “ “;cout << endl;

}

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Summary

Array is a collection of data of the same type.

Dimension (size) of a string must be a constant when it is first declared, because memory of array is allocated at compile time.

String is an array of characters.

C-style string always terminated with the null character.

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Exercise [1] Given 2 integer arrays A & B, of dimension M & N,

write a program to merge them into a single array C which contains every item from arrays A & B, in ascending order.

[2] Given 2 string s1 = “Merry X’mas” and s2 = “Happy New Year”, write a function with the prototype:void strConcat (char s1[ ] , char s2[ ]);so that the strings s1 & s2 will join and the result is stored in s1. Note: s1 must be large enough to hold the contents of both s1 & s2.

[3] Write a function:int len = strLength(char s[ ]);so that it can count the number of characters in the string s. Test your function in a main program withchar s[ ] = “Happy New Year”; 26

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Write a program that will allow 2 users to play tic-tac-toe. It should ask for moves alternately from player X and player O. The program displays the game positions by a JFrame in the Swing package. When there is a winner it will flash a message box to tell who have won the game. Following code is a suggestion how the program should organize. You should follow your own idea to develop the program.