Chapter 7 - A closer look at Arrays - PowerPoint Presentation

Slide1

Chapter 7 - A closer look at Arrays

one dimensional arrays (review)

copying arrays

arrays as function arguments

sorting and searching techniques

multi-dimensional arrays

representation and realization

initialization

as function arguments

Slide2

One dimensional ArraysDefine with square brackets []type array_name [num_elements][] is an operator with the highest precedenceexample: char name[20]char[0] is the first element, char[19] is the last“sizeof” determines storage requirement of array (e.g. sizeof (name) = 20).the scope and privacy is the same as for scalars

Slide3

Array bounds checkingC does not check that references to an array are valid, i.e. that the array subscript is between zero and num_elements-1advantages:faster program executiongreater reusability with function callsdisadvantages:easy to accidentally corrupt memoryreferring to array[num_elements] is a common programming error

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Initialization of ArraysZero initialization rules same as for scalarsGeneric form for explicit initialization:type name[num+1]={val_0, val_1, ..., val_num}too many initial values generates errortoo few initial values results in the remainder of the array values set to zero, regardless of scope.if the array length is not specified, the number of initialization values determines the array length.

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Copying arraysyou can not use the assignment operator (=) to copy an entire array. You must use a loop and element-by-element copying:#define SIZE 10...int i, y[SIZE], x[SIZE]={1,4,6,3,2,9,7,4,0,12};for (i=0;i<SIZE;i++) y[i]=x[i];

Slide6

Arrays as function argumentsArrays are passed (simulated) call by reference. This means that:actually the address of the FIRST element in the array is given to the called functionthe called function can modify the contents of the original array in the calling function.Since there is no bounds checking, the length of arrays need not be specified in the called function.

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Program 7.1 - bubble sort functionvoid bubble_sort(int num[], int count){ int temp, i, j; for(j=0;j<count-1;j++) for(i=j+1;i<count;i++) if(num

[i]>num[j]) temp=num[i], num[i]=num[j], num[j]=temp; return;}

Slide8

Program 7.1 - main function (part 1)

/* new bubble sort program of an integer array */

#include <

stdio.h

>

int

main(void)

{

void

bubble_sort

(

int

[],

int

);

int

num

[100],

i

, count;

for(count=0;count<100;count++) {

printf

("Enter a number: ");

if (

scanf

("%d",&

num

[count])==EOF)

break;

}

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Program 7.1 - main function (part 2) if (num[count]<=0) break; }// print array before we start to sort printf("\n\nThe unsorted array is:\n"); for(i=0;i<count;i++) printf("%c %d",i%10?' ':'\n',num[i]);// bubble_sort(num,count);

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Program 7.1 - main function (part 3)//// finished sort high to low; print results printf("\n\nThe SORTED array is:\n"); for(i=0;i<count;i++) printf("%c %d",i%10?' ':'\n',num[i]); printf("\n\n"); return 0;}

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Program 7.2 - array statisticsRead in an array of integers, find, and print the maximum, minimum, median, mean, and mode values.

Slide12

Program 7.2 - specification

start

declare variables

prompt for input

read in an array of numbers, stop at EOF

sort the numbers from high to low

compute and print

max, min, median, mean, mode

stop

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Program 7.2 - part 1/* program to calculate array statistics */#include <stdio.h>int main(void){ void bubble_sort(int[],int); int num[100], count, mean(int[],int), mode(int

[],int); for(count=0;count<100;count++) { printf("Enter a number (0 to exit): "); if (scanf("%d",&num[count])==EOF) break;

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Program 7.2 - part 2 if (num[count]<=0) break; }bubble_sort(num,count); printf("Array maximum = %d \n",num[0]); printf("Array minimum = %d \n",num[count-1]); printf("Array median = %d \n",num[count/2]);

printf("Array mean = %d \n",mean(num,count)); printf("Array mode = %d \n",mode(num,count)); return 0;}

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Program 7.2 - part 3int mean(int num[], int count){ int avg=0, i; for(i=0;i<count;avg+=num[i++]); return (avg/count);}

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Program 7.2 - part 4int mode(int num[], int count){ int i=0, omode=0, nmode, otemp, ntemp; while (i<count-1) { ntemp

=num[i]; for(nmode=1;i<(count-1)&&ntemp==num[++i];nmode++); if (nmode>omode) { otemp=ntemp; omode=nmode; } }

printf(“frequency = %d \n”,omode); return

otemp

;

}

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Array Searchingkey: is value present in array?if yes, return array locationif no, return “no match”search types:linear searchbinary search

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Linear searchstart at first value and move up through array until value is found.takes an average of n/2 comparisons (if key is present)good method for small or unsorted arraysfor (i=0,j=-1;i<max_num;i++) if (array[i]==key) { j=i; break; }return j;

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Binary searchmust have an ordered array (low-to-high or high-to-low)must first perform a sortcompare key to middle value (high-to-low).if key is the same, return resultif key is smaller, discard lower half of arrayif key is larger, discard upper half of arrayif array down to one value, return no match

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Program 7.3 - Binary search functionint binary_search(int num[],int count, int key){ int low=0, high=count-1, middle; while (low<=high) { middle=(low+high)/2; if (key > num[middle]) high=middle-1;

else if (key < num[middle]) low=middle+1; else return middle; } return -1;}

Slide21

Program 7.3 - main (part 1)/* new binary search program of an integer array */#include <stdio.h>int main(void){ void bubble_sort(int[],int); int binary_search(int[],int,int), num[100], i

, count, key, loc; for(count=0;count<100;count++) { printf("Enter a number (0 to exit): "); if (scanf("%d",&num[count])==EOF) break;

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Program 7.3 - main (part 2) if (num[count]<=0) break; } for(i=0;i<count;i++) printf("%c %d",i%10?' ':'\n',num[i]); bubble_sort(num,count); for(

i=0;i<count;i++) printf("%c %d",i%10?' ':'\n',num[i]); printf("\n Enter a key: "); scanf("%d",&key); loc=binary_search(num,count,key);

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Program 7.3 - main (part 3) if (loc==-1) printf("error: key was not found\n"); else printf("key found in position %d\n",loc); return 0;}

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Multi-dimensional arraysMulti-dimensional arrays are natural for many situations:chess board (2-D)matrices (2-D)arrays of strings (2-D)scalar fields in space (3-D)vector fields in space (9-D)We will only worry about 2-D arrays here

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2-D Arraysgeneral definition:type array_name[num_rows][num_columns]you can visualize it as a 2-D grid with num_rows rows by num_columns columns.In memory, they are actually stored linearly, with the outermost subscript (the column) varying most rapidly.

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2-D Array - exampleConsider the array: int num[3][4]. The numbers are stored in memory like so:num[0][0] (first) num[1][2]num[0][1] num[1][3]num[0][2] num[2][0]num[0][3] num[2][1]num[1][0]

num[2][2]num[1][1] num[2][3] (last)

Slide27

Accessing multi-dimensional arraysgiven our example: num[3][4], the usual way to reference the element in the second row and the third column is:num[1][2]NB: num[1,2] and num(1,2) are common errors!num[1] (without second subscript) references the entire second row (is a 1-D array)

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2-D array initializationExample:int num[3][4]={{1,2,3,4},{5,6,7},{9,10,11,12}};general rules:initial values are loaded by varying most rapidly the outer subscript and working inwardbraces enclose the entire initialization sequencebraces enclose each row of valuesif any values are missing from row data, the later column values are set to zero

Slide29

2-D array initialization - ctdif any rows are missing, the later rows are set to zerotoo many initial values is an errorif the number of columns is not specified, the number of rows is determined from the initial conditionsexample: we get three rows below:int num[][4]={{1,2,3,4},{5,6,7},{9,10,11,12}};

Slide30

Program 7.4 - simple 2-D array test/* 2-D integer array */#include <stdio.h>int main(void){ int num[4][4]={{1,2,3,4},{5,6,7},{9,10,11,12}}, i, j; for (i=0;i<4;i++) for(j=0;j<4;j++) printf("array element [%d][%d]=%d\n",i,j,num[

i][j]); return 0;}

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Multi-dimensional array argumentsshould have the same number of dimensions in the calling and called functionsthe number of elements in the inner most dimension is optional; all others must be specifiedlast statement is true for both the function prototype and the function header.

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Example 2-D argumentsin calling function: in called function:int main (void) int test(int num[][4]){ { int test(int[][4]), y; int x; int num

[3][4]; ... ... return x; y=test(num); } ...