Two-week ISTE workshop onEffective teaching/learning of computer

programming

Dr Deepak B PhatakSubrao Nilekani Chair Professor

Department of CSE, Kanwal Rekhi BuildingIIT Bombay

Lectures 9, Array examples

Friday 2 July 2010

Overview

A quiz Representation of character data

• Using arrays to represent character strings Example of another way of finding roots Accessing and manipulating elements of an array Analysis of the quiz from lecture 7

• Loop invariant code

A quiz

Q. Computers have memory to store instructions and data. Which technology was used in the earliest digital computer

A. Magnetic core

B. Electronic Valves

C. Semiconductor memory

D. Cathode ray tube

E. None of these

Character data type in C

Character constant is written as a single character within

quotes

‘a’ ‘P’ ‘o’ ‘O’ ‘5’ ‘0’ ‘\n’ ‘\0’

Internally, it is represented as an integer value equivalent to

its ASCII code.

It occupies one byte which is the smallest addressable unit of

memory

Character data type

Each character which you see on your terminal, or which you input using your keyboard is represented by an internal numerical “code”

Typically ASCII code is used (one byte long)• ‘a’ has a code value (in decimal) of 97• ‘z’ is 122 , ‘A’ is 65, ‘Z’ is 90• ‘ ’ (space or blank) is 32 ‘\n’ is 10

One can declare variables in a program of type char

char letter1; char letter2 = ‘Y’;

Character strings

The traditional way of representing character strings in c (and thus in c++) is to use a char array charEmployeeFirstName[60];

To indicate that such an array contains a string, a null value (\0) is stored in the location immediately after the last character of the string

Character strings as char arrays …

char sname[60]; // can hold 59 characters It will be wrong to use sname = “Rajesh Mashruwala”; Instead, each location has to be assigned a value

corresponding to a character sname[0] = ‘R’; sname[1] = ‘a’; …; sname[17] = ‘\0’; Such strings are called Null terminated strings

An array overflow may occur, against which our program must guard. So we must check that an index i in sname[i] does not cross 59

Character strings as char arrays …

Since a string stored in this fashion is not a pre-defined data type in C, we cannot perform normal operations like assignment, comparison, concatenation, etc. All these operations must be done by writing program code

// find the length of string stored in sname int i, length; for (i = 0; sname[i] != ‘\0’; i++){ continue; } length = i;

string library

The library <string.h> contains several useful functions to handle operations on such strings. We should use

#include <string.h>;Some of the functions are: strcpy (s1, s2) [strncpy(s1,s2,count)] copies s2 into s1 strcat (s1,s2) [strncat (s1, s2, count)] concatenates s2 onto the end of s1 strlen (s1) Returns the length of s1

string library …

strcmp(s1, s2)

Lexicographically compares strings s1 and s2. It returns an integer value which less than zero if s1 < s2, is 0 if s1 == s2, and is positive otherwise

strstr(s1, s2)

Returns a pointer to the first occurrence of s2 in s1. If no match found, a null pointer is returned

strchr(s1,ch) searches for ch instead of s2 memcpy (s1, s2, count)

copies count characters from s2 into s1

string library

Several functions which examine or change a single character (e.g., in a char variable ch)

isalnum(ch), isalpha(ch), iscntrl(ch), isdigit(ch),

isgraph(ch), islower(ch), isupper(ch), ispunct(ch)

isspace(ch)

(these return nonzero value if true, zero if false)

Searching for a value in an array

Roll Marks

1001 72

1002 45

1003 91

1004 86

1006 38

1008 53

1009 65

Findmarks.cpp

// given a roll number, find the marks int roll[100], marks[100], nstudents;int givenroll,foundmarks, position, i;// read all data in arrayscin >> givenroll;for (i=0; i<nstudents; i++){ if (roll[i] == givenroll){ foundmarks = marks[i];}cout << “Marks for ” << givenroll cout << “are ” << foundmarks;return (0);}

Searching for a given value in the array

Finding a root by bisection method

Finding a root by bisection method

Start with a lo and hi values such that f (lo) f (hi) < 0.∗ Compute mid and f (mid). while |if (mid)| > 0 (some small threshold value) Locate the next interval to be either [low,mid] or [mid,hi].

Consider x-axis as discrete point

Similarity with a sorted array

[0] 1001 72[1] 1002 45[2] 1003 91[3] 1004 86[4] 1006 38[5] 1008 53[6] 1009 65[7] 1011 95

Binary search

int lo =0, hi = n-1;mid = (lo + hi)/2; foundflag =0;While(roll[mid]>givenroll && hi > lo){ //recalculate mid if (roll[mid] > givenroll]) { // roll is towards upper half of the array hi = mid; } else {lo = mid;} mid = (lo+hi)/2;}If (roll[mid]==givenroll) foundflag =1;

Behaviour of Binary search 1

iteration lo hi mid

[0] 1001 72

[1] 1002 45

[2] 1003 91

[3] 1004 86

[4] 1006 38

[5] 1008 53

[6] 1009 65

[7] 1011 95

Behaviour of Binary search 2

iteration lo hi mid

[0] 1001 72

[1] 1002 45

[2] 1003 91

[3] 1004 86

[4] 1006 38

[5] 1008 53

[6] 1009 65

[7] 1011 95

Program for finding marks of a student

THANK YOU