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Programming in C++ Dale/Weems/Headington Chapter 12 Applied Arrays: Lists and Strings. 1. ‘H’ ‘e’ ‘l’ ‘l’ ‘o’ ‘\0’. message [0] [1] [2] [3] [4] [5] [6] [7]. String in C++. - PowerPoint PPT Presentation
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1
Programming in C++
Dale/Weems/HeadingtonChapter 12
Applied Arrays: Lists and Strings
2
String in C++
A string is an array of characters which contains a non-printing null character ‘\0’ ( with ASCII value 0 ) marking its end.
A string can be initialized in its declaration in two equivalent ways.
char message [ 8 ] = { ‘H’, ‘e’, ‘l’, ‘l’, ‘o’, ‘\0’ };
char message [ 8 ] = “Hello” ;
message [0] [1] [2] [3] [4] [5] [6] [7]
‘H’ ‘e’ ‘l’ ‘l’ ‘o’ ‘\0’
3
char vs. string
‘A’ has data type charand is stored in 1 byte
“A” is a string of 2 characters
and is stored in 2 bytes
5000
‘A’
6000
‘A’
6001
‘\0’
4
Recall that . . .
char message[8]; // this declaration allocates memory
To the compiler, the value of the identifier message alone is the base address of the array. We say message is a pointer (because its value is an address). It “points” to a memory location.
message [0] [1] [2] [3] [4] [5] [6] [7]
‘H’ ‘e’ ‘l’ ‘l’ ‘o’ ‘\0’
6000
5
Aggregate String I/O in C++
I/O of an entire string is possible using the array identifier with no subscripts and no looping.
EXAMPLE
char message [ 8 ] ; cin >> message ; cout << message ;
HOWEVER . . .
6
Extraction operator >>
When using the extraction operator ( >> ) to read input characters into a string variable,
the >> operator skips any leading whitespace characters such as blanks and newlines.
It then reads successive characters into the array, and stops at the first trailing whitespace character (which is not consumed, but remains waiting in the input stream).
The >> operator adds the null character to the end of the string.
7
Example using >>
char name [ 5 ] ;
cin >> name ;
Suppose input stream looks like this: J o e
name [0] name [1] name [2] name [3] name [4]
7000
total number of elements in the array
null character is added
‘J’ ‘o’ ‘e’ ‘\0’
8
Function get( ) Because the extraction operator stops reading at the
first trailing whitespace, >> cannot be used to input a string with blanks in it.
If your string’s declared size is not large enough to hold the input characters and add the ‘\0’, the extraction operator stores characters into memory beyond the end of the array.
Use get function with 2 parameters to overcome these obstacles.
EXAMPLE
char message [ 8 ] ;
cin.get ( message, 8 ) ; // inputs at most 7 characters plus ‘\0’
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inFileStream.get ( str, count + 1)
get does not skip leading whitespace characters such as blanks and newlines.
get reads successive characters (including blanks) into the array, and stops when it either has read count characters, or it reaches the newline character ‘\n’, whichever comes first.
get appends the null character to str.
If it is reached, newline is not consumed by get, but remains waiting in the input stream.
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Function ignore( )
can be used to consume any remaining characters up to and including the newline ‘\n’ left in the input stream by get
EXAMPLE
cin.get ( string1, 81 ) ; // inputs at most 80 characters
cin.ignore ( 30, ‘\n’ ) ; // skips at most 30 characters // but stops if ‘\n’ is
read
cin.get ( string2, 81 ) ;
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Another example using get( )char ch ;
char fullName [ 31 ] ;
char address [ 31 ] ;
cout << “Enter your full name: “ ;
cin.get ( fullName, 31 ) ;
cin.get (ch) ; // to consume the newline
cout << “Enter your address: “ ;
cin.get ( address, 31 ) ;
fullName [0]
‘N’ ‘e’ ‘l’ ‘l’ ‘ ’ ‘D’ ‘a’ ‘l’ ‘e’ ‘\0’ . . .
address [0]
‘A’ ‘u’ ‘s’ ‘t’ ‘i‘ ‘n’ ‘ ’ ‘T’ ‘X’ ‘\0’ . . .
12
String function prototypes in< string.h >
int strlen (char str [ ] ); // FCTNVAL == integer length of string str ( not including ‘\0’ )
int strcmp ( char str1 [ ], char str2 [ ] ); // FCTNVAL == negative, if str1 precedes str2 lexicographically
// == positive, if str1 follows str2 lexicographically
// == 0, if str1 and str2 characters same through ‘\0’
char * strcpy ( char toStr [ ], char fromStr [ ] );// FCTNVAL == base address of toStr ( usually ignored )
// POSTCONDITION : characters in string fromStr are copied to
// string toStr, up to and including ‘\0’,
// overwriting contents of string toStr
# include <string.h> . . .
char author [ 21 ] ;
int length ;
cin.get ( author , 21 ) ;
length = strlen ( author ) ; // What is the value of length ?
5000
author [0]
‘C’ ‘h’ ‘i’ ‘p’ ‘ ’ ‘W’ ‘e’ ‘e’ ‘m’ ‘s’ ‘\0’ . . . .
char myName [ 21 ] = “Huang” ; // WHAT IS OUTPUT?
char yourName [ 21 ] ;
cout << “Enter your last name : “ ;
cin.get ( yourName, 21 ) ;
if ( strcmp ( myName, yourName ) == 0 )
cout << “We have the same name! “ ;
else if ( strcmp ( myName, yourName ) < 0 )
cout << myName << “ comes before “ << yourName ;
else if ( strcmp ( myName, yourName ) > 0 )
cout << yourName << “comes before “ << myName ;
myName [0]
‘H’ ‘u’ ‘a’ ‘n’ ‘g’ ‘\0’ . . .
yourName [0]
‘H’ ‘e’ ‘a’ ‘d’ ‘i‘ ‘n’ ‘ g’ ‘t’ ‘o’ ‘n’ ‘\0’ . . .
char myName [ 21 ] = “Huang” ;
char yourName [ 21 ] ;
if ( myName == yourName ) // compares addresses only!
{ // That is, 4000 and 6000 here.
. // DOES NOT COMPARE CONTENTS! . .
}
myName [0]
‘H’ ‘u’ ‘a’ ‘n’ ‘g’ ‘\0’ . . .
yourName [0]
‘H’ ‘e’ ‘a’ ‘d’ ‘i‘ ‘n’ ‘ g’ ‘t’ ‘o’ ‘n’ ‘\0’ . . .
4000
6000
char myName [ 21 ] = “Huang” ;
char yourName [ 21 ] ;
cin.get ( yourName, 21 ) ;
yourName = myName; // DOES NOT COMPILE!
// What is the value of myName ?
myName [0]
‘H’ ‘u’ ‘a’ ‘n’ ‘g’ ‘\0’ . . .
yourName [0]
4000
6000
‘H’ ‘e’ ‘a’ ‘d’ ‘i‘ ‘n’ ‘ g’ ‘t’ ‘o’ ‘n’ ‘\0’ . . .
char myName [ 21 ] = “Huang” ;
char yourName [ 21 ] ;
cin.get ( yourName, 21 ) ;
strcpy ( yourName, myName ) ; // changes string yourName
// OVERWRITES CONTENTS!
myName [0]
‘H’ ‘u’ ‘a’ ‘n’ ‘g’ ‘\0’ . . .
yourName [0]
‘H’ ‘e’ ‘a’ ‘d’ ‘i‘ ‘n’ ‘ g’ ‘t’ ‘o’ ‘n’ ‘\0’ . . .
4000
6000 ‘u’ ‘n’ ‘g’ ‘\0’
18
Using typedef with arrays
typedef int Boolean ; // names Boolean as a data type
typedef char String20 [ 21 ] ; // names String20 as an array type
String20 myName ; // these declarations
String20 yourName ; // allocate memory for 3 variables
Boolean isSeniorCitizen ;
5000
7000
6000
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Write a program that will...
Read the ID numbers, hourly wages, and names, for up to 50 persons from a data file.
Then display the ID number and hourly wage for any person in the file whose name is entered at the keyboard, or indicate that the person was not located, if that is the case.
20
Assume file has this form with data for no more than 50 persons
4562 19.68 Dale Nell 1235 15.75 Weems Chip 6278 12.71 Headington Mark . . . . . . . . .
8754 17.96 Cooper Sonia2460 14.97 Huang Jeff
21
Parallel arrays hold related data
const int MAX_PERSONS = 50;
typedef char String20[ 21] ; // define data type
.
.
.
// declare 3 parallel arrays
int idNums[ MAX_PERSONS ] ;
float wages[ MAX_PERSONS ] ;
String20 names[ MAX_PERSONS ] ;
// holds up to 50 strings each with
// up to 20 characters plus null character ‘\0’
22
idNums[ 0 ] 4562 wages[ 0 ] 19.68 names[ 0 ] “Dale Nell”
idNums[ 1 ] 1235 wages[ 1 ] 15.75 names[ 1 ] “Weems Chip”
idNums[ 2 ] 6278 wages[ 2 ] 12.71 names[ 2 ] “Headington Mark”
. . . . . . . . . . . . . . . . . .
idNums[ 48] 8754 wages[ 48] 17.96 names[ 48] “Cooper Sonia”
idNums[ 49] 2460 wages[ 49] 14.97 names[ 49] “Huang Jeff”
int idNums [ MAX_PERSONS ] ; // parallel arrays
float wages [ MAX_PERSONS ] ;
String20 names [ MAX_PERSONS ] ;
23
#include < iomanip.h >#include < iostream.h >#include < fstream.h >#include < ctype.h >#include < string.h >#include “bool.h”
typedef char String20 [ 21 ] ;const int MAX_PERSONS = 50 ;
void GetData ( int [ ], float [ ], String20 [ ], int & ) ; // prototypes
void HandleRequests ( int [ ], float [ ], String20 [ ], int ) ;
void LookUp ( String20 [ ], String20, int, Boolean & , int & ) ;
Using array of strings
24
Main Program
int main (void){ int idNums [MAX_PERSONS] ; // holds up to 50 IDs float wages [MAX_PERSONS] ; // holds up to 50 wages String20 names [MAX_PERSONS] ; // holds up to 50 names int numPersons; // number of persons’ information in file
GetData ( idNums, wages, names, numPersons ) ;
HandleRequests ( idNums, wages, names, numPersons ) ;
cout << “End of Program.\n”;
return 0 ;}
25
Module Structure Chart
Main
GetData
LookUp
HandleRequests
names oneNamenumPersons
idNums wages names numPersons
idNums wages names numPersons
foundindex
void GetData ( /* out */ int ids[ ] , /* out*/ float wages[ ] , /* out */ String20 names[ ] , /* out */ int & howMany )
{ ifstream myInfile ; // Reads data from data fileint k = 0 ;
char ch ;
myInfile.open (“A:\\my.dat”) ; if ( ! myInfile ) { cout << “File opening error. Program terminated! “ << endl ; exit ( 1 ) ; } myInfile >> ids[ k ] >> wages [k] ; // get information for first person myInfile.get(ch) ; // read blank myInfile.get (names[ k ] , 21) ; myInfile.ignore(30, ‘\n’) ; // consume newline
while (myInfile) // while the last read was successful { k++ ;
myInfile >> ids[ k ] >> wages [k] ; myInfile.get(ch) ; // read blank myInfile.get (names[ k ] , 21) ;
myInfile.ignore(30, ‘\n’) ; // consume newline } howMany = k;}
void HandleRequests( const /* in */ int idNums[ ], const /* in */ float wages[ ] , const /* in */ String20 names[ ], /* in */ int numPersons )
{ String20 oneName ; // string to hold name of one person int index ; // will hold an array index value char response; // user’s response whether to continue Boolean found; // has oneName been located in array names
do { cout << “Enter name of person to find: ” ; cin.get (oneName, 21) ;
cin.ignore (100, ‘\n’); // consume newline
LookUp (names, oneName, numPersons, found, index );
if ( found ) cout << oneName << “ has ID # “ << idNums [index]
<< “ and hourly wage $ “ << wages [index] << endl;else cout << oneName << “ was not located. “ << endl;
cout << “Want to find another (Y/N)? “;cin >> response ;response = toupper ( response );
} while ( response == ‘Y’ );}
void LookUp ( const /* in */ String20 names [ ], const /* in */ String20 oneName, /* in */ int numPersons, /* out */ Boolean & found , /* out */ int &
index)
// Sequential search of unordered array. // POSTCONDITION:// IF oneName is in names array// found == true && names[index] == oneName// ELSE// found == false && index == numPersons{
index = 0; found = false; // initialize flag while ( ( ! found ) && ( index < numPersons ) ) // more to search
{if ( strcmp ( oneName, names[index] ) == 0 ) // match here
found = true ; // change flagelse
index ++ ; }
}
29
Some Questions
Assume that numPersons has value 50.
How many actual names in the array namesmust be examined before determining that oneName was not located?
Now suppose MAX_PERSONS is later changed to 1000.
How many actual names in the array names would need to be examined to determine that oneName cannot be located?
What are some ways to “speed up” this process?
30
Ways to improve efficiency of searching process
If the array names were sorted, the sequential search for oneName could be aborted as soon as a single name with greater lexicographic (dictionary) order is examined.
If the array names were sorted, a faster type of search, called a binary search, could be used instead of the slower sequential search.
31
Sorting
means arranging the list elements into some order (for instance, strings into alphabetical order, or numbers into ascending or descending order).
sorting
Dale NellWeems ChipHeadington MarkCooper SoniaHuang Jeff
Cooper Sonia Dale NellHeadington MarkHuang Jeff Weems Chip
32
Selection Sort Process
examines the entire list to select the smallest element. Then places that element where it belongs (with array subscript 0).
examines the remaining list to select the smallest element from it. Then places that element where it belongs (with array subscript 1).
. . .
examines the last 2 remaining list elements to select the smallest one. Then places that element where it belongs in the array.
33
Selection Sort Algorithm
FOR pass going from 0 through length - 2
Find minimum value in list [ pass . . length-1 ]
Swap minimum value with list [ pass ]
length = 5
names [ 0 ] Dale Nell Cooper Sonia names [ 1 ] Weems Chip Weems Chip names [ 2 ] Headington Mark Headington Mark names [ 3 ] Cooper Sonia Dale Nell names [ 4 ] Huang Jeff Huang Jeff
pass = 0
void SelSort ( /* inout */ String20 names [ ] , /* in */ int length )
// Selection sorts names into alphabetic order// Preconditions: length <= MAX_PERSONS// && names [0 . . length -1 ] are assigned// Postcondition: names [ 0 . . length -1 ] are rearranged into order
{ int pass; int place; int minIndex; String20 temp; for ( pass = 0 ; pass < length - 1 ; pass++ )
{ minIndex = pass; for ( place = pass + 1 ; place < length ; place ++ ) if ( strcmp ( names [ place ] , names [ minIndex ] ) < 0 )
minIndex = place; //swap names[pass] with names[minIndex]
strcpy ( temp , names [ minIndex ] ) ; strcpy ( names [ minIndex ] , names [ pass] ) ; strcpy ( names [ pass ] , temp ) ; }}
35
Binary Search in an Ordered List
Examines the element in the middle of the array. Is it the sought item? If so, stop searching. Is the middle element too small? Then start looking in second half of array. Is the middle element too large? Then begin looking in first half of the array.
Repeat the process in the half of the list that should be examined next.
Stop when item is found, or when there is nowhere else to look and it has not been located.
36
void BinSearch( /* in */ const ItemType list [ ] , /* in */ ItemType item, /* in */ int length, /* out */ int& index,
/* out */ Boolean & found )
// Searches list for item, returning index of item if found
// Precondition:// list [0 . . length - 1 ] are in ascending order // && length is assigned && item is assigned//// Postcondition:// IF item is in list// found == true && list[index] == item// ELSE// found == false && index is undefined
Heading for BinSearch function
37
{ int first = 0 ; // lower bound on list int last = length - 1 ; // upper bound on list int middle ; // middle index
found = false ; while ( ( last >= first ) && (! found ) )
{ middle = ( first + last ) / 2 ;
if ( item < list [ middle ] )last = middle - 1 ; // look in first half next
else if ( item > list [ middle ] )first = middle + 1; // look in second half next
else found = true ; } index = middle ; }
Body for BinSearch function
38
Trace of BinSearch function
list[0] [1] [2] [3] [4] [5] [6] [7] [8] [9]
15 26 38 57 62 78 84 91 108 119
item = 45
first middle last
list[0] [1] [2] [3] [4] [5] [6] [7] [8] [9]
15 26 38 57 62 78 84 91 108 119
first middle last
item < list [ middle ] last = middle - 1
item > list [ middle ] first = middle + 1
39
Trace continued
list[0] [1] [2] [3] [4] [5] [6] [7] [8] [9]
15 26 38 57 62 78 84 91 108 119
item = 45
first, middle, last
list[0] [1] [2] [3] [4] [5] [6] [7] [8] [9]
15 26 38 57 62 78 84 91 108 119
first, last middle
item > list [ middle ] first = middle + 1
item < list [ middle ] last = middle - 1
40
Trace concludes
list[0] [1] [2] [3] [4] [5] [6] [7] [8] [9]
15 26 38 57 62 78 84 91 108 119
item = 45
last first
last < first found = false