C-strings
and
C++ string Class
Topics
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C-Strings
Library Functions for Working with C-Strings
Conversions Between Numbers and Strings
Character Testing
Character Case Conversion
Writing Your Own C-String Handling Functions
C++ string Class
Strings
• C-string: array of characters terminated by
NULL character
• The C-string
"Hi there!"
would be stored in memory as shown:
H
12-3
i
t
h
e
r
e
!
\0
Array of char
• Array of char can be defined and initialized
to a C-string
char str1[20] = "hi there!";
• Array of char can be defined and later have
a string copied into it
char str2[20];
strcpy(str2, "hi there!");
12-4
Pointer to char
• Pointer to char can refer to C-strings defined
as arrays of char
char str[20] = "hi";
char *pStr = str;
cout << pStr; // prints hi
• Can dynamically allocate memory to be used
for C-string using new
12-5
Library Functions for C-Strings
(Requires #include <cstring>)
• int strlen(char *str)
// Returns length of a C-string:
cout << strlen("hello");
// Prints: 5
12-6
const int LENGTH = 30;
char line[LENGTH];
cout << "Enter a string (no more than "
<< LENGTH - 1 << " characters.)\n";
cin.getline(line, LENGTH);
cout <<
<<
<<
<<
"The C-string read was "
"\"" << line << "\""
" whose length was " << strlen(line)
endl;
strcpy
• char name[20] = “Mary” // This is OK
char name2[20];
name2 = “Nancy”;
char name3[20];
strcpy(name3, “Nancy”);
cout << name3;
12-8
// Now allowed
// Copy
// Prints Nancy
strcpy
• strcpy(char *dest, char *source)
// Copies a string from a source address
// to a destination address
char name[15];
strcpy(name, "Deborah");
cout << name; // prints Deborah
12-9
strcat
• strcat(char *dest, char *source)
// Takes two C-strings as input.
// Adds the contents of the second string to the
// end of the first string:
char str1[15] = "Good ";
char str2[30] = "Morning!";
strcat(str2, str1);
cout << str1; // prints: Good Morning!
• No automatic bounds checking: programmer must
ensure that str1 has enough room for result
12-10
strcmp
• int strcmp(char *str1, char*str2)
// Compares strings stored at two addresses
• Returns a value:
less than 0 if str1 precedes str2
equal to 0 if str1 equals str2
greater than 0 if str1 succeeds str2
12-11
strcmp
• Often used to test for equality
if(strcmp(str1, str2) == 0)
cout << "equal";
else
cout << "not equal";
• Also used to determine ordering of C-strings in sorting
applications
• Note that C-strings cannot be compared using == or >
if (str1 > str2) -- this is not allowed
(compares addresses of C-strings, not contents)
• If 12-12
Swapping
const int NAME_LENGTH = 20;
char name1[NAME_LENGTH];
char name2[NAME_LENGTH];
char temp[NAME_LENGTH];
strcpy(name1, "William");
strcpy(name2, "Catherine");
cout << "Originally...\n";
cout << "Name1: " << name1 << endl;
cout << "Name2: " << name2 << endl;
12-13
Swapping (cont.)
if (strcmp(name1, name2) > 0)
{
strcpy(temp, name1);
strcpy(name1, name2);
strcpy(name2, temp);
}
cout << "\nAfter switch...\n";
cout << "Name1: " << name1 << endl;
cout << "Name2: " << name2 << endl;
12-14
Character Testing
Requires #include <cctype>
FUNCTION MEANING
isalpha() true if arg. is a letter, false otherwise
isalnum() true if arg. is a letter or digit, false
otherwise
isdigit() true if arg. is a digit 0-9, false otherwise
islower() true if arg. is lowercase letter, false
otherwise
12-15
Character Testing
Require #include<cctype>
FUNCTION MEANING
isprint() true if arg. is a printable character, false
otherwise
ispunct() true if arg. is a punctuation character,
false otherwise
isupper() true if arg. is an uppercase letter, false
otherwise
isspace() true if arg. is a white space character,
false otherwise
12-16
C++ String Class
The C++ string Class
• The string class offers several advantages over
C-style strings:
large body of member functions
overloaded operators to simplify expressions
• Requires #include <string>
12-18
string class constructors
• string()
• string(string str) initializes string
object with str
• string(char *cstr) initializes string
ojbect with C-string
• Various other constructors
www.cplusplus.com
12-19
String Class Member Functions
• int find(string str)
// returns the position of str in the string object
• int find (char ch)
// returns the position of ch in the string object
• int find (string str, int x)
// returns the position of str beyond x
• int find (ch ch, intx)
// returns the position of ch beyond
String Class Member Functions (cont.)
• void insert (int x, string str)
// inserts str at position x
• void insert (int x, char ch)
// inserts char at position x
• string replace(int pos, int n, string str)
// replaces n characters starting at pos with
// substring str
• Other member functions www.cplusplus.com
Overloaded string Operators
OPERATOR
>>
<<
=
+=
MEANING
reads whitespace-delimited strings
into string object
outputs string object to a stream
assigns string on right to string
object on left
appends string on right to end of
contents of string on left
12-22
Overloaded string Operators (continued)
OPERATOR
+
[]
>, >=,
<, <=,
==, !=
MEANING
Returns concatenation of the two
strings
references character in string using
array notation
relational operators for string
comparison. Return true or false
12-23
Overloaded string Operators
string word1, phrase;
string word2 = " Dog";
cin >> word1; // user enters "Hot"
// word1 has "Hot"
phrase = word1 + word2; // phrase has
// "Hot Dog"
phrase += " on a bun";
for (int i = 0; i < 16; i++)
cout << phrase[i]; // displays
// "Hot Dog on a bun"
12-24
Swapping Two Strings
string name1 = "William";
string name2 = "Catherine";
string temp;
cout << "Originally...\n";
cout << "Name 1: " << name1 << endl;
cout << "Name 2: " << name2 << endl;
if (name1 > name2){
temp = name1;
name1 = name2;
name2 = temp;
}
cout << "\nAfter swap...\n";
cout << "Name 1: " << name1 << endl;
cout << "Name 2: " << name2 << endl;
Conversion to C-strings
• data() and c_str() both return the Cstring equivalent of a string object
• Useful when using a string object with a
function that is expecting a C-string
char greeting[20] = "Have a ";
string str("nice day");
strcat(greeting, str.data());
12-26
Modification of string objects
• str.insert(int pos, string s)
inserts s at position pos in str
• Convert constructor for string allows a
C-string to be passed in place of s
string str("Have a day");
str.insert(7, "nice ");
• insert is overloaded for flexibility
12-27
Modification of string objects
• str.append(string s)
appends contents of s to end of str
• Convert constructor for string allows a
C-string to be passed in place of s
string str("Have a ");
str.append("nice day");
• append is overloaded for flexibility
12-28
Your Turn
(Formatting Number)
• Problem: Design a function which formats a
string like “1234567.89” to
“$1,234,567.89”.
• Plan:
– Find the position dp of the decimal point
– Starting at dp, count backwards and insert a
comma every 3 places.
– At position 0, insert ‘$’.
Refinement
dp
0
1
2
3
4
5
6
7
8
9
1 2 3 4 5 6 7
.
9 5
• dp  position of decimal point in string
pos  dp
• Loop (while dp is greater than 3)
go back 3 places in the string
insert ‘,’ there
End Loop
• Insert “$” at front of string
Your Turn
• Write a program which inputs a long text
and counts the frequency of occurrence of
each letter in the text.
letters
frequency
0
1
2
3
4
5
…
23
24
25
8 1 2 2 20 3 … 1 7 0
In Main()
// Declarations
const int MAX = 26;
int frequency[MAX];
string text;
cout << "Please enter a “ +
“substantially long text.\n";
getline(cin, text);
string input(text);
// convert to
// c-string
letterFrequency(frequency, MAX, input);
Letter Frequency
• The quick brown fox jumps over the lazy
dog.
• Pack my box with five dozen liquor jugs.
• Now is the time for all good people to
come to the aid of their country.
letterFrequency()
void letterFrequency(int frequency[], int count, string
input) {
// Initialize requency
for (int i = 0; i < count; i++){
frequency[i] = 0;
}
// Count frequency
int length = input.length();
for (int i = 0; i < length; i++)
{
switch(input[i])
{
case 'a':
case 'A': frequency[0]++; break;
case 'b':
case 'B': frequency[1]++; break;
letterFrequency() (cont.)
case 'c':
case 'C':
case 'd':
case 'D':
case 'e':
case 'E':
. . .
. . .;
case 'x':
case 'X':
case 'y':
case 'Y':
case 'z':
case 'Z':
default:;
} // switch
}
}
frequency[2]++; break;
frequency[3]++; break;
frequency[4]++; break;
frequency[23]++; break;
frequency[24]++; break;
frequency[25]++; break;
String I/O
• string str1;
cin >> str1; // delimitted by
// white-space
cout << str1;
• string str2;
getline(cin, str2);
// delimitted by
// by eol
Your Turn
• Problem:
– Input a long sentence and count the number of
words.
• Assumption:
– All words are delimitted by a single space.
In Main()
• // Input text
• // wordCount = countWords(text)
• // Output results
countWords()
• int countWords(string text)
Loop (while length > 0 && !done)
ps  position of ‘ ‘ in text
If (ps = 0)
done  true
End If
Add one to count
Replace first part of text with “”
• End Loop