Chapter 9 C++ Programming Absolute C++ Lecture Slides

Absolute C++
Sixth Edition
Chapter 9
Strings
Copyright © 2016, 2013, 2010 Pearson Education, Inc. All Rights Reserved

Learning Objectives
9.1 An Array Type for Strings
9.1.1 C-Strings
9.2 Character Manipulation Tools
9.2.1 Character I/O
9.2.2 get, put member functions
9.2.3 putback, peek, ignore
9.3 Standard Class string
9.3.1 String processing

Introduction
• Two string types:
• C-strings
– Array with base type char
– End of string marked with null, “0”
– “Older” method inherited from C
• String class
– Uses templates

C-Strings
• Array with base type char
– One character per indexed variable
– One extra character: “0”
▪Called “null character”
▪End marker
• We’ve used c-strings
– Literal “Hello” stored as c-string

C-String Variable
• Array of characters:
char s[10];
– Declares a c-string variable to hold up to
9 characters
– + one null character
• Typically “partially-filled” array
– Declare large enough to hold max-size string
– Indicate end with null
• Only difference from standard array:
– Must contain null character

C-String Storage
• A standard array:
char s[10];
– If s contains string “Hi Mom”, stored as:

C-String Initialization
• Can initialize c-string:
char myMessage[20] = “Hi there.”;
– Needn’t fill entire array
– Initialization places “0” at end
• Can omit array-size:
char shortString[] = “abc”;
– Automatically makes size one more than
length of quoted string
– NOT same as:

C-String Indexes
• A c-string IS an array
• Can access indexed variables of:
char ourString[5] = “Hi”;

C-String Index Manipulation
• Can manipulate indexed variables
char happyString[7] = “DoBeDo”;
happyString[6] = “Z”;
– Be careful!
– Here, “0” (null) was overwritten by a “Z”!
• If null overwritten, c-string no longer “acts” like c-string!
– Unpredictable results!

Library
• Declaring c-strings
– Requires no C++ library
– Built into standard C++
• Manipulations
– Require library <cstring>
– Typically included when using c-strings
▪Normally want to do "fun" things with them

= and == with C-strings
• C-strings not like other variables
– Cannot assign or compare:
–Can ONLY use “=” at declaration of c-string!
• Must use library function for assignment:
strcpy(aString, “Hello”);
–Built-in function (in <cstring>)
–Sets value of aString equal to “Hello"
–NO checks for size!
▪Up to programmer, just like other arrays!

Comparing C-strings
• Also cannot use operator ==
–aString == anotherString; // NOT allowed!
• Must use library function again:

The <cstring> Library: Display 9.1 Some
Predefined C-String Functions in <cstring> (1 of 2)
• Full of string manipulation functions

Copyright © 2016,2013,2011 Pearson Education, Inc. All Rights Reserved
The <cstring> Library: Display 9.1 Some
Predefined C-String Functions in <cstring> (2 of 2)

C-string Functions: strlen()
• “String length”
• Often useful to know string length:
–Returns number of characters
▪Not including null
–Result here:
6

C-string Functions: strcat()
• strcat()
• “String concatenate”:
–Note result:
stringVar now contains “The rainin Spain”
–Be careful!
–Incorporate spaces as needed!

C-string Arguments and Parameters
• Recall: c-string is array
• So c-string parameter is array parameter
– C-strings passed to functions can be changed
by receiving function!
• Like all arrays, typical to send size as well
– Function “could” also use “0” to find end
– So size not necessary if function won’t change
c-string parameter
– Use “const” modifier to protect c-string arguments

C-String Output
• Can output with insertion operator, <<
• As we’ve been doing already:
–Where news is a c-string variable
• Possible because << operator is
overloaded for c-strings!

C-String Input
• Can input with extraction operator, >>
– Issues exist, however
• Whitespace is “delimiter”
– Tab, space, line breaks are “skipped”
– Input reading “stops” at delimiter
• Watch size of c-string
▪Must be large enough to hold entered string!
▪C++ gives no warnings of such issues!

C-String Input Example
• Dialogue offered:
Enter input: Do be do to you!
DobeEND OF OUTPUT
– Note: Underlined portion typed at keyboard
• C-string a receives: “do"
• C-string b receives: “be”

C-String Line Input
• Can receive entire line into c-string
• Use getline(), a predefined member function:
–Dialogue:
Enter input: Do be do to you!
Do be do to you! END OF INPUT

Example: Command Line Arguments (1 of 3)
• Programs invoked from the command line (e.g. a UNIX
shell, DOS command prompt) can be sent arguments
– Example: COPY C:FOO.TXT D:FOO2.TXT
▪This runs the program named “COPY” and sends in
two C-String parameters, “C:FOO.TXT” and “D:
FOO2.TXT”
▪It is up to the COPY program to process the inputs
presented to it; i.e. actually copy the files
• Arguments are passed as an array of C-Strings to the main
function

• Header for main
– int main(int argc, char *argv[])
– argc specifies how many arguments are supplied. The
name of the program counts, so argc will be at least 1.
– argv is an array of C-Strings.
▪argv[0] holds the name of the program that is
invoked
▪argv[1] holds the name of the first parameter
▪argv[2] holds the name of the second parameter
▪Etc.

More getline()
• Can explicitly tell length to receive:
–Results:
Enter input: dobedowap
dobeEND OF OUTPUT
–Forces FOUR characters only be read
▪Recall need for null character!

Character I/O
• Input and output data
– ALL treated as character data
– e.g., number 10 outputted as “1” and “0”
– Conversion done automatically
▪Uses low-level utilities
• Can use same low-level utilities ourselves as well

Member Function get()
• Reads one char at a time
• Member function of cin object:
–Reads next char & puts in variable
nextSymbol
–Argument must be char type
▪Not “string”!a

Member Function put()
• Outputs one character at a time
• Member function of cout object:
• Examples:
• Outputs letter “a” to screen
• Outputs letter “e” to screen

More Member Functions
• putback()
– Once read, might need to “put back”
– cin.putback(lastChar);
• peek()
– Returns next char, but leaves it there
– peekChar = cin.peek();
• ignore()
– Skip input, up to designated character
– Cin.ignore(1000, “n”);
▪Skips at most 1000 characters until “n”

Character-Manipulating Functions: Display
9.3 Some Functions in <cctype> (1 of 3)

Standard Class string
• Defined in library:
#include <string>
using namespace std;
• String variables and expressions
– Treated much like simple types
• Can assign, compare, add:
string s1, s2, s3;
s3 = s1 + s2; //Concatenation
s3 = “Hello Mom!” //Assignment
– Note c-string “Hello Mom!” automatically
converted to string type!

Display 9.4 Program Using the Class string

I/O with Class string
• Just like other types!
• Results:
User types in:
May the hair on your toes grow long and curly!
• Extraction still ignores whitespace:
s1 receives value “May”
s2 receives value “the”

getline() with Class string
• For complete lines:
string line;
• Dialogue produced:
Enter a line of input: Do be do to you!
Do be do to you! END OF INPUT
– Similar to c-string’s usage of getline()

Other getline() Versions
• Can specify “delimiter” character:
string line;
–Receives input until “?” encountered
• getline() actually returns reference
–string s1, s2;
getline(cin, s1) >> s2;
–Results in: (cin) >> s2;

Pitfall: Mixing Input Methods
• Be careful mixing cin >> var and getline
–If input is: 42
Hello hitchhiker.
▪Variable n set to 42
▪line set to empty string!
–Cin >> n skipped leading whitespace, leaving
“n” on stream for getline()!

Class string Processing
• Same operations available as c-strings
• And more!
– Over 100 members of standard string class
• Some member functions:
– .length()
▪Returns length of string variable
– .at(i)
▪Returns reference to char at position i

Display 9.7 Member Functions of the
Standard Class string (1 of 2)

Display 9.7 Member Functions of the
Standard Class string (2 of 2)

C-string and string Object Conversions
• Automatic type conversions
▪Perfectly legal and appropriate!
–aCString = stringVar;
▪ILLEGAL!
▪Cannot auto-convert to c-string
–Must use explicit conversion:
strcpy(aCString, stringVar.c_str());

Converting between string and numbers
• In C++11 it is simply a matter of calling stof, stod, stoi, or
stol to convert a string to a float, double, int, or long,
respectively.

Converting between numbers and string
objects
• In C++11 use to_string to convert a numeric type to a
string

Summary
• C-string variable is “array of characters”
– With addition of null character, “0”
• C-strings act like arrays
– Cannot assign, compare like simple variables
• Libraries <cctype> & <string> have useful manipulating
functions
• cin.get() reads next single character
• getline() versions allow full line reading
• Class string objects are better-behaved than c-strings

Copyright

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