1. std Library of C++

2. std Library Standard C++ Library
Has a lot of useful classes or abstract data types
cin, cout, and cerr – already seen. They are objects of istream or ostream classes.
string and vector are perhaps the next most important of the std library classes

3. std namespace
A namespace allows programmers to avoid collisions with same names for classes in different libraries.
Class names (and other names like objects cout, cin etc.) of Standard library are defined in the std namespace
cout of std namespace is different from, say, cout of xyz namespace.

4. std namespace …
std::cout specifies both namespace and object name within the namespace
Cumbersome to use std:: prefix everytime an object or class of std namespace is used
Solution is the ‘using declaration’

5. using declaration using namespace::name;
After the above declaration name can be used without specifying the namespace
#include <iostream>
// using declarations states our intent to use these names from the namespace std
using std::cout;
int main() {
cout << “Rama”; // ok: cout is now a synonym for std::cout }

6. using declaration … using namespace namespace-name;
After the above declaration compiler automatically searches in this namespace to resolve names
#include <iostream>
// using declarations states our intent to use these names from the namespace std
using namespace std;
int main() {
cout << “Rama”; // ok: compiler finds cout in std namespace }

7. Using declaration … using namespace std;
May result in unintended resolution of names to std namespace
So ‘using std::cout’ style is superior
But ‘using namespace std;’ is quite convenient
So can be used for our course work
And can also be used in real life C++ programs where too many namespaces are not involved

8. string class of std library
string supports variable-length character strings
Manages the memory associated with storing the characters of the string
Provides useful operations
Is efficient enough for general usage
#include <string>
using std::string;

9. string constructors string s1; string s2(s1); string s3("value");
//Default constructor; s1 is an empty string
string s2(s1);
//Initialize s2 as a copy of s1
string s3("value");
//Initialize s3 as a copy of the string literal
string s4(10, 'c');
//Initialize s4 with 10 copies of the character 'c'

10. Simple program using string
// book code: 3/string_io.cc
#include <string>
using namespace std;
int main() {
string s; // empty string
cin >> s; // read whitespace-separated string into s
cout << s << endl; // write s to the output
return 0; }

11. Reading strings till end of file
See word_echo.cc
Data file is data/word_echo
Note that:
even if multiple words are present on the same line in the input
each word is printed on a separate line in the output
The >> operator return the istream object that it read. That istream object evaluates to false on end of file/data.

12. Read a full line at a time
See getline.cc
getline is a function defined in <string>. It is not a member function of string class
getline takes an istream object (say cin) and a string object as arguments
getline reads a line of input including the newline character and
Stores the line of input excluding the newline character in its string argument.

13. Read a full line at a time
Empty line will be read into empty string
getline returns an istream object whose value can be tested for false (end of data returns false)
Compare word_echo with getline by running them with same data

14. string comparison s1 == s2 string big = "big", small = "small";
// Returns true if s1 and s2 are equal; false otherwise
string big = "big", small = "small";
if (big == small) // false
// ...

15. Observation 3Ob1.txt
Study and run the example programs that have been studied so far (and given below for your easy reference) and put down your observations in the file 3Ob1.txt. The example programs are:
string_io.cc
word_echo.cc
getline.cc

16. Assignment 3A
Write a program that reads words from standard input and writes it to standard output. However the following words should NOT be written to standard output:
and, or
in, on, of
The, the

17. Assignment 3B
Write a program that reads from standard input and outputs the number of words as well as the number of lines in it.

18. Common string operations
s.empty()
//Returns true if s is empty; otherwise returns false
s.size()
// Returns number of characters in s
s[n]
// Returns the character at position n in s; positions start at 0.
s1 + s2
// Returns a string equal to the concatenation of s1 and s2
s1 = s2
// Replaces characters in s1 by a copy of s2
s1 == s2
// Returns true if s1 and s2 are equal; false otherwise
!=, <, <=, >, and >=
Have their normal meanings

19. Size of string See string_size.cc
string::sizetype is the type of the return value of string::size()
We don’t need to know exact type of sizetype except that it is a numeric (unsigned) type and so can be used in all operations that an unsigned type can be used.
Sizetype allows us to be machine independent. E.g. On some machines it may be unsinged int and on some others it may be unsigned long.

20. string comparison string big = "big", small = "small";
string s1 = big; // s1 is a copy of big
if (big == small) // false
// ...
if (big <= s1) // true, they're equal, so big
// is less than or equal to s1

21. string comparison … If two strings have different lengths and
if every character in the shorter string is equal to the corresponding character of the longer string
then the shorter string is less than the longer one.
If the characters in two strings differ,
then we compare them by comparing the first character at which the strings differ.
As an example, given the strings
string substr = "Hello";
string phrase = "Hello World";
string slang = "Hiya";
then substr is less than phrase, and slang is greater than either substr or phrase
See strcompare.cc

22. string assignment
Assignment of one string to another results in a copy of one string in another
string st1;
string st2 = "The expense of spirit";
st1 = st2; // replace st1 by a copy of st2
st1 and st2 have different memory areas but their data contents are similar.
This is different from char pointer assignments

23. Char pointer assignment
char *s = null
char *p = “The expense of spirit”;
s = p;
Now both s and p point to same memory area

24. Adding strings string s1("hello, "); string s2("world\n");
string s3 = s1 + s2; // s3 is hello, world\n
s1 += s2; // equivalent to s1 = s1 + s2

25. Adding string literals and strings
string s1("hello");
string s2("world");
string s3 = s1 + ", " + s2 + "\n";
s3 will be “hello, world\n”

26. Adding strings and literals …
string s1 = "hello"; // no punctuation
string s2 = "world";
string s3 = s1 + ", "; // ok: adding a string and a
// literal string
s4 = "hello" + ", "; // error: no string operand
string s5 = s1 + ", " + "world"; // ok: each + has string
// operand string
s6 = "hello" + ", " + s2; // error: can't add string literals
s7 = string(“hello”) + “,” + s2; // ok

27. Fetching a char from a string
string str("some string");
for (string::size_type ix = 0; ix != str.size();
++ix)
cout << str[ix] << endl;
str[ix] = '*';

28. lvalue and rvalue string str(“Rama”); str[2] is an lvalue
lvalue (pronounced "ell-value"): An expression that is an lvalue may appear as either the left-hand or right-hand side of an assignment.
str[2] = ‘a’; //Is a valid expression as str[2]
//an lvalue
Variables are lvalues

29. rvalue
rvalue (pronounced "are-value"): An expression that is an rvalue may appear on the right- but not left-hand side of an assignment.
Literals are rvalues
“Rama” is an rvalue
“Rama” = “abc”; //Error. “Rama” is an
//rvalue and so cannot be on
//lhs of assignment expression

30. lvalue and rvalue
Sometimes compiler errors will refer to lvalue or rvalue.
So it is useful to know the terms

31. Functions dealing with char
<cctype> header defines the functions. The functions are:
isalnum(c) true if c is a letter or a digit
isalpha(c) true if c is a letter
iscntrl(c) true if c is a control character
isdigit(c) true if c is a digit
isgraph(c) true if c is not a space but is printable
islower(c) true if c is a lowercase letter

32. Functions dealing with char
isprint(c) true if c is a printable character
ispunct(c) true if c is a punctuation character
isspace(c) true if c is whitespace
isupper(c) true if c is an uppercase letter
isxdigit(c) true if c is a hexadecimal digit
tolower(c) If c is an uppercase letter, returns its
lowercase equivalent; otherwise
returns c unchanged
toupper(c) If c is a lowercase letter, returns its
uppercase equivalent; otherwise returns c
unchanged.

33. Example program for char functions
See cctype.cc

34. Assignments
Exercise 3.7: Write a program to read two strings and report whether the strings are equal. If not, report which of the two is the larger. Now, change the program to report whether the strings have the same length and if not report which is longer.
Exercise 3.8: Write a program to read strings from the standard input, concatenating what is read into one large string. Print the concatenated string. Next, change the program to separate adjacent input strings by a space.

35. Assignments …
Exercise 3.10: Write a program to strip the punctuation from a string. The input to the program should be a string of characters including punctuation; the output should be a string in which the punctuation is removed.

36. Book Source Code Copyright Note
The course book is C++ Primer, 4th Edition by Lippman, Lajoie and Moo. Any references in earlier files to source files, and use of code within those files, are of example code given in and/or along with the book. As these slides are freely accessible on the Internet, not-for-profit, and for educational purposes, based on the permission related statements in the source code, I have considered that permission has been granted to use them in these slides.