.Open The C++ Standard string library
C++ has a powerful and useful class of objects in its library.
They are called `strings`.  They contain characters.  You can use them to
model words and character data.   They can contain any characters or
no characters.  They can change size as you need.  You can input them
and output them.  You can also "slice and dice" them.

The characters in a string are numbered, starting at 0 -- don't ask why!

So  `string("12345")` has the character "1" in place 0, "2" in place 1,
and so on.

These notes give a simplified description of the ways they can be used.
. Include this library
The easy way to use strings is to put these lines at the start of your program
.As_is #include <string>
.As_is using namespace std;
. Declaring a string variable
.As_is string newVariable = "initial value";
.As_is string newVariable = anyStringExpression;
. Constructing a C++ string from a litteral
.As_is 		string ("example");

This is also use to construct a C++ string from an old C char*:
.As_is 		char * cp = "example";
.As_is 		string (cp);

. Accessing data about a string == length and empty
If you have a string called `s` then
.As_is 	s.length()
is the number of characters in the string, and
.As_is 	s.empty()
is true if the string is empty. So, for example,
.As_is 	string("example").length()
has value 7.

. Accessing the old-style C char* in a string
This expression gives you the array of chars,
terminated by a null '\0' char:
.As_is 		s.c_str()

. Concatenating two or more strings
You can use the "+" symbol to combine strings.  For example,
if we declare
.As_is 		string a = "abra";
.As_is 		string b = "ca";
.As_is 		string c = "dabra";
then
.As_is 		cout << a + b + c <<endl;
outputs "abracadabra".  Notice you have to have at least one string variable
or explicitly convert a old-style C string like "abra" into a modern string:
.As_is 		cout << string("abra") + "ca" + "dabra" <<endl;
otherwise C++ has problems with it.

. Copying and assigning strings
The normal assignment (=) works with strings.

. Extracting substrings
This
.As_is   string substr (int position, int length)
extracts a substring. So
.As_is   string("example").substr (0, 4)
is `string("exam")` and
.As_is   string("example"). substr (6, 1)
is `string("e")`.

By the way.... i first wrote "substr(7,1)" becasue I started counting
the characters with "1" not "0".   Look out for this "off by one"
error.

. Searching for a substring
There are many different ways of searching a string for a substring.
The simplest is `find`.
.As_is 	string("example").find("e")
has value 0 (the place of the first "e", and
.As_is 	string("example").find("e", 1)
has value 6 -- the next "e" starting from the "x".

.As_is   int find (substring)
.As_is   int find (substring, startingPoint)

If a matching substring is not found "find()" returns `string::npos` which is
a value that is never the palce of a substring in a string.

. Comparisons
Lexographic/alphabetical) -- same syntax as for ints and doubles.
.As_is    s1 == s2
.As_is    s1 != s2
.As_is    s1 < s2
.As_is    s1 > s2
.As_is    s1<=s2
.As_is    s1>=s2

Again one of the strings must be modern C++ string.   `"bad" < "cad"` does not
compare the contents of the literals.

. Input words into a string
This stops at the first whitespace character but does not store it.
.As_is 	cin >> word;
. Input lines into a string
Use
.Key getline
like this
.As_is 	getline (cin, line);
This works with all input streams.

. Output strings
.As_is 	cout << stringexpression ...

. Working on a character in a string
If `s` is a string and `i` is an positive integer that is less than `s`.lenght()
then
.As_is 	s[i]
is a character in the string and
.As_is 	s.substr(i,1)
is a string containing the `i`'th character in `s`.

Note: the characters are numbered 0,1,2,3,.....  For example,
.As_is 		string example="example";
means
.As_is 		example[1]
is 'x' not 'e'.  Similarly `example[6]` is an 'e' and the last valid character.

Warning: bad things happen if you go outsude the range 0,1,2,.... ,length()-1.

. Single characters are typed with single quotes
For historical reasons, 'x' is a character and "x" is a string of two characters!

. Working on each character in a string in turn
Use a 
.Key for loop.
Here is an example, that replaces each 'a' in a string `s` by `1`:
.As_is 		for(int i = 0; i < s.length(); i++)
.As_is 		{
.As_is 			if( s[i] == 'a' )
.As_is 				s[i] = '1';
.As_is 		}

Here is a way to use a for loop to output a string backwards
.As_is 		for(int i = 0; i < s.length(); i++)
.As_is 		{
.As_is 			cout << s[s.length()-i-1];
.As_is 		}
Exercise: try running this fragment by hand with string("abcd").

I tested the code in
.See ../cs201/back.cpp

Exercis:  can you right the for loop that starts at the end of the string and
works back to 0 (and no further)?

. Inputting one character at a time
You can read data from input one character at a time and put it
in a string if you want.  Here is a sample
.See ../cs201/backwards.cpp
of using `cin.get(c)`.

. How can I convert numbers to strings and strings to numbers
I've dug up a couple of advanced types of objects "$stringstreams"
that do the job if you need it in a project, for example.  Here
.See ../cs201/stringnumbers.cpp
is a program that demonstrates how to use
a couple of functions "convertDouble" and
"convertToDouble" that are defined in
.See ../cs201/stringnumbers.h
that you can download and #include in any of your programs.
.As_is string convertDouble(double d)
.As_is {
.As_is 	std::ostringstream s;
.As_is 	s << d;
.As_is 	return s.str();
.As_is }
.As_is double convertToDouble(string d)
.As_is {
.As_is 	double result;
.As_is 	std::istringstream s(d);
.As_is 	s >> result;
.As_is 	return result;
.As_is }

. stringstreams
Stringstreams are a very powerful feature.   Suppose you have a
line that describes a person. It has the first name, their
second name, there phone number, and their age.  Each
is a field with no spaces.  Between them is one or more spaces:
.As_is 		John Doe 909-883-1234 31
.As_is 		Jane Row 714-123-4567 21
Then we can write:
.As_is 		string line;
.As_is 		string first, second, phone; int age;
.As_is 		getline(input, line);
.As_is 		std::istringstream data(line);
.As_is 		data >> first >> second >> phone >> age;
to unpack the data.

. KISS file handling
Here is an example of the kind of simple code you can use that
combines stringstreams, objects, etc.
.See ../cs201/Widget.h
.See ../cs201/listWidgets.cpp


.Close The C++ Standard string library