The C String Library - string.h (aka cstring)

Strings as Character Arrays

Before C++ and classes, strings were stored in simple arrays of characters. Thus,

char name[12] = "Bob Dylan";

would be a simple way to create a string with an initial value. In memory, this array looks like:

--------------------------------------------------
| B | o | b |   | D | y | l | a | n | \0 |   |   |
--------------------------------------------------

The only thing special about this array is that an extra character, the nul character tells us where the string's value ends.

Note: In this case, the array is actually bigger than the string held in it.

Accessing/Changing Strings

Because a string is really just an array, we must access or change it by dealing with each element (like other arrays).

This means it becomes tedious to perform common string operations, i.e.,

To change the value of a string,

we must change several characters individually.

To find the length of the string,

we must search for the nul character.

What kind of language construct would we use to manipulate or search through the characters of a string?

Because these are such common operations, there are library functions that do these things for us...

string.h

The library string.h (also referred to as cstring) has several common functions for dealing with strings stored in arrays of characters. Following are the 4 basic string library functions that we'll discuss:

strlen(str)
Returns the number of characters in the string, not including the nul character.
strcmp(str1, str2)
This function takes two strings and compares them. If the strings are equal, it returns 0. If the first is greater than the 2nd, then it returns some value greater than 0. If the first is less than the 2nd, then it returns some value less than 0.
You might use strcmp() as in:
```
#include <string.h>

char str1[] = "garden";

if (strcmp(str1, "apple") == 0)
  cout << "Equal" << endl;
else 
  cout << "Not equal" << endl;

OR

if (strcmp(str1, "eden") > 0) 
  cout << "'" << str1 << "' comes after 'eden'" << endl;
```
The ordering for strings is lexical order based on the ASCII value of characters. Remember that the ASCII value of 'A' and 'a' (i.e., upper/lowercase) are not the same.

An easy way to remember how to use strcmp() to compare 2 strings (let's say a and b) is to use the following mnemonics:

Want... Use...

a == b strcmp(a, b) == 0

a < b strcmp(a, b) < 0

a >= b strcmp(a, b) >= 0

... ...
strcpy(dest, source)
Copies the contents of source into dest, as in:
```
#include <string.h>

char str1[10] = "initvalue";

strcpy(str1, "second");
```
Now, the string str1 contains the following:
```
-------------------------------------------
| s | e | c | o | n | d | \0 | u | e | \0 |
-------------------------------------------
```
and the word "initvalue" has been overwritten. Note that it is the first nul character (\0) that determines the end of the string.
When using strcpy(), make sure the destination is big enough to hold the new string.

Note: An easy way to remember that the destination comes first is because the order is the same as for assignment, e.g:
```
dest = source
```
Also, strcpy() returns the destination string, but that return value is often ignored.
strcat(dest, source)
Copies the contents of source onto the end of dest, as in:
```
#include <string.h>

char str2[10] = "first";

strcat(str2, " one");
```
Now, the string str2 contains the following:
```
------------------------------------------
| f | i | r | s | t |   | o | n | e | \0 |
------------------------------------------
```
When using strcat(), make sure the destination is big enough to hold the extra characters.

Note: Function strcat() also returns the destination string, but that return value is often ignored.

Want...	Use...
a == b	`strcmp(a, b) == 0`
a < b	`strcmp(a, b) < 0`
a >= b	`strcmp(a, b) >= 0`
...	...