Section1: String Fundamentals

     1.1 String Declaration

     1.2 The String Literal Pool

     1.3 String Initialization

Section2: Standard String Operations

     2.1 Comparing Strings

     2.2 Copying Strings

     2.3 Concatenating Strings

Section3: Passing Strings

     3.1 Passing a Simple String

     3.2 Passing a Pointer to a Constant char

     3.3 Passing a String to be Initialized

     3.4 Passing Arguments to an Application

Section4: Returning Strings

     4.1 Returning the Address of a Literal

     4.2 Returning the Address of Dynamically Allocated Memory

Section5: Function Pointers and Strings

    Arrays of char have been used to represent smaller integer units, such as boolean, to conserve memory space in an application.

     Two types of string in C:

     (1) Byte string: Consists of a sequence of char data type;  in string.h

     (2) Wide string: Consists of a sequence of wchar_t data type.   in wchar.h

          wchar_t is used for wide characters and may be either 16 or 32 bits in width.

     Both of these strings are terminated by the NUL character.

     The length of string does not include the NUL character.

PS:

     NULL : ((void*)0)

     NUL: ‘\0'

     Character constants are character sequences enclosed in single quotes.  In C, they are of type int.

         

          

     1.1 String Declaration

     three ways:

     (1) as a literal.  “adbde"

     (2) as an array of characters.  char header[32];

     (3) using a pointer to a character.  char *header;

     The string literal is a sequence of characters enclosed in double quotes.

     1.2 The String Literal Pool

     GCC uses a -fwritable-strings option to turn off string pooling.

     In Microsoft Visual Studio, the /GF option will turn on string pooling.

    

     In most compilers, a string literal is treated as a constant. However, in some compiler, such as GCC:

     char *tabHeader = “Sound”;

     *tabHeader = ‘L’;

     printf(“%s\n”, tabHeader);   // Displays “Lound"

     一般string当作常量，但如上所示, GCC可改。 一种做法是：

     const char* tabHeader = “Sound”;

     

     1.3 String Initialization

    

     1.3.1 Initializing an array of char

     (1) char header[] = “Media Player”;

     (2) using strcpy

          char header[13];

          strcpy(header, “Media Player”);

     1.3.2 Initializing a pointer to a char

    (1) normal

          char *header = (char*) malloc(strlen(“Media Player”) + 1);

          strcpy(header, “Media Player”);

     (2) directly

          char *header = “Media Player”;

     1.3.3 Initializing a String from standard input

          char *command = (char*) malloc(100);

          printf(“Enter a Command: “);

          scanf(“%s”, command);

     

     1.3.4 summary of string placement

    

     prototype:  int strcmp(const char *s1, const char *s2);

     returns:

          Negative : if s1 precedes s2 lexicographically (alphabetically)

          Zero: if the two strings are equal

          Positive : if s1 follows s2 lexicographically

          

     There are a couple of incorrect ways to compare two strings.

     (1) if (command = “Quit”)  // error, it is an assignment, set the address of string “Quit” to the array ( array name) - command;

     (2) if (command == “Quit”) // return false; 300 <> 600

     prototype:

          char* strcpy(char *s1, const char *s2)

    

     prototype:

          char* strcat(char* s1, const char* s2)

    

     

     

     

     size_t stringLength(char string[]) // 这是一样的

     size_t stringLength(const char* string);

     snprintf : http://www.cplusplus.com/reference/cstdio/snprintf/

    

    

    

     An alternative to this approach is to pass NULL;

     

     

    

    

  显然，也可以写成：

     int main(int argc, char* argv[ ]) {

     return a reference to either:

     (1) A literal

     (2) Dynamically allocated memory

     (3) A local string variable

     

    

    

     chapter3 section2  2.5 说明了a pointer to local variable, and then return it will cause error.

    

     

     1. tolower: http://www.cplusplus.com/reference/cctype/tolower/?kw=tolower

          it’s in the ctype.h

     2. strcmp : http://www.cplusplus.com/reference/cstring/strcmp/?kw=strcmp

          Returns an integral value indicating the relationship between the strings:

 负数表更小的值， acsii码越小，字母越靠前哪

ascii码表表示：从小到大顺序为  符号-> 数字->大写字母->小写字母。符号不一定，较乱。