字符串操作函數解析

在編程的時候我們經常會用到一些庫函數來操作字符串，例如strcmp，strlen，strcat, strstr, strcpy, strchar , memcpy, memmov, memset 接下來我們就看一看這些函數。

1 strcmp：

從msdn的解釋中可以看出字符串比較這個函數有兩個參數string1和string2，而且這倆參數都是常量字符串(const修飾)。返回值是一個int型。當string1小於string2的時候會返回一個小於0的數，當string1等於string2的時候會返回0，當string1小於string2的時候會返回一個大於0的數。我們可以根據返回值來判斷這兩個字符串的關係。

例如：

#define _CRT_SECURE_NO_DEPRECATE 1
#include <stdio.h>    
#include <stdlib.h>   
#include <string.h>
int main()
{
	char str1[]="hello future";
	char str2[]="hello my future";
	int ret;
	ret = strcmp(str1,str2);
	if(ret>0)
		printf("str1大於str2");
	else if(ret=0)
		printf("str1等於str2");
	else
		printf("str1小於str2");
	system("pause");
    return 0;
}

2 strlen：

從msdn的解釋中可以看出這個函數有一個參數 const char *string, 返回值是一個size_t（無符號整形），因爲字符串的個數根本就不可能是一個負數。

我們來看看msdn給的例子：

Example

/*  STRLEN.C */

#include <string.h>
#include <stdio.h>
#include <conio.h>
#include <dos.h>

void main( void )
{
   char buffer[61] = "How long am I?";
   int  len;
   len = strlen( buffer );
   printf( "'%s' is %d characters long\n", buffer, len );
}


Output

'How long am I?' is 14 characters long

3 strcat：

strcat函數實現的功能是把一個字符串連接到另一個字符串的後面。既然是兩個字符串連接就要有兩個參數即char *strDestination和const char *strSource; 返回值是一個char* 指針，指向strDestination的首元素地址，這樣有利於作爲鏈式訪問使用。

#define _CRT_SECURE_NO_DEPRECATE 1
#include <stdio.h>    
#include <stdlib.h>   
#include <string.h>
int main()
{
	char str1[]="Do you want to ";
        char str2[]="play baketball?";
	printf("%s  \n",strcat(str1,str2));//返回值char * 作爲首元素用於實現鏈式訪問
	system("pause");
        return 0;
}

當我們使用strcat的時候有一點要注意，不能使用strcat（str1，str1）即給一個字符串自身連接自己本身。否則會出現如下情況。

如果非要自身連接自身請使用strncat.

4 strstr：

這個函數的功能是在一個字符串裏面尋找另一個字符串是否存在。這個函數有兩個參數即const char *string ,const char *strcharset,返回值是一個char *指針。函數返回指向字符串中第一次出現strcharset，或null如果strcharset不出現在字符串。如果strcharset指向一個字符串的長度爲零，函數返回字符串。

我們來看一個msdn中的例子便於理解

Example

/* STRSTR.C */

#include <string.h>
#include <stdio.h>

char str[] =    "lazy";
char string[] = "The quick brown dog jumps over the lazy fox";
char fmt1[] =   "         1         2         3         4         5";
char fmt2[] =   "12345678901234567890123456789012345678901234567890";

void main( void )
{
   char *pdest;
   int  result;
   printf( "String to be searched:\n\t%s\n", string );
   printf( "\t%s\n\t%s\n\n", fmt1, fmt2 );
   pdest = strstr( string, str );
   result = pdest - string + 1;
   if( pdest != NULL )
      printf( "%s found at position %d\n\n", str, result );
   else
      printf( "%s not found\n", str );
}


Output

String to be searched:
   The quick brown dog jumps over the lazy fox
            1         2         3         4         5
   12345678901234567890123456789012345678901234567890

lazy found at position 36

5 strcpy：

這個函數實現的是字符串拷貝功能。有兩個參數 char *strDestination 和 const char *strSource。從這倆參數我們就可以直接看出是把const char *strSource中的字符串拷貝到char *strDestination中因爲是改變char *strDestination中的內容所以要傳址。而使用const的修飾char *strSource使之成爲常量字符串是爲了增加程序的健壯性（便於記憶倆參數的用處，放置倆參數放錯引起的不必要的錯誤）。而返回值char * 指向拷貝完成的字符串首元素。便於鏈式訪問。

看看msdn的例子

Example

/* STRCPY.C: This program uses strcpy
 * and strcat to build a phrase.
 */

#include <string.h>
#include <stdio.h>

void main( void )
{
   char string[80];
   strcpy( string, "Hello world from " );
   strcat( string, "strcpy " );
   strcat( string, "and " );
   strcat( string, "strcat!" );
   printf( "String = %s\n", string );
}


Output

String = Hello world from strcpy and strcat!

6 strchr：

函數功能：在一個字符串裏面尋找一個字符。這個函數有兩個參數 const修飾的常量字符串char *string，和int型變量c。c是用來定位這個字符在常量字符串string中的位置的。 char *指向的是要找的這個字符第一次出現在常量字符串string中的位置（可能存在多個該字符）。如果不存在則表示不存在。

看看例子

Example

/* STRCHR.C: This program illustrates searching for a character
 * with strchr (search forward) or strrchr (search backward).
 */

#include <string.h>
#include <stdio.h>

int  ch = 'r';

char string[] = "The quick brown dog jumps over the lazy fox";
char fmt1[] =   "         1         2         3         4         5";
char fmt2[] =   "12345678901234567890123456789012345678901234567890";

void main( void )
{
   char *pdest;
   int result;

   printf( "String to be searched: \n\t\t%s\n", string );
   printf( "\t\t%s\n\t\t%s\n\n", fmt1, fmt2 );
   printf( "Search char:\t%c\n", ch );

   /* Search forward. */
   pdest = strchr( string, ch );
   result = pdest - string + 1;
   if( pdest != NULL )
      printf( "Result:\tfirst %c found at position %d\n\n", 
              ch, result );
   else
      printf( "Result:\t%c not found\n" );

   /* Search backward. */
   pdest = strrchr( string, ch );
   result = pdest - string + 1;
   if( pdest != NULL )
      printf( "Result:\tlast %c found at position %d\n\n", ch, result );
   else
      printf( "Result:\t%c not found\n" );
}


Output

String to be searched: 
      The quick brown dog jumps over the lazy fox
               1         2         3         4         5
      12345678901234567890123456789012345678901234567890

Search char:   r
Result:   first r found at position 12

Result:   last r found at position 30

7 memcpy：

功能：從緩存中拷貝字符。參數 void *dest （需要拷貝到的目的地） const void *src（拷貝源）

size_t （需要拷貝的個數）返回值 void * （memcpy returns the value of dest.）

例子：msdn給的這個例子很長但是實現的功能確實非常簡單的，看一眼就能明白。

Example

/* MEMCPY.C: Illustrate overlapping copy: memmove
 * handles it correctly; memcpy does not.
 */

#include <memory.h>
#include <string.h>
#include <stdio.h>

char string1[60] = "The quick brown dog jumps over the lazy fox";
char string2[60] = "The quick brown fox jumps over the lazy dog";
/*                           1         2         3         4         5
 *                  12345678901234567890123456789012345678901234567890
 */

void main( void )
{
   printf( "Function:\tmemcpy without overlap\n" );
   printf( "Source:\t\t%s\n", string1 + 40 );
   printf( "Destination:\t%s\n", string1 + 16 );
   memcpy( string1 + 16, string1 + 40, 3 );
   printf( "Result:\t\t%s\n", string1 );
   printf( "Length:\t\t%d characters\n\n", strlen( string1 ) );

   /* Restore string1 to original contents */
   memcpy( string1 + 16, string2 + 40, 3 );

   printf( "Function:\tmemmove with overlap\n" );
   printf( "Source:\t\t%s\n", string2 + 4 );
   printf( "Destination:\t%s\n", string2 + 10 );
   memmove( string2 + 10, string2 + 4, 40 );
   printf( "Result:\t\t%s\n", string2 );
   printf( "Length:\t\t%d characters\n\n", strlen( string2 ) );

   printf( "Function:\tmemcpy with overlap\n" );
   printf( "Source:\t\t%s\n", string1 + 4 );
   printf( "Destination:\t%s\n", string1 + 10 );
   memcpy( string1 + 10, string1 + 4, 40 );
   printf( "Result:\t\t%s\n", string1 );
   printf( "Length:\t\t%d characters\n\n", strlen( string1 ) );
}


Output

Function:   memcpy without overlap
Source:      fox
Destination:   dog jumps over the lazy fox
Result:      The quick brown fox jumps over the lazy fox
Length:      43 characters

Function:   memmove with overlap
Source:      quick brown fox jumps over the lazy dog
Destination:   brown fox jumps over the lazy dog
Result:      The quick quick brown fox jumps over the lazy dog
Length:      49 characters

Function:   memcpy with overlap
Source:      quick brown dog jumps over the lazy fox
Destination:   brown dog jumps over the lazy fox
Result:      The quick quick brown dog jumps over the lazy fox
Length:      49 characters

8 memmov：

函數功能：緩衝區數據移動。參數 void *dest （需要移動到的目的地） const void *src（需要移動的數據源） size_t count （需要拷貝的類型字節數）返回值 void * （memmove returns the value of dest.）

例子：

Example

/* MEMCPY.C: Illustrate overlapping copy: memmove
 * handles it correctly; memcpy does not.
 */

#include <memory.h>
#include <string.h>
#include <stdio.h>

char string1[60] = "The quick brown dog jumps over the lazy fox";
char string2[60] = "The quick brown fox jumps over the lazy dog";
/*                           1         2         3         4         5
 *                  12345678901234567890123456789012345678901234567890
 */

void main( void )
{
   printf( "Function:\tmemcpy without overlap\n" );
   printf( "Source:\t\t%s\n", string1 + 40 );
   printf( "Destination:\t%s\n", string1 + 16 );
   memcpy( string1 + 16, string1 + 40, 3 );
   printf( "Result:\t\t%s\n", string1 );
   printf( "Length:\t\t%d characters\n\n", strlen( string1 ) );

   /* Restore string1 to original contents */
   memcpy( string1 + 16, string2 + 40, 3 );

   printf( "Function:\tmemmove with overlap\n" );
   printf( "Source:\t\t%s\n", string2 + 4 );
   printf( "Destination:\t%s\n", string2 + 10 );
   memmove( string2 + 10, string2 + 4, 40 );
   printf( "Result:\t\t%s\n", string2 );
   printf( "Length:\t\t%d characters\n\n", strlen( string2 ) );

   printf( "Function:\tmemcpy with overlap\n" );
   printf( "Source:\t\t%s\n", string1 + 4 );
   printf( "Destination:\t%s\n", string1 + 10 );
   memcpy( string1 + 10, string1 + 4, 40 );
   printf( "Result:\t\t%s\n", string1 );
   printf( "Length:\t\t%d characters\n\n", strlen( string1 ) );
}

Output

Function:   memcpy without overlap
Source:      fox
Destination:   dog jumps over the lazy fox
Result:      The quick brown fox jumps over the lazy fox
Length:      43 characters

Function:   memmove with overlap
Source:      quick brown fox jumps over the lazy dog
Destination:   brown fox jumps over the lazy dog
Result:      The quick quick brown fox jumps over the lazy dog
Length:      49 characters

Function:   memcpy with overlap
Source:      quick brown dog jumps over the lazy fox
Destination:   brown dog jumps over the lazy fox
Result:      The quick quick brown dog jumps over the lazy fox
Length:      49 characters

9 memset ：

功能:把指定的緩存空間設置爲指定的字符。參數 void *dest （需要設置的目的地） c:需要設置的字符類型 size_t count （需要設置的指定類型的個數）返回值 void * （memmove returns the value of dest.）

例子：比起之前幾個的繁瑣相當精簡

Example

/* MEMSET.C: This program uses memset to
 * set the first four bytes of buffer to "*".
 */

#include <memory.h>
#include <stdio.h>

void main( void )
{
   char buffer[] = "This is a test of the memset function";

   printf( "Before: %s\n", buffer );
   memset( buffer, '*', 4 );
   printf( "After:  %s\n", buffer );
}

Output

Before: This is a test of the memset function
After:  **** is a test of the memset function

在使用這些字符串操作函數的時候我們需要注意的地方是：

1 各個參數的意義，我們可以通過const修飾減少一些不必要的錯誤引起的失。

2 巧妙的利用返回值，使程序更加的巧妙。

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