C語言筆記——字符串和文本02

C語言沒有提供字符串數據類型，這與其他編程語言不同。
C語言使用char類型的數組元素存儲字符串

1.1字符串定義

放在” “中的任何內容都被編譯器視爲字符串

結果
This is on
two lines!

注意：C語言中字符串總是由\0字符結尾,所以字符串長度永遠比字符串中的字符多1。

1.2處理字符串和文本的方法

char saying[20];  
//聲明字符串的數組，其大小至少要比所存儲的字符是多1，因爲編譯器會自動在字符串常量的末尾添加\0
char saying[] = "This is a string.";
//或者直接初始化，不用指定大小

1.3字符串操作

1.3.1連接兩個字符串

要對字符串執行算術賦值操作，必須逐個元素地賦值

//連接兩個字符串
#include <stdio.h>

int main(void)
{
  char str1[40] = "To be or not to be";
  char str2[] = ",that is the question";
  int count1 = 0;                /* Length of str1 */
  int count2 = 0;                /* Length of str2 */

  /* find the length of the first string */
  while (str1[count1])          /* Increment count till we reach the string */
    count1++;                   /* terminating character.                 */

  /* Find the length of the second string */
  while (str2[count2])          /* Count characters in second string      */
    count2++;

  /* Check that we have enough space for both strings  */
  if(sizeof str1 < count1 + count2 + 1)
    printf("\nYou can't put a quart into a pint pot.");
  else
  {  /* Copy 2nd string to end of the first  */

     count2 = 0;                 /* Reset index for str2 to 0   */
    while(str2[count2])          /* Copy up to null from str2   */
      str1[count1++] = str2[count2++];

    str1[count1] = '\0';         /* Make sure we add terminator */
    printf("\n%s\n", str1 );     /* Output combined string      */

  }
  return 0;

連接兩個字符，strcat

#include <stdio.h>
#include <string.h>
#define STR_LENGTH 40

int main(void)
{
  char str1[STR_LENGTH] = "To be or not to be";
  char str2[STR_LENGTH] = ",that is the question";

  if(STR_LENGTH > strlen(str1) + strlen(str2)) /* Enough space ?             */
    printf("\n%s\n", strcat(str1, str2));   /* yes, so display joined string */
  else
    printf("\nYou can't put a quart into a pint pot.");
  return 0;
}

1.3.2比較字符串strcmp

/* Comparing strings */
#include <stdio.h>
#include <string.h>

int main(void)
{
  char word1[20];                /* Stores the first word  */
  char word2[20];                /* Stores the second word */

  printf("\nType in the first word:\n 1: ");
  scanf("%19s", word1);            /* Read the first word    */
  printf("Type in the second word:\n 2: ");
  scanf("%19s", word2);           /* Read the second word   */

  /* Compare the two words */
  if(strcmp(word1,word2) == 0)
    printf("You have entered identical words");
  else
    printf("%s precedes %s",
                    (strcmp(word1, word2) < 0) ? word1 : word2,
                    (strcmp(word1, word2) < 0) ? word2 : word1);
  return 0;
}

1.3.3字符串中尋找字符或字符串

/*  找字符串中的某個字符串  */
#include <stdio.h>
#include <string.h>
int main(void)
{
  char str1[] = "This string contains the holy grail.";
  char str2[] = "the holy grail";
  char str3[] = "the holy grill";

  /* Search str1 for the occurrence of str2 */
  if(strstr(str1, str2) == NULL)
    printf("\n\"%s\" was not found.", str2);
  else
    printf("\n\"%s\" was found in \"%s\"",str2, str1);

  /* Search str1 for the occurrence of str3 */
  if(strstr(str1, str3) == NULL)
    printf("\n\"%s\" was not found.", str3);
  else
    printf("\nWe shouldn't get to here!");
  return 0;
}

1.4分析和轉換字符串

如果需要檢查字符串內部的內容，可以在頭文件

/* 測試輸入字符串中有多少個數字和字母 */
#include <stdio.h>
#include <ctype.h>

int main(void)
{
  char buffer[80];               /* Input buffer               */
  int i = 0;                     /* Buffer index               */
  int num_letters = 0;           /* Number of letters in input */
  int num_digits = 0;            /* Number of digits in input  */

  printf("\nEnter an interesting string of less than 80 characters:\n");
  gets(buffer);                  /* Read a string into buffer  */


  while(buffer[i] != '\0')
  {
    if(isalpha(buffer[i]))
      num_letters++;             /* Increment letter count     */
    if(isdigit(buffer[i++]))
      num_digits++;              /* Increment digit count      */
  }
  printf("\nYour string contained %d letters and %d digits.\n",
                                              num_letters, num_digits);
  return 0;
}

轉換字符
使用函數toupper()和函數strstr()可以確定一個字符串中是否出現在哎另一個字符串中（忽略大小寫）

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

int main(void)
{
  char text[100];                /* Input buffer for string to be searched */
  char substring[40];            /* Input buffer for string sought         */

  printf("\nEnter the string to be searched(less than 100 characters):\n");
  fgets(text, sizeof(text), stdin);

  printf("\nEnter the string sought (less than 40 characters ):\n");
  fgets(substring, sizeof(substring), stdin);

  /* overwrite the newline character in each string */
  text[strlen(text)-1] = '\0';
  substring[strlen(substring)-1] = '\0';

  printf("\nFirst string entered:\n%s\n", text);
  printf("\nSecond string entered:\n%s\n", substring);

  /* Convert both strings to uppercase. */
  for(int i = 0 ; (text[i] = toupper(text[i])) ; i++);
  for(int i = 0 ; (substring[i] = toupper(substring[i])) ; i++);

   printf("\nThe second string %s found in the first.",
              ((strstr(text, substring) == NULL) ? "was not" : "was"));
  return 0;
}

6.4.2將字符串轉換成數值

在頭文件

char value_str[] = "98.4";
double value = 0;
value = atof(valuestr);//將string轉換成浮點數

將數字轉換成字符

char *itoa( int value, char *string,int radix);[1] 原型說明： value：欲轉換的數據。 string：目標字符串的地址。 radix：轉換後的進制數，可以是10進制、16進制等。 C語言提供了幾個標準庫函數，可以將任意類型(整型、長整型、浮點型等)的數字轉換爲字符串。以下是用itoa()函數將整數轉換爲字符串的一個例子：

 # include <stdio. h> 
 # include <stdlib. h> 
{      
    int num = 100;     
    char str[25];  
    itoa(num, str, 10); 
    printf("The number 'num' is %d and the string 'str' is %s. \n" ,                        num, str); }