C语言－第六章－字符串

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这个跟文本打交道的，使用起来常出错！！！汗！！！

字符串和文本的应用

C语言使用char类型的数组元素存储字符串。

1 什么是字符串

双引号之间的任何字符，包括转义后的符号，空格，回车等字符。字符串以”\0”结束。

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#include <stdio.h>

int main()
{
    printf("The character \0 is used for terminate a string."); //显示"The character "
    return 0;
}

2 处理字符串和文本的方法

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char saying[20];

这条语句可以存储一个至多包含19个字符的字符串。

用const将字符串声明为常量：

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const char message[]="The end of the world is nigh.";

2.1 确定字符串长度

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#include <stdio.h>

int main()
{
    const char message[]="uruir.";
    int i=0;
    while(message[i])   //"\0" 的ASCII码就是 0
        i++;
    printf("%d", i);
    return 0;
}

2.2 用strlen确定长度

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#include <stdio.h>

int main()
{
    const char message[]="uruir.";
    printf("%d", strlen(message));
    return 0;
}

3 字符串操作

将字符串赋值给另一字符串，不能像int那样用”=”，而要用专门函数来将字符一个个传入。

3.1 连接字符串

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#include <stdio.h>

int main()
{
    char s1[50]="to be or not to be";
    char s2[]=", that is the question.";
    int i=strlen(s1), j=strlen(s2);
    if(i+j+1>sizeof s1)
        printf("can't put a quart into a prin pot.");
    else {
        j=0;
        while(s2[j])
            s1[i++] = s2[j++];  //i的原来位置是"\0"，正好用s2[]的第一个字符覆盖。
        s1[i]='\0'; //末尾加上结束符。
        printf("%s", s1);
    }
    return 0;
}

第12、13行可用下列一行代替： 

while(s1[i++] = s2[j++]);

因为括号内是一个赋值语句，如果s1[i++]不是”\0”，那条件就为真，继续复制，直到s2将”\0”传过来。

3.2 字符串数组

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#include <stdio.h>

int main()
{
    char sayings[2][32] = {
        "My name is uruir.",
        "What's your name?"
        };
    int i=0;
    for(; i<2; i++)
    printf("%s\n", sayings[i]);
    return 0;
}
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#include <stdio.h>

int main()
{
    char say[][50] = {
                      "to be or not to be",
                      ", that is the question."
    };
    int i=0,count[] = {0, 0};
    for(; i<2; i++)
        while(say[i][count[i]])
            count[i]++;
    if(count[0]+count[1]+1>sizeof say[0])
        printf("can't put a quart into a prin pot.");
    else {
        count[1]=0;
        while((say[0][count[0]++] = say[1][count[1]++]));
        say[0][count[0]]='\0';
        printf("%s", say[0]);
    }
    return 0;
}

4 字符串库函数

字符串函数在<string.h>头文件中

4.1 使用库函数复制字符串

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if(sizeof(string2) <= sizeof(string1))
    strcpy(string1, string2);   //2 -> 1

复制前n个字符：

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#include <stdio.h>

int main()
{
    char destination[] = "This string will be replaced";
    char source[] = "This string will be copied in part";
    size_t n = 26;
    strncpy(destination, source, n);
    printf("%d, %s", sizeof(destination), destination);
    return 0;
}

由于string是从source直接copy到destination中，所以如果sizeof(source) > sizeof(destination)，那destination中就没有’\0’了，所以上例中的n也不要大于sizeof(destination)。

4.2 使用库函数确定字符串的长度

#include <stddef.h>

size_t count = 0;
count = strlen(string);

这个计数不包括’\0’，而sizeof(string)包括。

4.3 使用库函数连接字符串

strcat(str1, str2); //copy str2 to the end of str1

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#include <stdio.h>
#include <stddef.h>

int main()
{
    char destination[] = "This string will be replaced, ";
    char source[] = "This string will be copied.";
    strcat(destination, source);
    printf("%d, %s", strlen(destination), destination); //57, balabala
    return 0;
}

如果用sizeof代替strlen，则57 -> 31。

4.4 比较字符串

strcmp(string1, string2);   //以ASCII码确定大小

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#include <stdio.h>
#include <stddef.h>

int main()
{
    char destination[20];
    char source[20];
    printf("输入第一个字符串:\n");
    scanf("%19s", destination); //因为destination本来就是数组，所以不用"&"。
    printf("第二个\n");
    scanf("%19s", source);  //因为变量是一个地址，所以用"&"来让scanf写入到变量指向的空间。
    if(strcmp(destination, source) == 0)
        printf("两串一样。");
    else
        printf("%s 比 %s 小。", (strcmp(destination, source) < 0) ? destination : source, (strcmp(destination, source) > 0) ? destination : source);
    return 0;
}

4.5 搜索字符串

4.5.1 指针的概念

指针是变量类型，它是含有地址的变量。

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int number = 99;
int *pnumber = &number;

number里含有的是地址，该地址里的内存含有99这个常量。&number现在是指向99的地址，是一个地址！pnumber是一个指针变量，用”int *”定义，所以pnumber这个变量存的是number这个常量所在的地址！

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#include <stdio.h>

int main()
{
    int i = 333;
    int *p = &i;
    printf("%p\n", i);  //%p用于显示地址，这里显示8个16进制组成的地址，该地址里存放333这个常量。本机32位。
    printf("%p", *p);   //和上面一行显示相同的地址，也就是p这个变量存放的地址。指针变量p指向变量i，变量i里存放333。
    return 0;
}

比如，假设我的内存是4GB，那就有约4,000,000,000个byte。那么”0000,0000”就指向第一个byte，”ffff,ffff”指向最后一byte。再假设”int i”时，系统给”i”分配的是第15号空间，即”0000,000f”，因为int占4个字节，所以”0000,000f”,”0000,0010”,”0000,0011”和”0000,0012”这4个字节里存放的就是333这个int常量。因为p指向i，所以p里存放”0000,000f”这么一个地址。当系统通过p来使用333时，先看p，即”0000,000f”，就转向地址为”0000,000f”的内存空间。*是取消引用运算符，其作用是访问指针指定的地址中存储的数据。

4.5.2 在字符串中查找子字符串
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#include <stdio.h>
#include <string.h>

int main()
{
    char text[] = "Every dog has his day";
    char str1[] = "dog";
    char str2[] = "Dog";
    char *p1 = NULL;
    char *p2 = NULL;

    if(strstr(text,str1) == NULL)
        printf("%s was not found.\n", str1);
    else
        printf("%s was found in \"%s.\", %p, %p\n", str1, text, text, strstr(text,str1));    //text的地址和str1的地址

    if(strstr(text,str2) == NULL)
        printf("%s was not found.\n", str2);
    else
        printf("%s was found in \"%s.\", %p, %p\n", str2, text, text, strstr(text,str2));

    return 0;
}

5 分析和转换字符串

5.1 用gets读入字符串

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#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); //scanf以空格或回车结束，gets能读入空格直到回车


  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;
}

使用fgets函数可以确保写入安全。fgets(buffer, sizeof(text), stdin)是从stdin获取大小为text串大小的串，然后只要buffer[strlen(text)-1]=’\0’，读入就安全了。

5.2 将字符串转换成数值

atof()  //字符串参数转换成double类型的值
atoi()  //int
atol()  //long
atoll() //long long

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#include <stdlib.h>

char value_str[] = "98.4";
double value = 0;
value = atof(value_str);

6 小程序

6.1 问题

从键盘读入任意长度的一段文本，确定该文本中每个单词出现的频率（忽略大小写）。

6.2 分析

  • 使用fgets()
  • 文本包含标点，删除之
  • 用空格来区分单词，将单词存入另一个数组中，小写保存

6.3 解决方案

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

#define TEXTLEN  10000      /* Maximum length of text            */
#define BUFFERSIZE 100      /* Input buffer size                 */
#define MAXWORDS   500      /* Maximum number of different words */
#define WORDLEN     15      /* Maximum word length               */

int main(void)
{
  char text[TEXTLEN+1];
  char buffer[BUFFERSIZE];
  char endstr[] = "*\n";          /* Signals end of input       */

  const char space = ' ';
  const char quote = '\'';

  char words[MAXWORDS][WORDLEN+1];
  int nword[MAXWORDS];            /* Number of word occurrences */
  char word[WORDLEN+1];           /* Stores a single word       */
  int wordlen = 0;                /* Length of a word           */
  int wordcount = 0;              /* Number of words stored     */


  printf("Enter text on an arbitrary number of lines.");
  printf("\nEnter a line containing just an asterisk to end input:\n\n");

  /* Read an arbitrary number of lines of text */
  while(true)
  {
    /* A string containing an asterisk followed by newline */
    /* signals end of input                                */
    if(!strcmp(fgets(buffer, BUFFERSIZE, stdin), endstr))
      break;

    /* Check if we have space for latest input */
    if(strlen(text)+strlen(buffer)+1 > TEXTLEN)
      {
        printf("Maximum capacity for text exceeded. Terminating program.");
        return 1;
      }
    strcat(text, buffer);
  }

  /* Replace everything except alpha and single quote characters by spaces */
  for(int i = 0 ; i < strlen(text) ; i++)
  {
    if(text[i] == quote || isalnum(text[i]))
      continue;
    text[i] = space;
  }

  /* Find unique words and store in words array */
  int index = 0;
  while(true)
  {
    /* Ignore any leading spaces before a word */
    while(text[index] == space)
      ++index;

    /* If we are at the end of text, we are done */
    if(text[index] == '\0')
      break;

    /* Extract a word */
    wordlen = 0;          /* Reset word length */
    while(text[index] == quote || isalpha(text[index]))
    {
      /* Check if word is too long */
      if(wordlen == WORDLEN)
      {
        printf("Maximum word length exceeded. Terminating program.");
        return 1;
      }
      word[wordlen++] = tolower(text[index++]);  /* Copy as lowercase     */
    }
    word[wordlen] = '\0';                        /* Add string terminator */

    /* Check for word already stored */
    bool isnew = true;
    for(int i = 0 ; i< wordcount ; i++)
      if(strcmp(word, words[i]) == 0)
      {
        ++nword[i];
        isnew = false;
        break;
      }

    if(isnew)
    {
      /* Check if we have space for another word */
      if(wordcount >= MAXWORDS)
      {
        printf("\n Maximum word count exceeded. Terminating program.");
        return 1;
      }

      strcpy(words[wordcount], word);   /* Store the new word  */
      nword[wordcount++] = 1;           /* Set its count to 1  */
    }
  }

  /* Output the words and frequencies */
  for(int i = 0 ; i<wordcount ; i++)
  {
    if( !(i%3) )                       /* Three words to a line */
      printf("\n");
    printf("  %-15s%5d", words[i], nword[i]);
  }

  return 0;
}

7 练习

7.1 数值以英文显示

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

int main(void)
{
  char *unit_words[] = {"zero", "one","two","three","four","five","six","seven","eight","nine"};
  char *teen_words[] = {"ten", "eleven","twelve","thirteen","fourteen","fifteen","sixteen","seventeen","eighteen","nineteen"};
  char *ten_words[] = {"error", "error","twenty","thirty","forty","fifty","sixty","seventy","eighty","ninety"};
  char hundred[] = " hundred";
  char and[] = " and ";
  char value_str[50] = "";
  int value = 0;                     /* Integer to be converted        */
  int digits[] = {0,0,0};            /* Stores digits of value entered */
  int i = 0;

  printf("Enter a positive integer less than 1000: ");
  scanf("%d",&value);
  if(value >= 1000)
    value = 999;
  else if(value < 1)
    value = 1;

  while(value > 0)
  {
    digits[i++] = value%10;
    value /= 10;
  }

  if(digits[2] > 0)
  {
    strcat(strcat(value_str,unit_words[digits[2]]), hundred);
    if(digits[1]>0 || digits[0]>0)
      strcat(value_str, and);
  }
  if(digits[1] > 0)
  {
    if(digits[1] == 1)
      strcat(value_str,teen_words[digits[0]]);
    else
    {
      strcat(value_str,ten_words[digits[1]]);
      if(digits[0] > 0)
        strcat(strcat(value_str, " "), unit_words[digits[0]]);
    }
  }
  else
    if(digits[0] > 0)
      strcat(value_str, unit_words[digits[0]]);
  printf("\n%s\n", value_str);
  return 0;
}

7.2 一行显示一个单词

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

int main(void)
{
  char list[5000];         /* Stores the list of comma separated words   */
  char words[500][20];     /* Array for 500 words of up to 20 characters */
  const char comma = ',';
  const char space = ' ';
  int word_count = 0;      /* Number of words found */
  int word_length = 0;     /* Current word length   */
  int index = 0;           /* Character position    */

  printf("Enter a comma separated list of words:\n");
  gets(list);              /* Read the list of words */

  /* Find words in list */
  while(list[index] != '\0')
  {
    /* Skip over spaces and commas */
    while(list[index] == space || list[index] == comma)
      ++index;

    /* Copy characters that are not space, comma or \0 as part of a word */
    while(list[index] != space && list[index] != comma && list[index] != '\0')
     words[word_count][word_length++] = list[index++];

    words[word_count++][word_length] = '\0';    /* Append terminator         */
    word_length = 0;                            /* Reset ready for next word */
  }

  /* List the words that were found */
  printf("\nThe words in the list are:\n");
  for(index = 0 ; index<word_count ; index++)
    printf("%s\n",words[index]);

    return 0;
}

7.3 随机显示一段文本

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#include <stdio.h>
#include <time.h>
#include <stdlib.h>

int main(void)
{
  char thoughts[][50] = {"Wherever you go, there you are!",
                         "A nod is as good as a wink to a blind horse.",
                         "Many hands make light work.",
                         "Too many cooks spoil the broth.",
                         "A rolling stone gathers no moss.",
                         "A wise man will cover the hole in his carpet."};

  srand((unsigned int)time(NULL));

  printf("Today's thought is:\n%s\n", thoughts[rand()%(sizeof thoughts/sizeof thoughts[0])]);
  return 0;
}

7.4 回文

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

int main(void)
{
  char sentence[500];             /* Stores the sentence to be tested                   */
  char sentence_chars[500];       /* Stores the sentence without punctuation and spaces */
  size_t j = 0;                   /* Index to character position */
  size_t length = 0;              /* Length of a string          */

  printf("Enter a sentence to be tested:\n");
  gets(sentence);

  /* Copy only letters as lowercase */
  for (size_t i = 0 ; i< strlen(sentence) ; i++)
    if(isalpha(sentence[i]))
      sentence_chars[j++] = tolower(sentence[i]);
  sentence_chars[j] = '\0';        /* Append string terminator */

  length = strlen(sentence_chars); /* Get the satring length   */

  /* Compare matching characters in the string                */
  /* If any pair are not the same, then it's not a palindrome */
  for(size_t i = 0 ; i<length/2 ; i++)
    if(sentence_chars[i] != sentence_chars[length-1-i])
    {
      printf("\n The sentence you entered is not a palindrome.\n");
      return 0;
    }
    /* If we arrive here all matching pairs of characters are equal */
    printf("\n The sentence you entered is a palindrome.\n");
    return 0;
}