9. 字符串 Charater Strings

9.1 基础

字符串，由一维字符数组组成（字符型）。 1 代表文本。2 验证用户输入。3 创建格式化字符串。用双引号包含。"This is a string", "Hello World", "xyz 123 *!#@&"都是字符串。

存储：在C中，存储成由一个指定的字符串末端的名为NULL的符号敞亮种植的字符型数组。NULL常量的数值是'\0'（转义序列符）。双引号不被存储为string的一部分。可以利用下标、指针符对string（数组）进行处理，NULL字符能用来监测string末端。

i/o

＃include <stdio.h>
getchar()和putchar()提供single字符的io。
gets()和puts()由上面两个函数编写而来，能按完整单元处理字符串。

#include <stdio.h>
#define MSIZE 81//够80个字符＋'\0'（合计81个）使用。

int main()
{

    char message[MSIZE]; 
    
    printf("Enter a string:\n");
    gets(message);
    printf("The string just entered is:\n");
    puts(message);
    //printf("s%\n", message)
    
    return 0;
}

1 按Enter，产生一个\n，gets()解释这个字符为输入末端。除了\n，所有的输入都被存储在了message数组中
2 printf()能代替puts()，但scanf()不能代替gets()

processing

1 strcopy()把string2内容复制到string1
当string2[i]不等于'\0'时，循环，string1[i] = string2[i]。
＊但需确保string1有足够的长度保存string2。

#include <stdio.h>

void strcopy(char [], char []); /* expects two arrays of chars */

int main()
{
#define LSIZE 81
    char message[LSIZE];  /* enough storage for 80 characters plus '\0' */
    char newMessage[LSIZE]; /* enough storage for a copy of message */
    
    printf("Enter a sentence: ");
    gets(message);
    
    strcopy(newMessage, message); /* pass two array addresses */
    
    puts(newMessage);
    
    return 0;
}

/* copy string2 to string 1 */
void strcopy (char string1[], char string2[]) /* two arrays are passed */
{
    int i = 0;  /* i will be used as a subscript */
    
    while (string2[i] != '\0') /* check for the end-of-string *///检测字符串末端
    {
        string1[i] = string2[i]; /* copy the element to string1 */
        i++;
    }
    string1[i] = '\0'; /* terminate the copied string */
}

把赋值语句包含在while语句的测试部分内

#include <stdio.h>
int main()
{
#define LSIZE 81
    char message[LSIZE];  /* enough storage for 80 characters plus '\0' */
    char c;
    int i;
    
    printf("Enter a string:\n");
////////
    i = 0;
    while(i < (LSIZE-1) && (c = getchar()) != '\n')
    {
        message[i] = c; /* store the character entered */
        i++;
    }
    message[i] = '\0'; /* terminate the string */
////////取代了上个代码中的gets()函数。构成一个从一个终端输入一正航字符的独立单元，因此这些语句能从main()函数移走。
    printf("The string just entered is: \n");
    puts(message);
    
    return 0;
}

9.2 库函数

C标准库中，有大量字符串处理函数和字符处理函数。

字符串库函数(string.h)

名称	描述
strcpy(str1, str2)	复制str2到str1，包括'\0'
strcat(str1, str2)	附加str2到str1的末端
strlen(sting)	返回string的长度，不包括'\0'
strccmp(str1, str2)	比str2 str1长，1<2返回-1,==返回0,>返回正整数
strncpy(str1, str2, n)	str2的n字符复制到str1，若str2<n，用'\0'补
strncmp(str1, str2, n)	比str1 str2最多n个字符
strchr(string, char)	char在string中第一次出现的位置，返回这个字符地址

"Behop"大于"Beehive"，因为h>e
字母排在后面的大，小写字母>大写字母

#include  <stdio.h>
#include <string.h> /* required for the string function library */

int main()
{
#define MAXELS 50
    char string1[MAXELS] = "Hello";
    char string2[MAXELS] = "Hello there";
    int n;
    
    n = strcmp(string1, string2);
    
    if (n < 0)
        printf("%s is less than %s\n\n", string1, string2);
    else if (n == 0)
        printf("%s is equal to %s\n\n", string1, string2);
    else
        printf("%s is greater than %s\n\n", string1, string2);
    
    printf("The length of string1 is %d characters\n", strlen(string1));
    printf("The length of string2 is %d characters\n\n", strlen(string2));
    
    strcat(string1," there World!");
    
    printf("After concatenation, string1 contains the string value\n");
    printf("%s\n", string1);
    printf("The length of this string is %d characters\n\n",
           strlen(string1));
    
    printf("Type in a sequence of characters for string2:\n");
    gets(string2);
    
    strcpy(string1, string2);
    
    printf("After copying string2 to string1");
    printf(" the string value in string1 is:\n");
    printf("%s\n", string1);
    printf("The length of this string is %d characters\n\n",
           strlen(string1));
    printf("\nThe starting address of the string1 string is: %d\n",
           (void *) string1);
    return 0;
}

字符函数

ctype.h

原型	描述
int isalpha(char)	是字母返!0，or 0
int isupper(char)	是大写返!0，or 0
int islower(char)	是小写返!0，or 0
int isdigit(char)	是0-9返!0，or 0
int isascii(char)	是ASCII返!0，or 0
int isspace(char)	是空格返!0，or 0
int isprint(char)	是能打印返!0，or 0
int iscntrl(char)	是控制字符返!0，or 0
int ispunct(char)	是标点字符返!0，or 0
int toupper(char)	是小写，则返回相应大写，or 不变
int tolower(char)	是大写，则返回相应小写，or 不变

#include <stdio.h>
#include <ctype.h> /* required for the character function library */

int main()
{
#define MAXCHARS 100
    char message[MAXCHARS];
    void convertToUpper(char []);  /* function  prototype */
    
    printf("\nType in any sequence of characters:\n");
    gets(message);
    
    convertToUpper(message);
    
    printf("The characters just entered, in uppercase are:\n%s\n", message);
    
    return 0;
}

// this function converts all lowercase characters to uppercase
void convertToUpper(char message[])
{
    int i;
    for(i = 0; message[i] != '\0'; i++)
        message[i] = toupper(message[i]);
}

转换函数

stdlib.h

原型	描述
int atoi(string)	转换ASCII字符串为整数，在第一个非整数字符处停止转换
double atof(string)	转换ASCII字符串为双精数值，在第一个不能被解释为双精的字符处停止转换
char[] itoa(string)	转换一个整数为ASCII字符串。分配足够空间。

atoi和atof的使用

#include <stdio.h>
#include <string.h>
#include <stdlib.h> // required for test conversion function library
#define MAXELS 20

int main()
{
  char test[MAXELS] = "1234";
  int num;
  double dnum;

  num = atoi(test);
  printf("The string %s as an integer number is %d\n", test,num);
  printf("This number divided by 3 is: %d\n", num/3);
  
  strcat(test, ".96");

  dnum = atof(test);
  printf("\nThe string %s as a double number is: %f\n", test,dnum);
  printf("This number divided by 3 is: %f\n", dnum/3);
  
  return 0;
}

9.3 输入数据验证

验证输入的是否为整数
1

#include <stdio.h>
#include <stdlib.h>  /* needed to convert a string to an integer */
#define MAXCHARS 40
#define TRUE 1
#define FALSE 0

int isvalidInt(char []);  /* function prototype */

int main()
{
    
    char value[MAXCHARS];
    int number;
    
    printf("Enter an integer: ");
    gets(value);
    
    if (isvalidInt(value)== TRUE)
    {
        number = atoi(value);
        printf("The number you entered is %d\n", number);
    }
    else
        printf("The number you entered is not a valid integer.\n");
    
    return 0;
}

int isvalidInt(char val[])
{
    int start = 0;
    int i;
    int valid = TRUE;
    int sign = FALSE;
    
    /* check for an empty string */
    if (val[0] == '\0') valid = FALSE;
    
    /* check for a leading sign */
    if (val[0] == '-' || val[0] == '+')
    {
        sign = TRUE;
        start = 1;  /* start checking for digits after the sign */
    }
    
    /* check that there is at least one character after the sign */
    if(sign == TRUE && val[1] == '\0') valid = FALSE;
    
    /*now check the string, which we know has at least one non-sign char */
    i = start;
    while(valid == TRUE && val[i] != '\0')
    {
        if (val[i] < '0' || val[i] > '9') /* check for a non-digit */
            valid = FALSE;    
        i++;
    }
    
    return valid;
}

优化

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

int getanInt();  /* function prototype */

int main()
{  
  int value;

  printf("Enter an integer value: ");
  value = getanInt();
  printf("The integer entered is: %d\n", value);

  return 0;
}

#define TRUE 1
#define FALSE 0
#define MAXCHARS 40
int getanInt()
{
  int isvalidInt(char []);  /* function prototype */

  int isanInt = FALSE;
  char value[MAXCHARS];

  do
  {
    gets(value);
    if (isvalidInt(value) == FALSE)
    {
      printf("Invalid integer - Please re-enter: ");
      continue; /* send control to the do-while expression test */
    }
    isanInt = TRUE;
  }while (isanInt == FALSE);

  return (atoi(value));  /* convert to an integer */
}

int isvalidInt(char val[])
{
  int start = 0;
  int i;
  int valid = TRUE;
  int sign = FALSE;

  /* check for an empty string */
  if (val[0] == '\0') valid = FALSE;

  /* check for a leading sign */
  if (val[0] == '-' || val[0] == '+')
  {
    sign = TRUE;
    start = 1;  /* start checking for digits after the sign */
  }

  /* check that there is at least one character after the sign */
  if(sign == TRUE && val[1] == '\0') valid = FALSE;

  /*now check the string, which we know has at least one non-sign char */
  i = start;
  while(valid == TRUE && val[i] != '\0')
  {
    if (val[i] < '0' || val[i] > '9') /* check for a non-digit */
        valid = FALSE;    
    i++;
  }

   return valid;
}

＊9.4 格式化字符串

print("|% 数字s|")

正数，右对齐
负数，左对齐

9.5 字符和单词计数

字符计数

#include <stdio.h>
#define MAXNUM 1000

int countchar(char []);  /* function prototype */

int main()
{
    char message[MAXNUM];
    int numchar;
    
    printf("\nType in any number of characters: ");
    gets(message);
    numchar = countchar(message);
    printf("The number of characters just entered is %d\n", numchar);
    
    return 0;
}

int countchar(char list[])
{
    int i, count = 0;
    
    for(i = 0; list[i] != '\0'; i++)
        count++;
    
    return(count);
}

单词计数

#include <stdio.h>
#define MAXNUM 1000 

int countword(char []);  /* function prototype */

int main()
{
  char message[MAXNUM];
  int numchar;
  
  printf("\nType in any number of words: ");
  gets(message);
  numchar = countword(message);
  printf("The number of words just entered is %d\n", numchar);

  return 0;
}

int countword(char list[])
#define YES 1
#define NO 0
{
  int i, inaword, count = 0;

  inaword = NO;
  for(i = 0; list[i] != '\0'; i++)
  {
    if (list[i] == ' ')
      inaword = NO;
    else if (inaword == NO)
    {
      inaword = YES;
      count++;
    }
  }

  return(count);
}