The C programming language (2nd) -- Notes -- 1.9

1.9 A character array

         The character array is C The most common array type in . The following programming explains the usage of character arrays and functions that operate character arrays . The program reads in a set of text lines , And print the longest line of text . Algorithm framework ：      

  while（ There are still unprocessed rows ）
  if ( This row is longer than the longest row that has been processed )
      Save the longest line of this behavior 
      Save the line length 
   Print the longest line 

        It can be seen from the above that , The program is divided into several sections , , respectively, Read in new line 、 Test read in rows 、 Save the row , The rest is Control this process .

        Because the division method is more reasonable , Press this to program . First compile an independent function getline Read Enter the next line . Try to keep this function available in other occasions . At least ,getline The function must return a signal when it reads to the end of the text ; It can even return the length of a line of text when reading it , Or return when encountering the text terminator 0. because 0 Is not a valid line length , Therefore, it can be used as the return value to mark the end of the file . Each line contains at least one character ; Even lines that contain only newline characters , The length of 1.

        When a Newly read line Compared with what I read before Longest line For a long time , This line needs to be saved . namely , The second function is required ,copy function , Copy the new line to a safe location .

        Last , Need one main Main program to control getline and copy function . Programming ：

#include <stdio.h>
#define MAXLINE 1000                /* Maximum input line length.*/

int getline(char line[], int maxline);
void copy (char to[], char from[]);

/* Print the longest input line.*/
int main()
{
    int len;                        /* [Current line] Length.*/
    int max;                        /* Maximum length [seen so far]. */
    char line[MAXLINE];             /* Current input*/
    char longest[MAXLINE];          /* Longest line saved here.*/

    max = 0;
    while ((len = getline(line, MAXLINE))>0)
        if (len > max)
        {
            max = len;
            copy (longest, line);
        }
    if (max > 0)                    /* There was a line.*/
        printf("%s, longest");
    return 0;
}


/*getline:  Read a line into s,   Return length.*/
int getline(char s[], int lim)
{
    int c, i;

    for ( i = 0; i < lim-1&(c = getchar()) != EOF &c!='\n'; ++i)
        s[i] = c;
    if (c == '\n')
        {
            s[i] = c;
            ++i;
        }
        s[i] = '\0';
        return i ;
}


/*copy:   Copy 'from' into 'to';  Assume to is big enough.*/ 
void copy(char to[], char from[])
{
    int i;

    i = 0;
    while ((to[i] =from[i]) != '\0')
    {
        ++i; 
    }    
}   

        stay Beginning of program The statement getline and copy function , Suppose they are in the same file .

        main And getline Functions pass through A pair of parameters And A return value Exchange data .getline function Two parameters Through the program line

int getline(char s[], int lim)

Statement ： Parameters 1—— s Declared as an array , Parameters 2—— lim Declare as integer , The statement provides Array size Is the purpose of Leave storage space . stay getline The function does not specify an array s The length of is because the size of the array is main Function settings . Same as power function ,getline The function uses a return Statement will Value is returned to its caller . The above program line also declares getline The return value type of the function is int. Because the default return value of the function is int, therefore int You can omit .

        There are some The function returns Useful values , And some functions ( Such as copy) Only used to perform some actions , and No return value .copy Function return value type is void, It explicitly states ： This function does not return any value .

        getline Function handle character '\0'（ That is, empty characters , Its value 0） Insert at the end of the array it creates , Mark the end of the string . This agreement has been C Language adoption ：C Language appears similar to

"hello\0"

Of Character constant , it Will be stored as a character array , Each element of the array stores each character of the string , And '\0' Marks the end of the string .

h

e

l

l

o

\n

\0

printf Normal format in function ——%s Regulations ： The corresponding parameter must be a string in this form .copy The function implementation depends on the input parameters '\0' End this fact , It will '\0' Copy to output parameters .（ namely , Null character '\0' Not part of ordinary text .）

        It is worth mentioning , Even such a small program as the above , There are also some troublesome design problems when passing parameters . Such as , When the length of the read line is greater than the maximum allowed ,main How should the function handle ？getline Function execution is safe , Whether the newline character is reached or not , Array full It will stop reading characters .main The function can determine whether the current line is too long by testing the length of the line and checking the last character returned , Then deal with it according to the specific situation . To simplify the procedure , This problem is not considered here .

        call getline The program of the function cannot predict the length of the input line , therefore getline The function needs to check whether it overflows . On the other hand , call copy Function program knows （ Or you can find out ） Length of string , Choose not to add error checking to this function .

        practice 1-16         Modify the main program of the program that prints the longest line of text main, Make it possible to print the length of input lines of any length , And print as much text as possible .

        practice 1-17        Programming , Print length greater than 80 All input lines of characters .

        practice 1-18        Programming , Delete spaces and tabs at the end of each input line , And delete lines that are completely blank .

        practice 1-19        Write function reverse(s), The string s The order of the characters in is reversed . Write a program with this function , Reverse the order of characters in one input line at a time .