C - Write the program tail which prints the last n lines of its input

素颜马尾好姑娘i 2022-11-14 11:52 22阅读 0赞

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/*
     * Write the program tail, which prints the last n lines of its input. By default,
     * n is set to 10, let us say, but it can be changed by an optional argument so that
     * tail -n
     * prints the last n lines. The program should behave rationally no matter how
     * unreasonable the input or the value of n. Write the program so it makes the best
     * use of available storage.
     *
     * Tail.c - by FreeMan
     */
    
    #include <stdio.h>
    #include <stdlib.h>
    
    #define MAXLINES        1024
    #define MAXINPUT        10240
    #define DEF_NUM_LINES   10
    
    int  getlines(char *);
    void parse_args(int, char **);
    
    char  linestr[MAXINPUT];
    char *lineptr[MAXLINES];
    int num_of_lines = DEF_NUM_LINES;
    
    int main(int argc, char *argv[])
    {
    	int lines;
    	int ltp = 0;
    	int i;
    	int c;
    
    	*lineptr = linestr;
    
    	/* Parse the argument strings passed to the program */
    	if (argc > 1) {
    		parse_args(argc, argv);
    	}
    
    	/* Get the input from the user */
    	lines = getlines(linestr);
    
    	if (num_of_lines == 0) {
    		num_of_lines = 10;
    	}
    	ltp = lines < num_of_lines ? lines : num_of_lines;
    	printf("\n>>> Printing %d line(s):\n", ltp);
    	if (ltp == DEF_NUM_LINES) {
    		printf(">>> The default number of lines to print is ");
    		printf("%d\n", DEF_NUM_LINES);
    	}
    	printf("\n");
    
    	for (i = lines; i > 0; i--)
    	{
    		while ((c = *lineptr[0]++) != '\n')
    		{
    			if (i <= ltp)
    			{
    				printf("%c", c);
    			}
    		}
    		if (c == '\n' && i <= ltp)
    		{
    			printf("\n");
    		}
    	}
    
    	return 0;
    }
    
    int getlines(char *buffer)
    {
    	int i, count = 0;
    	char c;
    
    	for (i = 0; (c = getchar()) != EOF && i < MAXINPUT; i++) {
    		*buffer++ = c;
    		if (c == '\n') {
    			lineptr[++count] = buffer;
    		}
    
    	}
    	*buffer++ = '\0';
    
    	return count;
    }
    
    void parse_args(int argc, char **argv)
    {
    	char c;
    
    	while (--argc > 0 && (*++argv)[0] == '-') {
    		c = *++argv[0];
    		switch (c) {
    		case 'n':
    			num_of_lines = atoi(*(argv + 1));
    			break;
    		}
    	}
    }

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