《C和指针》---【美】Kenneth A.Reek 著 读书笔记（一）快速上手

1.1 简介

    从一个例子程序开始：

   /*
** This program reads input lines from the standard input and prints
** each input line, followed by just some portions of the lines, to
** the standard output.
**
** The first input is a list of column numbers, which ends with a
** negative number.  The column numbers are paired and specify
** ranges of columns from the input line that are to be printed.
** For example, 0 3 10 12 -1 indicates that only columns 0 through 3
** and columns 10 through 12 will be printed.
*/


#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_COLS 20 /* max # of columns to process */
#define MAX_INPUT 1000 /* max len of input & output lines */


int read_column_numbers( int columns[], int max );
void rearrange( char *output, char const *input,
   int n_columns, int const columns[] );


int main( void )
{
int n_columns; /* # of columns to process */
int columns[MAX_COLS]; /* the columns to process */
char input[MAX_INPUT]; /* array for input line */
char output[MAX_INPUT]; /* array for output line */


/*
** Read the list of column numbers
*/
n_columns = read_column_numbers( columns, MAX_COLS );


/*
** Read, process and print the remaining lines of input.
*/
while( gets( input ) != NULL ){
printf( "Original input : %s\n", input );
rearrange( output, input, n_columns, columns );
printf( "Rearranged line: %s\n", output );
}


return EXIT_SUCCESS;
}


/*
** Read the list of column numbers, ignoring any beyond the specified
** maximum.
*/
int
read_column_numbers( int columns[], int max )
{
int num = 0;
int ch;


/*
** Get the numbers, stopping at eof or when a number is < 0.
*/
while( num < max && scanf( "%d", &columns[num] ) == 1
   && columns[num] >= 0 )
num += 1;


/*
** Make sure we have an even number of inputs, as they are
** supposed to be paired.
*/
if( num % 2 != 0 ){
puts( "Last column number is not paired." );
exit( EXIT_FAILURE );
}


/*
** Discard the rest of the line that contained the final
** number.
*/
while( (ch = getchar()) != EOF && ch != '\n' )
;


return num;
}


/*
** Process a line of input by concatenating the characters from
** the indicated columns.  The output line is then NUL terminated.
*/
void
rearrange( char *output, char const *input,
    int n_columns, int const columns[] )
{
int col; /* subscript for columns array */
int output_col; /* output column counter */
int len; /* length of input line */


len = strlen( input );
output_col = 0;


/*
** Process each pair of column numbers.
*/
for( col = 0; col < n_columns; col += 2 ){
int nchars = columns[col + 1] - columns[col] + 1;


/*
** If the input line isn't this long or the output
** array is full, we're done.
*/
if( columns[col] >= len ||
   output_col == MAX_INPUT - 1 )
break;


/*
** If there isn't room in the output array, only copy
** what will fit.
*/
if( output_col + nchars > MAX_INPUT - 1 )
nchars = MAX_INPUT - output_col - 1;


/*
** Copy the relevant data.
*/
strncpy( output + output_col, input + columns[col],
   nchars );
output_col += nchars;
}


output[output_col] = '\0';

}

1.注释

 /*开始  */结束 注释不能嵌套

注释段代码 

#if 0

 statements

#endif

2.预处理指令

3.函数原型