c和指针 第一章的不懂的1,output_col == MAX_INPUT - 1 这个output_col是怎么回事 2, if( output_col +
/*** 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
#include
#include
#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 )//大于等于0的数字，这个数字所在数组columns中的列号小于20
num += 1;
/*
** Make sure we have an even number of inputs, as they are
** supposed to be paired.
*/
if( num % 2 != 0 ){ //判断输入的数字个数是否为奇数
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' )//EOF是个宏，在stdil.h中定义，值为 -1，也称结束标志。
; //当输入 ctrl+z 或 ctrl+c等组合键，getchar返回结束标志。
return num; //返回符合条件的数的个数
}
/*
** 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; //字符列区间
/*
** 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 ) //如果输入行结束或者或者输出行数组已满，就结束任务
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';
}
