获取字符串的Ｎ段子串

例如输入：

0 3  5  7 -1

input:0123456789

output:0123567

程序分析：

输入两段字串间隔如：（0　3）　（5　7）　－１是结束符

/*
** 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 : %sn", input );
rearrange( output, input, n_columns, columns );
printf( "Rearranged line: %sn", 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] = '';
}

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