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 : %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';
}

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