for example...

>seq A
aagagagagtctctgtagatgatcgctgctcgctgct
agctcgatcgtacgcatactacgactgacgactacgcat
gctacgcatacgactcagcatgatcatgacgtactacg
>seq B
aagagagagtctctgtagatgatcgctgctcgctgct
agctcgatcgtacgcatactacgactgacgactacgcat
gctacgcatacgactcagcatgatcatgacgtactacg
>seq C
aagagagagtctctgtagatgatcgctgctcgctgct
agctcgatcgtacgcatactacgactgacgactacgcat
gctacgcatacgactcagcatgatcatgacgtactacg


and rank file contains

seqA :0.5
seqB :1.5
seqc :2.5


Then the file that gets the seq A, must get this :
>seq A
aagagagagtctctgtagatgatcgctgctcgctgct
agctcgatcgtacgcatactacgactgacgactacgcat
gctacgcatacgactcagcatgatcatgacgtactacg

#n
/^seq A/,+3w fileA
/^seq B/,+3w fileB
/^seq C/,+3w fileC
...
/^seq Q/,+3w fileA

jschiwal@hpamd64:~/tmpdir> cat datafile
>seq A
aagagagagtctctgtagatgatcgctgctcgctgct
agctcgatcgtacgcatactacgactgacgactacgcat
gctacgcatacgactcagcatgatcatgacgtactacg
>seq B
aagagagagtctctgtagatgatcgctgctcgctgct
agctcgatcgtacgcatactacgactgacgactacgcat
gctacgcatacgactcagcatgatcatgacgtactacg
>seq C
aagagagagtctctgtagatgatcgctgctcgctgct
agctcgatcgtacgcatactacgactgacgactacgcat
gctacgcatacgactcagcatgatcatgacgtactacg
>seq Q
aagagagagtctctgtagatgatcgctgctcgctgct
agctcgatcgtacgcatactacgactgacgactacgcat
gctacgcatacgactcagcatgatcatgacgtactacg
jschiwal@hpamd64:~/tmpdir> cat selseq.sed
#n
/^>seq A/,+3 w fileA
/^>seq B/,+3 w fileB
/^>seq C/,+3 w fileC
/^>seq Q/,+3 w fileA

jschiwal@hpamd64:~/tmpdir> sed -f selseq.sed datafile
jschiwal@hpamd64:~/tmpdir> cat fileA
>seq A
aagagagagtctctgtagatgatcgctgctcgctgct
agctcgatcgtacgcatactacgactgacgactacgcat
gctacgcatacgactcagcatgatcatgacgtactacg
>seq Q
aagagagagtctctgtagatgatcgctgctcgctgct
agctcgatcgtacgcatactacgactgacgactacgcat
gctacgcatacgactcagcatgatcatgacgtactacg
jschiwal@hpamd64:~/tmpdir> cat fileB
>seq B
aagagagagtctctgtagatgatcgctgctcgctgct
agctcgatcgtacgcatactacgactgacgactacgcat
gctacgcatacgactcagcatgatcatgacgtactacg
jschiwal@hpamd64:~/tmpdir> cat fileC
>seq C
aagagagagtctctgtagatgatcgctgctcgctgct
agctcgatcgtacgcatactacgactgacgactacgcat
gctacgcatacgactcagcatgatcatgacgtactacg

/* this is the function to parse the input data */
static int parse_input_file(const char *fFile)
{
    /* open the file provided */
    FILE *input_file = fopen(fFile, "r");
    if (!input_file) return -1;

    /* this is to hold input data from the file */
    char current_line[1024];

    /* indicator of the current file */
    int current_file = -1;

    while (!feof(input_file))
    /* this loop keeps reading lines from the file until it reaches the end */
    {
    /* if there are less than 3 characters, the line is ignored ('\n' + '\0' = 2) */
    if (!fgets(current_line, 1024, input_file)) break;
    if (strlen(current_line) < 3) continue;

    /* to allow comparison to the list, remove the newline */
    current_line[strlen(current_line) - 1] = 0;

    /* if the line denotes a label, find the correct file and change it */
    if (current_line[0] == '>') current_file = determine_file(value(current_line + 1));

    /*else*/
    /* otherwise, send the output to the file and replace the newline */
    /* {*/
    send_file_output(current_line, strlen(current_line), current_file);
    send_file_output("\n", 1, current_file);
    /* }*/
    }

    return 0;
}

#!/usr/bin/python

import math

datafname="data.txt"
rankfname="rank.txt"
outfbase="outfile_"

outmap = {}
outfiles = []
for i in (1,2,3,4,5,6,7,8,9,10) :
        outfiles.append(open(outfbase + '%02d' % i, 'w'))

rankfile = open(rankfname, 'r')
for line in rankfile.xreadlines() :
        tag = line.split(':')[0].strip()
        rank = float(line.split(':')[1].strip())

        rankno = int(math.ceil(rank * 10))
        if rankno == 0 :
                rankno = 1

        if rankno < 0 or rankno > 10 :
                print "Invalid rank: " + line
                continue

        outmap[tag] = outfiles[rankno-1]

rankfile.close()

tag=""
datafile = open(datafname, 'r')
for line in datafile.xreadlines() :
        if line[0] == '>' :
                tag = line[1:].strip()
                outfile = outmap.get(tag,"")
                if not outfile :
                        print "Unranked seq: " + tag
                        tag=""
                        continue

        if len(tag) > 0 :
                outfile.write(line)

datafile.close()
for f in outfiles :
        f.close()

#include <string>
#include <vector>


struct tabled_value
{
	tabled_value() : value() {}

	tabled_value(const std::string lLabel, double vValue) :
	label(lLabel), value(vValue) {}

	inline bool operator == (const char *lLabel) const
	{ return !strcmp(&this->label[0], lLabel); }

	std::string label;
	double      value;
};


class multi_table3
{
public:
	void add_value(const std::string lLabel, double vValue)
	{
	unsigned int X = 256, Y = 256, Z = 256, size = lLabel.size();

	if (size > 0) X = lLabel[0];
	if (size > 1) Y = lLabel[1];
	if (size > 2) Z = lLabel[2];

	master_table[X][Y][Z].insert(master_table[X][Y][Z].begin(), tabled_value());

	if (size > 3)
	master_table[X][Y][Z][0].label = lLabel.substr(3, size - 3);

	master_table[X][Y][Z][0].value = vValue;
	}

	double retrieve_value(const char *lLabel) const
	{
	unsigned int X = 256, Y = 256, Z = 256, size = strlen(lLabel);

	const char empty[] = "";
	const char *compare = empty;

	if (size > 0) X = lLabel[0];
	if (size > 1) Y = lLabel[1];
	if (size > 2) Z = lLabel[2];
	if (size > 3) compare = lLabel + 3;

	std::vector <tabled_value> ::const_iterator begin = master_table[X][Y][Z].begin(),
	                                            end   = master_table[X][Y][Z].end();

	while (begin < end)
	if (*begin == compare) return begin->value;
	else ++begin;

	return 0.0;
	}

private:
	std::vector <tabled_value> master_table[257][257][257];
};