Extract every nth character from a string

Question

I'm trying to extract every 21st character from this text, s (given below), to create new strings of all 1st characters, 2nd characters, etc.

I have already separated the long string into substrings of 21 characters each using

    splitstring[String : str_, n_] := 
    StringJoin @@@ Partition[Characters[str], n, n, 1, {}]

giving:

    {"ALJJJEAQZJMZKOZDKEHBL", "XLPXNEHZCSEJVVLWHTUDJ", \
    "WFYXKKMWNNTNPHDTMGIOP", "OOSYPXGTLOHOPHTDHBHWO", \
    "MWGSKXSTNNEYSQHRSGPKP", "CJBNVIYCZHIVPFSWCKFPJ", \
    "OZQLNGPTLCIALHMBIGUOP", "ESYNDGACTURTALHLSGFBR", \
    "LPRMYKFQFXTEEZQHIUMOC", "CLSUIKWYLRPRRJZWCKUOW", \
    "WJLLVYEJXNIEMDQYQTDFC", "HJQLYKFQJTKUCICIRKOWX", \
    "PHLWYCCDRKVPAHCYPZFRL", "CNYBIOEWWGHQQBCDHGPRW", \
    "WHWIUOQTYOJLHGLFRTTVL", "CQCIUEHZYJKJEWHOVMYOM", \
    "JOBTIHCOSGCZVZJFEYHAC", "JQCQFSTWLOHYPXZDHBHTW", \
    "TLGIUJIWEXSJGKLTMKAFF", "PYGYICGSPRPLPOZNKPUSH", \
    "NSMQCGPWHILVXLZARZLIS", "POBDNEXJYMESQDTAQWKWL", \
    "WPBIDCCJXKHQCOTAJXBVZ", "WWDNCCCCWACQHZJREEHRO", \
    "LQSSRCVPFHCJCGLURBHFM", "POYKFKWRHKVGLLSYRTISC", \
    "ESYKXCEYFNSAHUHYIBJVL", "WZLKPJHQJEQQVHNHSEHTO", \
    "TNFJCRMZCCIEXKSALTMFP", "YTLGCORTMRIAILOISWKRJ", \
    "DTMRXGVYHEHRSIPRIWKOX", "SLBQVJHSZTMZMIHRAFFTJ", \
    "CTMSYGADQEHQXUZIQHLZJ", "OOGRZGVHLKPTIUEHHKHKT", \
    "CHCNCMMYLIMBWWHNKXBQP", "DEWQCIVZRNETVVPFCVYST", \
    "RTYZYEPWZTMQHLNHSGPRO", "TNFJCRRLNNPRHGYARRJVW", \
    "HJBIFTAPWRVUCZODYYLGF", "COBDWKMDTGJCEBDTVRDRO", \
    "HJQLPLVHJYKNJSLGEBZDL", "OOWKROWOOOJRXKRAMYMMM", \
    "FOBFTGTSLHIGLQLWVVLTL", "TDYBEGVNJLEAQDPRQXKRH", \
    "WOGIUCPLCJEAJBYGLTSCY", "OCUDECCQCURAELOAPEHKP", \
    "YJBIVOPWZTEVHJHNEYNMX", "CFSHYKPPWCGUMEWYKNHZW", \
    "JQSWCRANSOALVIJLPRZDL", "YOFDJVCDHTAVAHTRMTBHP", "RJZBIOEOSCR"}

How can I make strings of all the first characters, second characters, etc.?

    s=ALJJJEAQZJMZKOZDKEHBLXLPXNEHZCSEJVVLWHTUDJWFYXKKMWNNTNPHDTMGIOPOOSYPXG\
    TLOHOPHTDHBHWOMWGSKXSTNNEYSQHRSGPKPCJBNVIYCZHIVPFSWCKFPJOZQLNGPTLCIALH\
    MBIGUOPESYNDGACTURTALHLSGFBRLPRMYKFQFXTEEZQHIUMOCCLSUIKWYLRPRRJZWCKUOW\
    WJLLVYEJXNIEMDQYQTDFCHJQLYKFQJTKUCICIRKOWXPHLWYCCDRKVPAHCYPZFRLCNYBIOE\
    WWGHQQBCDHGPRWWHWIUOQTYOJLHGLFRTTVLCQCIUEHZYJKJEWHOVMYOMJOBTIHCOSGCZVZ\
    JFEYHACJQCQFSTWLOHYPXZDHBHTWTLGIUJIWEXSJGKLTMKAFFPYGYICGSPRPLPOZNKPUSH\
    NSMQCGPWHILVXLZARZLISPOBDNEXJYMESQDTAQWKWLWPBIDCCJXKHQCOTAJXBVZWWDNCCC\
    CWACQHZJREEHROLQSSRCVPFHCJCGLURBHFMPOYKFKWRHKVGLLSYRTISCESYKXCEYFNSAHU\
    HYIBJVLWZLKPJHQJEQQVHNHSEHTOTNFJCRMZCCIEXKSALTMFPYTLGCORTMRIAILOISWKRJ\
    DTMRXGVYHEHRSIPRIWKOXSLBQVJHSZTMZMIHRAFFTJCTMSYGADQEHQXUZIQHLZJOOGRZGV\
    HLKPTIUEHHKHKTCHCNCMMYLIMBWWHNKXBQPDEWQCIVZRNETVVPFCVYSTRTYZYEPWZTMQHL\
    NHSGPROTNFJCRRLNNPRHGYARRJVWHJBIFTAPWRVUCZODYYLGFCOBDWKMDTGJCEBDTVRDRO\
    HJQLPLVHJYKNJSLGEBZDLOOWKROWOOOJRXKRAMYMMMFOBFTGTSLHIGLQLWVVLTLTDYBEGV\
    NJLEAQDPRQXKRHWOGIUCPLCJEAJBYGLTSCYOCUDECCQCURAELOAPEHKPYJBIVOPWZTEVHJ\
    HNEYNMXCFSHYKPPWCGUMEWYKNHZWJQSWCRANSOALVIJLPRZDLYOFDJVCDHTAVAHTRMTBHP\
    RJZBIOEOSCR

Answer 1

StringTake and Span would be useful.

For example, to get the second characters:

StringTake[s, 2 ;; ;; 21]
(* "LLFOWJZSPLJJHNHQOQLYSOPWQOSZNTTLTOHETNJOJOODOCJFQOJ" *)

To get all of the strings:

StringTake[s, Array[# ;; ;; 21&, 21]]

Another approach would be a combination of Characters and Part:

StringJoin[Characters[s][[2 ;; ;; 21]]]
(* "LLFOWJZSPLJJHNHQOQLYSOPWQOSZNTTLTOHETNJOJOODOCJFQOJ" *)

Again, to get all of the strings:

Array[StringJoin[Characters[s][[# ;; ;; 21]]]&, 21]

Answer 2

The last substring does not contain 21 characters but you can use the padded form of Partition to get sublist of equal length with padding. Then it is just a Transpose and StringJoin to the result. I use "&" for padding which you can substitute for any character.

StringJoin@Flatten@Transpose@Partition[Characters[s], 21, 21, {1, 1}, "&"]

Hope this helps.

Answer 3

StringJoin /@ Transpose[Characters @ StringPartition[s,21]]