Calculate the characters from a combination












1












$begingroup$


Given a table with position - 2 character combination pairs like this:



1. aa
2. ab
3. ac
4. ad
...
27. ba
28. bb
29. bc
30. bd


Assuming there are unique combinations with two characters from the alphabet following the same pattern.



How do I calculate what is the first and second character of the combination, if I know the position. That is, how do I find position 28 holds bb?





EDIT:



The answer that also describes how to do the reverse, i.e find the position of a combination based on the pair is going to be the most useful answer.










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    should ba be $27$ if you are starting from $1$ and including aa?
    $endgroup$
    – gt6989b
    Dec 10 '18 at 14:03










  • $begingroup$
    @gt6989b Oh, I missed one character when counting. (Missed to write it). Sorry.
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:05


















1












$begingroup$


Given a table with position - 2 character combination pairs like this:



1. aa
2. ab
3. ac
4. ad
...
27. ba
28. bb
29. bc
30. bd


Assuming there are unique combinations with two characters from the alphabet following the same pattern.



How do I calculate what is the first and second character of the combination, if I know the position. That is, how do I find position 28 holds bb?





EDIT:



The answer that also describes how to do the reverse, i.e find the position of a combination based on the pair is going to be the most useful answer.










share|cite|improve this question











$endgroup$








  • 1




    $begingroup$
    should ba be $27$ if you are starting from $1$ and including aa?
    $endgroup$
    – gt6989b
    Dec 10 '18 at 14:03










  • $begingroup$
    @gt6989b Oh, I missed one character when counting. (Missed to write it). Sorry.
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:05
















1












1








1





$begingroup$


Given a table with position - 2 character combination pairs like this:



1. aa
2. ab
3. ac
4. ad
...
27. ba
28. bb
29. bc
30. bd


Assuming there are unique combinations with two characters from the alphabet following the same pattern.



How do I calculate what is the first and second character of the combination, if I know the position. That is, how do I find position 28 holds bb?





EDIT:



The answer that also describes how to do the reverse, i.e find the position of a combination based on the pair is going to be the most useful answer.










share|cite|improve this question











$endgroup$




Given a table with position - 2 character combination pairs like this:



1. aa
2. ab
3. ac
4. ad
...
27. ba
28. bb
29. bc
30. bd


Assuming there are unique combinations with two characters from the alphabet following the same pattern.



How do I calculate what is the first and second character of the combination, if I know the position. That is, how do I find position 28 holds bb?





EDIT:



The answer that also describes how to do the reverse, i.e find the position of a combination based on the pair is going to be the most useful answer.







arithmetic index-notation






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Dec 11 '18 at 0:10







Edenia

















asked Dec 10 '18 at 14:00









EdeniaEdenia

1085




1085








  • 1




    $begingroup$
    should ba be $27$ if you are starting from $1$ and including aa?
    $endgroup$
    – gt6989b
    Dec 10 '18 at 14:03










  • $begingroup$
    @gt6989b Oh, I missed one character when counting. (Missed to write it). Sorry.
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:05
















  • 1




    $begingroup$
    should ba be $27$ if you are starting from $1$ and including aa?
    $endgroup$
    – gt6989b
    Dec 10 '18 at 14:03










  • $begingroup$
    @gt6989b Oh, I missed one character when counting. (Missed to write it). Sorry.
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:05










1




1




$begingroup$
should ba be $27$ if you are starting from $1$ and including aa?
$endgroup$
– gt6989b
Dec 10 '18 at 14:03




$begingroup$
should ba be $27$ if you are starting from $1$ and including aa?
$endgroup$
– gt6989b
Dec 10 '18 at 14:03












$begingroup$
@gt6989b Oh, I missed one character when counting. (Missed to write it). Sorry.
$endgroup$
– Edenia
Dec 10 '18 at 14:05






$begingroup$
@gt6989b Oh, I missed one character when counting. (Missed to write it). Sorry.
$endgroup$
– Edenia
Dec 10 '18 at 14:05












2 Answers
2






active

oldest

votes


















1












$begingroup$

Assuming you have an alphabet A = ['a','b',...'z'] and a permutation list L = ['aa', 'ab', ..., 'az', 'ba', ... , 'zz'], with the first index starting from 1. (BTW the arithmetic would be much cleaner if you start indexing from $0$, like in the C language, for example.)



Then, the permutations come in groups of $26$, so if you are given an index $k$, then




  • the second letter is given by the offset of $k$ in the current 26-long block, so it is $A[(k-1)pmod{26} + 1]$ and

  • the first letter is given by $Aleft[lfloor(k-1)/26rfloor+1right]$.




If you were to number starting with $0$, the formulae become




  • the second letter is given by $A[k pmod{26}]$ and

  • the first letter is given by $Aleft[lfloor k/26rfloorright]$.




Another free update you get with C, for example, is that if you declare int k, d = 26; then k/d computes $lfloor k/d rfloor$ automatically, no extra flooring is needed.





UPDATE



The inversion is simple. If first letter has index $F$ and last has index $L$, the offset is given by





  • $26F + L$ for $0$-based indexing (like C)


  • $26(F+1) + (L+1) = 26F + L + 27$ for $1$-based indexing






share|cite|improve this answer











$endgroup$









  • 1




    $begingroup$
    Haha, I am implementing algorithms in C, bt dubs.
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:17










  • $begingroup$
    @Edenia see updates
    $endgroup$
    – gt6989b
    Dec 10 '18 at 14:20










  • $begingroup$
    Hmm A[pos % 26] doesn't return the correct first character. Afirst[(pos / sz) - 1] and Asecond[pos % sz] works though
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:32












  • $begingroup$
    sz being the 26.
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:38










  • $begingroup$
    @gt6989b I think you wrote the reverse: the first letter is given by $A[lfloor k/26rfloor]$
    $endgroup$
    – Shubham Johri
    Dec 10 '18 at 14:46



















2












$begingroup$

For the index $n$, the $(lfloor frac{n-1}{26}rfloor+1)^{th}$ letter of the alphabet is the first character, $(nmod26)^{th}$ letter of the alphabet is the second character.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    so for the second, index's reminder of the length of the alphabet. Got it. Really useful, thank you. What is this called though? Didn't know what to search for.
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:12












  • $begingroup$
    It's even nicer with 0-based numbering (and doesn't require the unusual interpretation of $26 bmod 26$ as 26).
    $endgroup$
    – LSpice
    Dec 10 '18 at 14:13











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2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









1












$begingroup$

Assuming you have an alphabet A = ['a','b',...'z'] and a permutation list L = ['aa', 'ab', ..., 'az', 'ba', ... , 'zz'], with the first index starting from 1. (BTW the arithmetic would be much cleaner if you start indexing from $0$, like in the C language, for example.)



Then, the permutations come in groups of $26$, so if you are given an index $k$, then




  • the second letter is given by the offset of $k$ in the current 26-long block, so it is $A[(k-1)pmod{26} + 1]$ and

  • the first letter is given by $Aleft[lfloor(k-1)/26rfloor+1right]$.




If you were to number starting with $0$, the formulae become




  • the second letter is given by $A[k pmod{26}]$ and

  • the first letter is given by $Aleft[lfloor k/26rfloorright]$.




Another free update you get with C, for example, is that if you declare int k, d = 26; then k/d computes $lfloor k/d rfloor$ automatically, no extra flooring is needed.





UPDATE



The inversion is simple. If first letter has index $F$ and last has index $L$, the offset is given by





  • $26F + L$ for $0$-based indexing (like C)


  • $26(F+1) + (L+1) = 26F + L + 27$ for $1$-based indexing






share|cite|improve this answer











$endgroup$









  • 1




    $begingroup$
    Haha, I am implementing algorithms in C, bt dubs.
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:17










  • $begingroup$
    @Edenia see updates
    $endgroup$
    – gt6989b
    Dec 10 '18 at 14:20










  • $begingroup$
    Hmm A[pos % 26] doesn't return the correct first character. Afirst[(pos / sz) - 1] and Asecond[pos % sz] works though
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:32












  • $begingroup$
    sz being the 26.
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:38










  • $begingroup$
    @gt6989b I think you wrote the reverse: the first letter is given by $A[lfloor k/26rfloor]$
    $endgroup$
    – Shubham Johri
    Dec 10 '18 at 14:46
















1












$begingroup$

Assuming you have an alphabet A = ['a','b',...'z'] and a permutation list L = ['aa', 'ab', ..., 'az', 'ba', ... , 'zz'], with the first index starting from 1. (BTW the arithmetic would be much cleaner if you start indexing from $0$, like in the C language, for example.)



Then, the permutations come in groups of $26$, so if you are given an index $k$, then




  • the second letter is given by the offset of $k$ in the current 26-long block, so it is $A[(k-1)pmod{26} + 1]$ and

  • the first letter is given by $Aleft[lfloor(k-1)/26rfloor+1right]$.




If you were to number starting with $0$, the formulae become




  • the second letter is given by $A[k pmod{26}]$ and

  • the first letter is given by $Aleft[lfloor k/26rfloorright]$.




Another free update you get with C, for example, is that if you declare int k, d = 26; then k/d computes $lfloor k/d rfloor$ automatically, no extra flooring is needed.





UPDATE



The inversion is simple. If first letter has index $F$ and last has index $L$, the offset is given by





  • $26F + L$ for $0$-based indexing (like C)


  • $26(F+1) + (L+1) = 26F + L + 27$ for $1$-based indexing






share|cite|improve this answer











$endgroup$









  • 1




    $begingroup$
    Haha, I am implementing algorithms in C, bt dubs.
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:17










  • $begingroup$
    @Edenia see updates
    $endgroup$
    – gt6989b
    Dec 10 '18 at 14:20










  • $begingroup$
    Hmm A[pos % 26] doesn't return the correct first character. Afirst[(pos / sz) - 1] and Asecond[pos % sz] works though
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:32












  • $begingroup$
    sz being the 26.
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:38










  • $begingroup$
    @gt6989b I think you wrote the reverse: the first letter is given by $A[lfloor k/26rfloor]$
    $endgroup$
    – Shubham Johri
    Dec 10 '18 at 14:46














1












1








1





$begingroup$

Assuming you have an alphabet A = ['a','b',...'z'] and a permutation list L = ['aa', 'ab', ..., 'az', 'ba', ... , 'zz'], with the first index starting from 1. (BTW the arithmetic would be much cleaner if you start indexing from $0$, like in the C language, for example.)



Then, the permutations come in groups of $26$, so if you are given an index $k$, then




  • the second letter is given by the offset of $k$ in the current 26-long block, so it is $A[(k-1)pmod{26} + 1]$ and

  • the first letter is given by $Aleft[lfloor(k-1)/26rfloor+1right]$.




If you were to number starting with $0$, the formulae become




  • the second letter is given by $A[k pmod{26}]$ and

  • the first letter is given by $Aleft[lfloor k/26rfloorright]$.




Another free update you get with C, for example, is that if you declare int k, d = 26; then k/d computes $lfloor k/d rfloor$ automatically, no extra flooring is needed.





UPDATE



The inversion is simple. If first letter has index $F$ and last has index $L$, the offset is given by





  • $26F + L$ for $0$-based indexing (like C)


  • $26(F+1) + (L+1) = 26F + L + 27$ for $1$-based indexing






share|cite|improve this answer











$endgroup$



Assuming you have an alphabet A = ['a','b',...'z'] and a permutation list L = ['aa', 'ab', ..., 'az', 'ba', ... , 'zz'], with the first index starting from 1. (BTW the arithmetic would be much cleaner if you start indexing from $0$, like in the C language, for example.)



Then, the permutations come in groups of $26$, so if you are given an index $k$, then




  • the second letter is given by the offset of $k$ in the current 26-long block, so it is $A[(k-1)pmod{26} + 1]$ and

  • the first letter is given by $Aleft[lfloor(k-1)/26rfloor+1right]$.




If you were to number starting with $0$, the formulae become




  • the second letter is given by $A[k pmod{26}]$ and

  • the first letter is given by $Aleft[lfloor k/26rfloorright]$.




Another free update you get with C, for example, is that if you declare int k, d = 26; then k/d computes $lfloor k/d rfloor$ automatically, no extra flooring is needed.





UPDATE



The inversion is simple. If first letter has index $F$ and last has index $L$, the offset is given by





  • $26F + L$ for $0$-based indexing (like C)


  • $26(F+1) + (L+1) = 26F + L + 27$ for $1$-based indexing







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Dec 11 '18 at 4:13

























answered Dec 10 '18 at 14:16









gt6989bgt6989b

33.9k22455




33.9k22455








  • 1




    $begingroup$
    Haha, I am implementing algorithms in C, bt dubs.
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:17










  • $begingroup$
    @Edenia see updates
    $endgroup$
    – gt6989b
    Dec 10 '18 at 14:20










  • $begingroup$
    Hmm A[pos % 26] doesn't return the correct first character. Afirst[(pos / sz) - 1] and Asecond[pos % sz] works though
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:32












  • $begingroup$
    sz being the 26.
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:38










  • $begingroup$
    @gt6989b I think you wrote the reverse: the first letter is given by $A[lfloor k/26rfloor]$
    $endgroup$
    – Shubham Johri
    Dec 10 '18 at 14:46














  • 1




    $begingroup$
    Haha, I am implementing algorithms in C, bt dubs.
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:17










  • $begingroup$
    @Edenia see updates
    $endgroup$
    – gt6989b
    Dec 10 '18 at 14:20










  • $begingroup$
    Hmm A[pos % 26] doesn't return the correct first character. Afirst[(pos / sz) - 1] and Asecond[pos % sz] works though
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:32












  • $begingroup$
    sz being the 26.
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:38










  • $begingroup$
    @gt6989b I think you wrote the reverse: the first letter is given by $A[lfloor k/26rfloor]$
    $endgroup$
    – Shubham Johri
    Dec 10 '18 at 14:46








1




1




$begingroup$
Haha, I am implementing algorithms in C, bt dubs.
$endgroup$
– Edenia
Dec 10 '18 at 14:17




$begingroup$
Haha, I am implementing algorithms in C, bt dubs.
$endgroup$
– Edenia
Dec 10 '18 at 14:17












$begingroup$
@Edenia see updates
$endgroup$
– gt6989b
Dec 10 '18 at 14:20




$begingroup$
@Edenia see updates
$endgroup$
– gt6989b
Dec 10 '18 at 14:20












$begingroup$
Hmm A[pos % 26] doesn't return the correct first character. Afirst[(pos / sz) - 1] and Asecond[pos % sz] works though
$endgroup$
– Edenia
Dec 10 '18 at 14:32






$begingroup$
Hmm A[pos % 26] doesn't return the correct first character. Afirst[(pos / sz) - 1] and Asecond[pos % sz] works though
$endgroup$
– Edenia
Dec 10 '18 at 14:32














$begingroup$
sz being the 26.
$endgroup$
– Edenia
Dec 10 '18 at 14:38




$begingroup$
sz being the 26.
$endgroup$
– Edenia
Dec 10 '18 at 14:38












$begingroup$
@gt6989b I think you wrote the reverse: the first letter is given by $A[lfloor k/26rfloor]$
$endgroup$
– Shubham Johri
Dec 10 '18 at 14:46




$begingroup$
@gt6989b I think you wrote the reverse: the first letter is given by $A[lfloor k/26rfloor]$
$endgroup$
– Shubham Johri
Dec 10 '18 at 14:46











2












$begingroup$

For the index $n$, the $(lfloor frac{n-1}{26}rfloor+1)^{th}$ letter of the alphabet is the first character, $(nmod26)^{th}$ letter of the alphabet is the second character.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    so for the second, index's reminder of the length of the alphabet. Got it. Really useful, thank you. What is this called though? Didn't know what to search for.
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:12












  • $begingroup$
    It's even nicer with 0-based numbering (and doesn't require the unusual interpretation of $26 bmod 26$ as 26).
    $endgroup$
    – LSpice
    Dec 10 '18 at 14:13
















2












$begingroup$

For the index $n$, the $(lfloor frac{n-1}{26}rfloor+1)^{th}$ letter of the alphabet is the first character, $(nmod26)^{th}$ letter of the alphabet is the second character.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    so for the second, index's reminder of the length of the alphabet. Got it. Really useful, thank you. What is this called though? Didn't know what to search for.
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:12












  • $begingroup$
    It's even nicer with 0-based numbering (and doesn't require the unusual interpretation of $26 bmod 26$ as 26).
    $endgroup$
    – LSpice
    Dec 10 '18 at 14:13














2












2








2





$begingroup$

For the index $n$, the $(lfloor frac{n-1}{26}rfloor+1)^{th}$ letter of the alphabet is the first character, $(nmod26)^{th}$ letter of the alphabet is the second character.






share|cite|improve this answer









$endgroup$



For the index $n$, the $(lfloor frac{n-1}{26}rfloor+1)^{th}$ letter of the alphabet is the first character, $(nmod26)^{th}$ letter of the alphabet is the second character.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Dec 10 '18 at 14:08









Shubham JohriShubham Johri

4,992717




4,992717












  • $begingroup$
    so for the second, index's reminder of the length of the alphabet. Got it. Really useful, thank you. What is this called though? Didn't know what to search for.
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:12












  • $begingroup$
    It's even nicer with 0-based numbering (and doesn't require the unusual interpretation of $26 bmod 26$ as 26).
    $endgroup$
    – LSpice
    Dec 10 '18 at 14:13


















  • $begingroup$
    so for the second, index's reminder of the length of the alphabet. Got it. Really useful, thank you. What is this called though? Didn't know what to search for.
    $endgroup$
    – Edenia
    Dec 10 '18 at 14:12












  • $begingroup$
    It's even nicer with 0-based numbering (and doesn't require the unusual interpretation of $26 bmod 26$ as 26).
    $endgroup$
    – LSpice
    Dec 10 '18 at 14:13
















$begingroup$
so for the second, index's reminder of the length of the alphabet. Got it. Really useful, thank you. What is this called though? Didn't know what to search for.
$endgroup$
– Edenia
Dec 10 '18 at 14:12






$begingroup$
so for the second, index's reminder of the length of the alphabet. Got it. Really useful, thank you. What is this called though? Didn't know what to search for.
$endgroup$
– Edenia
Dec 10 '18 at 14:12














$begingroup$
It's even nicer with 0-based numbering (and doesn't require the unusual interpretation of $26 bmod 26$ as 26).
$endgroup$
– LSpice
Dec 10 '18 at 14:13




$begingroup$
It's even nicer with 0-based numbering (and doesn't require the unusual interpretation of $26 bmod 26$ as 26).
$endgroup$
– LSpice
Dec 10 '18 at 14:13


















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