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“Add the option -f to fold upper and lower case together, so that case distinctions are not made during sorting; for example, a and A compare equal.”

Excerpt From: Brian W. Kernighan. “The C Programming Language, Second Edition.

I am not sure what this means exactly. I was thinking about two options:

  • convert all uppercase to lowercase letters or vice versa
  • keep the cases as they are and merely treat them as equal

Does the expression to fold something together in this case have a specific meaning?
Or am I free to choose an interpretation?

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  • The clause so that case distinctions are not made during sorting; for example, a and A compare equal defines what folding means here. – Jim Jan 3 '16 at 2:24
  • So merely treat them as if they were equal, but not turning them into equal? I am not sure if I am understood. Do you know what I mean? – Ely Jan 3 '16 at 2:27
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    Correct. Treat them as equal but do not change them. Thus, the set: { BB,ba,aB,cA,Cb,Aa,CC,ac} would sort to: {Aa,aB,ac,ba,BB,cA,Cb,CC} and not {ac,aB,ba,ca,Aa,BB,Cb,CC}. – Jim Jan 3 '16 at 2:35
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This is from an exercise in C programming, in which the student is asked to modify a line sorting program. "Fold together" here means to treat each lower-case letter and its corresponding upper-case letter as the same value for purposes of the sort.

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  • So merely treat them as if they were equal, but not turning them into equal? I am not sure if I am understood. Do you know what I mean? – Ely Jan 3 '16 at 2:27
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    I think so. The program sorts the lines as though all uppercase letters were lowercase letters but outputs the lines unchanged. – deadrat Jan 3 '16 at 3:19
  • Yes, "fold" was an unfortunately poetic choice of words, especially for a manual, because it implies action where there is no action. The only action is the reordering of the entries. – lauir Jan 3 '16 at 5:53

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