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I'm not sure what's common for conditional statements, with and without alternative consequent clauses, in speech—even my own—but as a computer programmer, I'm niggled whenever I have to write English instructions that follow the form of a programmatic if (condition) doThis(); else doThat();. I was just wondering what structure seemed clearest and least ambiguous (not necessarily restricted to the following), keeping in mind that the place-holders (condition,alternative,consequent) might include punctuation such as one or more commas, or even a number of sub-clauses:

  • If <condition>, <consequent>.
  • If <condition>, then <consequent>.

With alternative:

  • <If...>, otherwise <alternative>.
  • <If...>; otherwise, <alternative>.
  • <If...>. Otherwise, <alternative>.
  • If ... is ..., <consequent>. If ... isn't ..., <alternative>.
  • <If...>, or else <alternative>.
    • I've determined "or else" to be poor style. "Else" on its own would be better but still sounds inferior to otherwise or negating the condition.
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As you have set forth, there are many ways to express these statements. The appropriate style will depend upon the context. Can you provide more detail? How many statements of this type will there be? Is there any nested logic involved? How technical or non-technical is the audience? – Jonathan Van Matre Dec 19 '11 at 20:30
Correct use of the subjunctive tense is also important in conditional usage... – Ben Voigt Dec 19 '11 at 22:15
I was asking for a general case, which now seems obvious that it's very dependent on the context, although an enumeration of different cases and best style for each would be nice to have. The original incident that prompted me to ask was when I wrote to someone something like, "If the following report is still valid, let me know; otherwise just ignore it." – Mark Cidade Dec 20 '11 at 2:01
@Mark Cidade: If you replaced "otherwise" with "or else" in your "something like" above, strictly speaking I would interpret that to me "PROVIDING it's still valid, you're free to choose whether to let me know or ignore it." The implication being that if it's NOT valid, you must come back to me for further instructions anyway. – FumbleFingers Dec 20 '11 at 16:00
I agree. Or else isn't a valid option for good style in just about any case. – Mark Cidade Dec 20 '11 at 16:51
up vote 2 down vote accepted

The basic format is If condition consequence. It's largely a matter of style whether you insert a comma and/or the word "then" between condition and consequence.

If the statement continues with an alternative, it should be preceded by a comma or period (or sometimes a dash). The most common form is otherwise consequence2 (optionally separated by a comma), but the word "otherwise" may be replaced by In all other cases, If not, Failing that, etc.

Variations on the second alternative involving the word "else" are normally either archaic, or you're writing program code.

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I find "or else" to be current. – Colin Fine Dec 20 '11 at 0:16
Even in if constructions with alternative? I think it's fine in Do this, or else I'll do that, but not in If it's hot I'll go swimming, or else I'll read a book. – FumbleFingers Dec 20 '11 at 0:22
"Or else" is still common as a threat, often with nothing following else. – Mark Cidade Dec 20 '11 at 2:06
@Mark Cidade: Yes, but as "Do this, or else I'll kill you", not "If you do this, I'll kill you. Or else I'll marry you". I don't think "else" works after "if" except in programming languages. – FumbleFingers Dec 20 '11 at 4:59
@FumbleFingers: yes, I find that natural. – Colin Fine Dec 20 '11 at 11:23

Your first two examples are both sound, though the first sounds more natural (read: casual):

If you heat water to 100 degrees Celsius, it boils.

If you heat water to 100 degrees Celsius, then it boils.

The three otherwise statements are quite similar, though the first is not optimal, since it is harder to parse and understand:

If it rains, we won't be able to attend, otherwise I see nothing against it.

If it rains, we won't be able to attend; otherwise, I see nothing against it.

If it rains, we won't be able to attend. Otherwise, I see nothing against it.

But the third does not fit the pattern:

If it rains, we won't be able to attend, or else I see nothing against it.

This means [If ... then this or that] rather than [If ... then this, otherwise that].

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The first otherwise example (If it rains, we won't be able to attend, otherwise I see nothing against it.) is a run-on. – onomatomaniak Dec 19 '11 at 20:56
That's only because the purpose of the commas are ambiguous. If there was no other comma, I see nothing against it. – Mark Cidade Dec 20 '11 at 2:03

It depends on context. If your audience is not trained in logic, then writing a series of syllogisms, while technically more correct, might not be as helpful as something like this:

Whenever you (condition), the program will (consequent).

We will (consequent) as long as (condition).

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If would still be appropriate if it's expected only once (but following the consequent is common in non-technical speech or languages like PERL or Ruby). – Mark Cidade Dec 20 '11 at 2:08

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