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Executing 

filter(i -> !ismissing(i.dep_delay > 60), select(flights, (:carrier, :dep_delay)))

should return the carriers with delay more than 60mins(my understanding!). But it returns carriers with all positive and negative dep_delays.

Why it behaves so?

AVA
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1 Answers1

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I understand you rather wanted to write:

filter(i -> coalesce(i.dep_delay > 60, false), select(flights, (:carrier, :dep_delay)))

If you write !ismissing(i.dep_delay > 60) you will get true if i.dep_delay is not missing.

Bogumił Kamiński
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  • sir, a short note on functionality of coalesce(), please! – AVA May 22 '19 at 06:05
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    It is a standard function in Base. If you look up its help you get: return the first value in the arguments which is not equal to `missing`, if any. Otherwise return `missing`. In a typical application (like the one you have) it is used to replace a `missing` with some value. So essentially `coalesce(v, v2)` is the same as `ismissing(v) ? v2 : v`. The difference is that `coalesce` can accept any number of arguments, but two-argument form is the most common. – Bogumił Kamiński May 22 '19 at 06:13