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For example I have a vector 'H2DH2DH', is there a way to count the number of 'H' scalars that appear?

Shawn C.
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Richard Adams
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2 Answers2

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I assume you mean you have the character vector 'H2DH2DH' and want to count how many scalar 'H' characters are in there.

'H'='H2DH2DH will give you a Boolean vector indicating where the desired characters are. +/'H'='H2DH2DH' will sum that, and give you the count.

Play with it on TryAPL!

You can also define a generalised function which takes a scalar as left argument and a vector as right argument and counts the number of occurrences of the scalar in the vector.

Some APL systems (e.g. Dyalog, GNU, and ngn) will let you write CountIn←{+/⍺=⍵} where ⍺ stands for the left argument, and ⍵ for the right argument. Try it online! The remaining systems (e.g. APLX, APL+WIN, and APL2) will have you type:

∇ count←char CountIn text
  count←+/char=text
∇

Try it online!

Adám
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  • @RichardAdams: can you pls. accept the answer, so that the question is no longer shown as "open" (just tick the check-mark next to the reply). – MBaas Mar 26 '18 at 07:50
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Or, if you prefer, to count the occurrences using inner product:

'H'+.='H2DH2DH'

(Years ago, on other implementations, +/'H'='H2DH2DH' may have been slightly faster, perhaps this is still true)

Lobachevsky
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