count
Counts the number of elements with a certain property in a container.
Syntax
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count(X, p)-
Xis a container -
pis a predicate
-
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count(X, x[, ε])-
Xis a container -
xis an object -
εis a positive number
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Description
If X is a container and p a predicate, then count(X, p) returns the number of elements x in X for which p(x) is true.
If X is a container and x an object, then count(X, x) returns the number of elements in X that are equal to x. count(X, x, ε) does the same, but uses the specified epsilon when comparing floating-point numbers. Both overloads compare mathematical values, so, for instance, the integer 5 is equal to the real number 5.0. However, only the latter overload, with an appropriate ε, is guaranteed to consider sin(π) and 0 to be equal.
Examples
ℙ ≔ compute(prime(n), n, 1, 1000000);
count(ℙ, p ↦ sum(digits(p)) = 10)
1121
count(ℙ, p ↦ (d ≔ digits(p); ∑(d) = ∏(d)))
23
count(ℙ, p ↦ (s ≔ string(p); s = reverse(s)))
781
count(digits(2^50), 5)
1
s ≔ ExampleData("Alice in Wonderland");
count(s, ChrIsPunctuation) / #s
0.0563018101844 (=1664/29555)