What is the idiomatic way of counting the number of elements matching predicate?

Question

Is there a better way to count the number of elements for which the predicate function is true, other than this: ``` PredCount[lst_, pred_] := Length@Select[lst, pred]; ``` I'm asking because it seems inefficient to construct a subset of `lst` with `Select[]`, and because `Count[]` only works with patterns. In my use case, the function `PredCount` is called many times with a large `lst`.

Accepted Answer

You can often do this by turning your predicate into a pattern with a condition. For example:

```
Count[list, x_/;x>5]
```

would count the number of elements in list which are greater than 5.

What is the idiomatic way of counting the number of elements matching predicate?

Solution

Problem