Counting the elements of a matrix satisfying a property: count

### 6.45.12 Counting the elements of a matrix satisfying a property: count

The count applies a function to the elements of a matrix or list and adds the result. Hence, if the function is a boolean function, then count will count the number of elements satifying the property that the function tests for.

- count takes two arguments:
  - f, a real-valued function.
  - A, a matrix or a list.
- count(f,A) returns the sum of the function f applied to the elements of A.

Examples.

- Input:
  count(x->x,[[2,12],[45,3],[7,78]])
  Output:

  147

  Indeed: 2+12+45+3+7+78=147.
- Input:
  count(x->x<10,[[2,12],[45,3],[7,78]])
  Output:

  3