math-stat.md

Mathematical and Aggregation Operators

• count(predicate?)
• reduce(accumulator, seed?)
• max(comparer?)
• min(comparer?)

count(predicate?)

• predicate: (value, index, sourceObservable)

const numbers = range(1, 7);
numbers.pipe(
	count(i => i % 2 == 1)
).subscribe(console.log);
// 4

interval(1000).pipe(
	takeUntil(
		fromEvent(document, 'click')
	)
).pipe(
	count()
)

TIME	0.5s	1s	1.5s	2s	2.5s	3s	3.5s	4s	4.5s	5s
click									x
intv		0		1		2		3
obs.									4

reduce(accumulator, seed?)

• accumulator: (acc, value, index?)

of(1, 2, 3, 4).pipe(
	reduce((acc, val) => acc + val)
).subscribe(console.log);
// 10

// number of clicks
// that happened in 5 seconds

const clicks = fromEvent(document, 'click').pipe(
	takeUntil(interval(5000))
);
const ones = clicks.pipe(mapTo(1));
const seed = 0;

const _count = ones.pipe(
	reduce(
		(acc, one) => acc + one,
		seed
	)
).subscribe(console.log);

max(comparer?)

• comparer: (x, y)

of(5, 4, 7, 2, 8).pipe(
	max()
).subscribe(console.log);
// 8

of(
	{age: 7, name: 'Foo'},
	{age: 5, name: 'Bar'},
	{age: 9, name: 'Beer'}
).pipe(
	max(
		(a, b) => a.age < b.age ? -1 : 1
	)
).subscribe(x => console.log(x.name));
// Beer

min(comparer?)

• comparer: (x, y)

of(5, 4, 7, 2, 8).pipe(
	min()
).subscribe(console.log);
// 2

of(
	{age: 7, name: 'Foo'},
	{age: 5, name: 'Bar'},
	{age: 9, name: 'Beer'}
).pipe(
	min(
		(a, b) => a.age < b.age ? -1 : 1
	)
).subscribe(x => console.log(x.name));
// Bar