### 1. [rant][benchmark] Sieve of Eratosthenes

- Posted by _tom (admin) Jan 08, 2019
- 549 views

The **Sieve of Eratosthenes** is "a classic algorithm for finding prime numbers."

I found a Python algorithm that uses "list comprehension" which results in a compact program. [ *Python without Fear* Brian Overland, 2018 Pearson Education ]

The program outputs the largest primes in the first 20_000 numbers.

# Python code n = 20000 comp_list = [j for i in range(2,n) for j in range(i*i, n, i )] prime_list = [i for i in range(2,n) if i not in comp_list] print( prime_list[-10:] )

The Euphoria version looks like this:

-- Euphoria Code integer n = 20000 sequence b_lst = repeat( 1, n ) sequence prime_lst = {} for prime = 2 to n do if b_lst[prime] then prime_lst = append(prime_lst, prime ) for i = prime * 2 to n by prime do b_lst[i] = 0 end for end if end for ? prime_lst[$-9..$]

My benchmark testing program is:

atom t puts(1, "phix\n" ) t = time() ? system_exec( "p sieve.ex" ) printf(1, "%g time\n\n", time() - t ) puts(1, "oE\n" ) t = time() ? system_exec( "eui sieve.ex" ) printf(1, "%g time\n\n", time() - t ) puts(1, "python\n" ) t = time() ? system_exec( "python sieve.py" ) printf(1, "%g time\n\n", time() - t ) puts(1, "phix compiled\n" ) t = time() ? system_exec( "./Sieve" ) printf(1, "%g time\n\n", time() - t ) puts(1, "oE compiled\n" ) t = time() ? system_exec( "./sieve" ) printf(1, "%g time\n\n", time() - t )

The results (netbook) are:

phix {19927,19937,19949,19961,19963,19973,19979,19991,19993,19997} 0 0.126013 time oE {19927,19937,19949,19961,19963,19973,19979,19991,19993,19997} 0 0.0479414 time python [19927, 19937, 19949, 19961, 19963, 19973, 19979, 19991, 19993, 19997] 0 33.448 time phix compiled {19927,19937,19949,19961,19963,19973,19979,19991,19993,19997} 0 0.00570715 time oE compiled {19927,19937,19949,19961,19963,19973,19979,19991,19993,19997} 0 0.0102074 time

In conclusion:

- I found a benchmark that is extremely unfair to Python
- I still do not understand the advantage of "list comprehension"
- as long as I use oE|Phix my netbook is
*fast enough*

_tom

### 2. Re: [rant][benchmark] Sieve of Eratosthenes

- Posted by petelomax Jan 08, 2019
- 546 views

Have fun: http://www.rosettacode.org/wiki/Sieve_of_Eratosthenes#Python

I recently got a bit of a slapping for not implementing wheels at http://www.rosettacode.org/wiki/Extensible_prime_generator#Phix - oh, wait, there's more on that page too .

The phix docs include this: http://phix.x10.mx/docs/html/phixvsfl.htm

To be fair, list comprehensions are probably more useful for lazy evaluation (not that I'm a fan of that either).

### 3. Re: [rant][benchmark] Sieve of Eratosthenes

- Posted by ChrisB (moderator) Jan 08, 2019
- 531 views

Hi

So you can tell I'm not a professional programmer.

How on earth does this calculate primes?

-- Euphoria Code integer n = 20000 sequence b_lst = repeat( 1, n ) sequence prime_lst = {} for prime = 2 to n do if b_lst[prime] then prime_lst = append(prime_lst, prime ) for i = prime * 2 to n by prime do b_lst[i] = 0 end for end if end for ? prime_lst[$-9..$]

Chris

### 4. Re: [rant][benchmark] Sieve of Eratosthenes

- Posted by petelomax Jan 08, 2019
- 535 views

How on earth does this calculate primes?

b_lst (the "sieve") is initially an array of 20,000 "true".

For every [prime] number it marks all multiples of that number (prime*2 by prime) as false.

Any b_lst[i] still true by the time we get to it means that i must be prime, since by then we've processed every number smaller that it.

Invented by some ancient Greek bloke a very very long time ago.

Pete

### 5. Re: [rant][benchmark] Sieve of Eratosthenes

- Posted by _tom (admin) Jan 09, 2019
- 511 views

A **factor** is "an integer value that exactly divides the test number `i`."

3 is (1)(3) -- a prime number 6 is (1)(2)(3) -- a composite number -- because `2` is an extra factor

A **prime** "number `i` has only one `1` and itself `i` as factors." A **composite** number "has a factor (or more) other than the numbers one `1` and `i`."

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 | | | | | | | | | primes

For example the value two `2` is prime. That means any multiple of two `2` is not prime.

┌───┬───┬───┬───┬─────┬─────┬─────┬─────┬─────┬──── composite values to ignore 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 | prime

We use False `0` as a marker.

the revised number line to search for primes ┌───┬───┬───┬───┬─────┬─────┬─────┬─────┬─────┬──── composite values to ignore 1 2 3 0 5 0 7 0 9 0 11 0 13 0 15 0 17 0 19 0 | prime

┌─────┬─────┬───────┬────────┬────────┬─────────── possible composite values to ignore 1 2 3 0 5 0 7 0 9 0 11 0 13 0 15 0 17 0 19 0 | prime

The revised number line to search for primes is now simpler.

1 2 3 0 5 0 7 0 0 0 11 0 13 0 0 0 17 0 19 0 | prime

As you locate prime numbers you also increasingly eliminate future values that you will not have to test.

# As a Euphoria program

We will search for prime numbers in the first `n` numbers.

integer n = 2000

We mark *all* values as prime to start with. We make a list of numbers from one to n and mark them as True. Thus we have a "boolean" list of values which I call `b_lst`.

The "trick" here is that the index value is a substitute for having a separate sequence of numbers ( 1,2,3,4,5,6,...,n ).

sequence b_lst = repeat(1, n ) --> { 1,1,1,1,1, ...

We save prime numbers as we locate them.

sequence prime_lst = {}

We know in advance that one `1` is a prime number (so we ignore this value). We will test *every* number from two to `n` to see if it is prime.

for prime = 2 to n do ... end for

We look at the `b_lst` to discover if a value is prime. If a value is marked as True `1` then we add it to our `prime_lst`. If a value is marked as False `0` then we ignore that value. The index value `prime` is our connection between the number and its T|F status.

if b_lst[prime] then ...

So if the value *is* prime we add it to our `prime_lst`

prime_lst = append(prime_lst, prime )

We must eliminate all multiples of this prime number from future searches. The next number *we know is not prime* is larger by the factor two `2`; so we start with `prime*2` and go to the end of the list of numbers to `n`; we mark known composite numbers as False `0`.

for i = prime * 2 to n by prime do b_lst[i] = 0 end for

- If prime is `2` this series looks like
`4, 6, 8, 10, 12, ...`; these are all numbers we can eliminate as not prime numbers. - If prime is `3` this series looks like
`6, 9, 12, 15, 18, ...`these are all numbers we can eliminate as not prime numbers.

The algorithm moves along; we find a prime number and then eliminate all future values that we can ignore as composite values. The more primes we find the more values are eliminated from our brute force search.

Finally, we display the last ten prime values

? prime_lst[$-9..$]

This output has to be the same as that for the python algorithm.

The *Python without Fear* book told me that "The sieve is frequently used as a benchmark to demonstrate how fast a computer program is." I called their bluff. Speed measured on a netbook; the slow atom chip magnifies speed differences between algorithms.

Language | Time (seconds) | Notice This |
---|---|---|

Phix | 0.13 | |

oE | 0.05 | |

Python | 33 | * |

Phix compiled | 0.006 | |

oE compiled | 0.01 |

The speed differences between Phix and oE are a cat and mouse game. When the differences are small you can even argue that they are not significant because this is not a rigorous test. However, Python is dramatically slower in this benchmark.

_tom