diff --git a/project_euler/problem_058/__init__.py b/project_euler/problem_058/__init__.py new file mode 100644 index 0000000000000000000000000000000000000000..792d6005489ebee62cde02066f19c5521e620451 --- /dev/null +++ b/project_euler/problem_058/__init__.py @@ -0,0 +1 @@ +# diff --git a/project_euler/problem_058/sol1.py b/project_euler/problem_058/sol1.py new file mode 100644 index 0000000000000000000000000000000000000000..d3b15157fbbd0b07ced126d9929bbac93512daa8 --- /dev/null +++ b/project_euler/problem_058/sol1.py @@ -0,0 +1,86 @@ +""" +Project Euler Problem 58:https://projecteuler.net/problem=58 + + +Starting with 1 and spiralling anticlockwise in the following way, +a square spiral with side length 7 is formed. + +37 36 35 34 33 32 31 +38 17 16 15 14 13 30 +39 18 5 4 3 12 29 +40 19 6 1 2 11 28 +41 20 7 8 9 10 27 +42 21 22 23 24 25 26 +43 44 45 46 47 48 49 + +It is interesting to note that the odd squares lie along the bottom right +diagonal ,but what is more interesting is that 8 out of the 13 numbers +lying along both diagonals are prime; that is, a ratio of 8/13 ≈ 62%. + +If one complete new layer is wrapped around the spiral above, +a square spiral with side length 9 will be formed. +If this process is continued, +what is the side length of the square spiral for which +the ratio of primes along both diagonals first falls below 10%? + +Solution: We have to find an odd length side for which square falls below +10%. With every layer we add 4 elements are being added to the diagonals +,lets say we have a square spiral of odd length with side length j, +then if we move from j to j+2, we are adding j*j+j+1,j*j+2*(j+1),j*j+3*(j+1) +j*j+4*(j+1). Out of these 4 only the first three can become prime +because last one reduces to (j+2)*(j+2). +So we check individually each one of these before incrementing our +count of current primes. + +""" + + +def isprime(d: int) -> int: + """ + returns whether the given digit is prime or not + >>> isprime(1) + 0 + >>> isprime(17) + 1 + >>> isprime(10000) + 0 + """ + if d == 1: + return 0 + + i = 2 + while i * i <= d: + if d % i == 0: + return 0 + i = i + 1 + return 1 + + +def solution(ratio: float = 0.1) -> int: + """ + returns the side length of the square spiral of odd length greater + than 1 for which the ratio of primes along both diagonals + first falls below the given ratio. + >>> solution(.5) + 11 + >>> solution(.2) + 309 + >>> solution(.111) + 11317 + """ + + j = 3 + primes = 3 + + while primes / (2 * j - 1) >= ratio: + for i in range(j * j + j + 1, (j + 2) * (j + 2), j + 1): + primes = primes + isprime(i) + + j = j + 2 + return j + + +if __name__ == "__main__": + import doctest + + doctest.testmod()