# Find the nth Fibohexaprime

The challenge this time is to find the nth Fibohexaprime. The definition of a Fibohexaprime is as following:

We first observe a list with Fibonacci numbers:

N  | Fibonacci number
1  | 1
2  | 1
3  | 2
4  | 3
5  | 5
6  | 8
7  | 13
8  | 21
9  | 34
10 | 55
11 | 89
12 | 144
13 | 233
14 | 377
15 | 610
16 | 987
17 | 1597

After that, we convert the numbers to hexadecimal:

N  | Fib  | Hex
1  | 1    | 1
2  | 1    | 1
3  | 2    | 2
4  | 3    | 3
5  | 5    | 5
6  | 8    | 8
7  | 13   | D
8  | 21   | 15
9  | 34   | 22
10 | 55   | 37
11 | 89   | 59
12 | 144  | 90
13 | 233  | E9
14 | 377  | 179
15 | 610  | 262
16 | 987  | 3DB
17 | 1597 | 63D

From the hexadecimal numbers, we filter out the letters. All we are left with are numbers. We need to check if these numbers are prime:

hex |  filtered |  is prime? |  N =
1   >  1        >  false
1   >  1        >  false
2   >  2        >  true         1
3   >  3        >  true         2
5   >  5        >  true         3
8   >  8        >  false
D   >  0        >  false
15  >  15       >  false
22  >  22       >  false
37  >  37       >  true         4
59  >  59       >  true         5
90  >  90       >  false
E9  >  9        >  false
179 >  179      >  true         6
262 >  262      >  false
3DB >  3        >  true         7
63D >  63       >  false

If the filtered number is a prime, we call this a Fibohexaprime. You can see that for N = 7, the related fibonacci number is 987.

The task is simple, when given an input using STDIN or an acceptable alternative, write a program or a function which outputs the nth Fibohexaprime using STDOUT or an acceptable alternative.

Test cases

Input - Output
1     - 2
2     - 3
3     - 5
4     - 55
5     - 89
6     - 377
7     - 987
8     - 28657
9     - 75025
10    - 121393
11    - 317811
12    - 5702887
13    - 9227465
14    - 39088169
15    - 102334155
16    - 32951280099
17    - 4052739537881
18    - 806515533049393
19    - 7540113804746346429

The rules:

• Given an integer between 1 and 19 (the values above 20 exceed the max value for a 64-bit signed integer), output the corresponding value.
• You may write a function or a program.
• This is , so the submission with the least amount of bytes wins!
• The way it's worded, it sounds like functions must also read from STDIN and write to STDOUT. Is that correct? Typically we allow functions to accept arguments and return values as convenient. – Alex A. Dec 12 '15 at 21:50
• @AlexA. Both are acceptable alternatives. Reading from STDIN and using STDOUT is not mandatory. – Adnan Dec 12 '15 at 21:56

# Pyth, 27 bytes

Leu,eGsGbU2ye.fq1lPs-.HyZGQ

Demonstration

y computes the nth Fibonacci number. A .f loop finds the fibohexaprime according to the input.

## MATL, 28 bytes

This uses MATL version 1.0.0, which was published in Esolangs on December 12, earlier than this challenge.

> 10
121393

### Explanation

The code is similar to that in Martin Büttner's answer.

1           % number literal
H           % paste from clipboard H. Initial contents: 2
i:          % vector of equally spaced values from 1 to input value
"           % for
`         % do...while
t       % duplicate
b       % bubble up element in stack
t       % duplicate
16YA    % convert integer to string representation in base 16
t       % duplicate
58      % number literal: first ASCII code after '9'
<       % is less than? (element-wise)
)       % reference () indexing with logical index from previous comparison
Yt      % convert string to number
Zp      % true for prime numbers
~       % logical 'not'
]         % end
]           % end
1$% input specification for final implicit display function • The world's first MATL answer! Good job, Luis! – beaker Dec 12 '15 at 17:12 • Hurray for MATL! Welcome to the world of code golfing! – rayryeng Dec 13 '15 at 5:37 ## CJam, 28 bytes TXri{{_@+_Gb{A<},Abmp!}g}*p; ### Explanation TX e# Push 0 and 1 to initialise Fibonacci computation. ri e# Read input and convert to integer N. { e# Run this block N times... { e# While the condition on top of the stack is truthy... _@+ e# Compute next Fibonacci number (dropping the second-to-last one). _Gb e# Duplicate and convert to base 16. {A<}, e# Keep only digits less than 10. Ab e# Convert from base 10. mp! e# Check that it's not a prime. }g }* p; e# Print the last number we found and discard the one before. # Perl 6, 62 bytes My first pass of just get it to work was: {(grep *[1].is-prime,map {$_,+[~] .base(16)~~m:g/\d/},(1,1,*+*...*))[$_-1;0]} # 77 By combining the grep and the map, I can remove 10 bytes {(map {$_ if is-prime [~] .base(16)~~m:g/\d/},(1,1,*+*...*))[$_-1]} # 67 If I use grep instead of map, I save 5 more bytes: {(grep {is-prime [~] .base(16)~~m:g/\d/},(1,1,*+*...*))[$_-1]} # 62

usage:

# give it a name
my &code = {...}

say code \$_ for 1..^20;

2
3
5
55
89
377
987
28657
75025
121393
317811
5702887
9227465
39088169
102334155
32951280099
4052739537881
806515533049393
7540113804746346429

# Mathematica 111 bytes

There may still be room for additional golfing.

t=Table[Fibonacci@k,{k,1600}];f@n_:=PrimeQ@FromDigits[Select[n~IntegerDigits~16,#<10&]];
g@k_:=Select[t,f][[k]]

g[7]

987

g[19]

7540113804746346429

# Julia, 123 bytes

n->(a=[];i=1;while endof(a)<n b=([1 1;1 0]^i)[1];(s=filter(isdigit,hex(b)))>""&&isprime(parse(s))&&push!(a,b);i+=1end;a[n])

This is an anonymous function that accepts an integer and returns an integer. To call it, give it a name, e.g. f=n->....

Ungolfed:

function f(n::Integer)
# Initialize an array and an index
a = []
i = 1

# Loop while we've generated fewer than n fibohexaprimes
while endof(a) < n
# Get the ith Fibonacci number
b = ([1 1; 1 0]^i)[1]

# Filter the hexadecimal representation to digits only
s = filter(isdigit, hex(b))

# If there are digits to parse, parse them into an
# integer, check primality, and push the Fibonacci
# number if prime
s > "" && isprime(parse(s)) && push!(a, b)

# Next
i += 1
end

# Return the last generated
return a[n]
end

# GAP, 204 Bytes

This answer is pretty unremarkable, except that GAP is cool enough to be able to find the next couple fibohexaprimes (and cooler still, it finds these in milliseconds with the given code).

gap>f(20);
31940434634990099905
gap> f(21);
12776523572924732586037033894655031898659556447352249
gap> f(22);
971183874599339129547649988289594072811608739584170445
gap> f(23);
1324695516964754142521850507284930515811378128425638237225
gap> f(24);
187341518601536966291015050946540312701895836604078191803255601777

Note that f(24) is between 2^216 and 2^217.

Here is the code:

f:=function(n)local c,i,x;c:=1;i:=0;while c<=n do x:=HexStringInt(Fibonacci(i));RemoveCharacters(x,"ABCDEFGHIJKLMNOPQRSTUVWXYZ");x:=Int(x);if IsPrime(x) then c:=c+1;fi;i:=i+1;od;Print(Fibonacci(i-1));end;

There's probably still some golfing that could be done. I think the implementation is pretty straightforward.

Ungolfed:

f:=function(n)
local counter,i,x;
counter:=1;i:=0;
while counter<=n do
x:=HexStringInt(Fibonacci(i));
RemoveCharacters(x,"ABCDEFGHIJKLMNOPQRSTUVWXYZ");
x:=Int(x);
if IsPrime(x) then
counter:=counter+1;
fi;
i:=i+1;
od;
Print(Fibonacci(i-1));
end;

## C, 186 183 bytes

#include<stddef.h>
size_t a,b,c,d,m,x;size_t F(n){a=0,b=1;while(n){x=b;b+=a;a=x;c=0,m=1;while(x)d=x%16,m*=d<10?c+=m*d,10:1,x/=16;d=c>1;x=2;while(x<c)if(c%x++==0)d=0;d&&--n;}return a;}

The primality test is very inefficient, so the computation will struggle a bit for n > 16 and become painfully long for n = 19. Nevertheless it works and gives the expected results.

The code assumes that size_t is a 64bit type, which is true for both 64bit Linux and Windows.

Bonus: unfortunately we are required to use 64bit types, which lead to an overhead of 33 bytes. The following version works for n <= 15 using int and is 150 byte long:

a,b,c,d,m,x;F(n){a=0,b=1;while(n){x=b;b+=a;a=x;c=0,m=1;while(x)d=x%16,m*=d<10?c+=m*d,10:1,x/=16;d=c>1;x=2;while(x<c)if(c%x++==0)d=0;d&&--n;}return a;}

Test main:

#include <stdio.h>

int main() {
printf("Input - Output\n");
for (int i = 1; i < 20; ++i) {
printf("%2d    - %ld\n", i, F(i));
}
}
• Could you save a bit by using size_t and dropping the include? It's implementation-specific, but seems to be 64-bit in both 64-bit Linux and Windows gcc (and since when did we care about portability in codegolf?). (side note: %ld is not 64-bit in 64-bit Windows; needs %lld) – Bob Dec 14 '15 at 22:36
• @Bob I thought about it, but size_t is not a builtin, it's defined in stddef.h (which in turn is directly or indirectly included by virtually any other header). One way or the other, I need an #include. I can still save 2 bytes by using size_t instead of uint64_t, though :) – Stefano Sanfilippo Dec 15 '15 at 0:25
• Also thanks for the lld bit, I didn't get the chance to test it on Windows (but portability doesn't matter, right?) – Stefano Sanfilippo Dec 15 '15 at 0:31
• Hm, it must've come from stdio.h while I was testing. In any case - you could still save a couple by including math.h instead of stddef.h. – Bob Dec 15 '15 at 0:46
• math.h doesn't do the trick for me (GCC 4.9 with GNU libc) – Stefano Sanfilippo Dec 15 '15 at 9:47

## Python 2, 127 bytes

N=input();a,b=0,1
while N:a,b=b,a+b;t=int(''.join(c for c in hex(b)if ord(c)<65));N-=(t>1)*all(t%x for x in range(2,t))
print b

The algorithm could be a lot more efficient. In particular, the primality check (t>1)*all(t%x for x in range(2,t)) checks potential factors all the way up to t-1, when really it would only have to check up to the floor of the square root. Since range stores an entire list in memory in Python 2 , this leads to a MemoryError at N=17 (on my machine using default settings).

# Ruby, 160 bytes

->i{t,o,k=[],[],0;f=->n{t[n]||=n<3?1:f[n-2]+f[n-1]};(r=('%x'%f[k]).scan(/\d/).join.to_i;(r>1&&(r==2||(2...r).none?{|j|r%j==0}))&&o<<r;k+=1)while !o[i-1];t[k-1]}

Ungolfed:

-> i {
t, o, k = [], [], 0
f = -> n {
t[n] ||= n < 3 ? 1 : f[n-2] + f[n-1]
}
while !o[i-1] do
r=('%x'%f[k]).scan(/\d/).join.to_i
o << r if (r > 1 && (r == 2 || (2...r).none?{|j| r%j == 0 }))
k+=1
end
t[k-1]
}

Usage:

# Assign the anonymous function to a variable
m = ->i{t,o,k=[],[],0;f=->n{t[n]||=n<3?1:f[n-2]+f[n-1]};(r=('%x'%f[k]).scan(/\d/).join.to_i;(r>1&&(r==2||(2...r).none?{|j|r%j==0}))&&o<<r;k+=1)while !o[i-1];t[k-1]}

m[2]
=> 3
m[19]
=> 7540113804746346429

# R, 164 bytes

g=function(n){f=function(m)ifelse(m<3,1,f(m-1)+f(m-2));p=0;while(n){p=p+1;x=gsub("\\D","",sprintf("%x",f(p)));x[x==""]=1;y=1:x;if(sum(!tail(y,1)%%y)==2)n=n-1};f(p)}

Indented, with new lines:

g=function(n){
f = function(m)ifelse(m<3,1,f(m-1)+f(m-2)) #Fibonacci function
p = 0
while(n){
p = p+1
x = gsub("\\D","",sprintf("%x",f(p))) #To Hex, and get rid of non-digits
x[x==""] = 1 #If x is empty string
y = 1:x #Converts to integer(!) and save the 1-to-x sequence to a variable
if(sum(!tail(y,1)%%y)==2) n = n-1 #If prime, decrements counter
}
f(p)
}

Examples:

> g(1)
[1] 2
> g(5)
[1] 89
> g(10)
[1] 121393
> g(12)
[1] 5702887