CHALLENGE : TRUNCATABLE PRIMES 💻⌨️🖱️ | Sololearn: Learn to code for FREE!
+ 41


Truncatable prime is a prime number that when you successively remove digits from one end of the prime, you are left with a new prime number. Examples: 997 is called a left-truncatable prime as the numbers 997, 97, and 7 are all prime. 7393 is a right-truncatable prime as the numbers 7393, 739, 73, and 7 formed by removing digits from its right are also prime. The task is to find the largest left-truncatable and right-truncatable primes less than 1.000.000 No zeroes are allowed in truncatable prime.

5th Jan 2018, 7:33 AM
LukArToDo - avatar
25 ответов
5th Jan 2018, 7:33 AM
LukArToDo - avatar
+ 21
//not some different logic😕 //mine initial logic failed ... worst ever logic thinked in the world
5th Jan 2018, 2:56 PM
Gaurav Agrawal
Gaurav Agrawal - avatar
+ 17
Thamk you @LukArToDo for your interesting challenge! Here is my try using C++:
5th Jan 2018, 1:17 PM
Vahid Shirbisheh
Vahid Shirbisheh - avatar
5th Jan 2018, 8:18 AM
David Akhihiero
David Akhihiero - avatar
6th Jan 2018, 10:28 AM
Danijel Ivanović
Danijel Ivanović - avatar
7th Jan 2018, 8:55 AM
Danijel Ivanović
Danijel Ivanović - avatar
+ 12
5th Jan 2018, 2:35 PM
Käzî Mrîdùl Høssäîn
Käzî Mrîdùl Høssäîn - avatar
+ 11
Gives largest left and right truncatable primes less than or equal to input number
5th Jan 2018, 8:22 PM
David Ashton
David Ashton - avatar
+ 9
@LukArTodo : I love this kind of challenges - congrats! got a simiiar solution like VcC leftTruncated Prime: a 2-digit must end with 3 or 7 and has to be a prime this set is p2 a 3-digit has to end with a p2 and be a prime this set is p3.... finally this is my prog
5th Jan 2018, 3:37 PM
Oma Falk
Oma Falk - avatar
+ 7
Why go for millions when you can do billions or trillions ? just enter number of digits (6 to answer the question but higher values work up to 12 digits or 1000000000000)
5th Jan 2018, 1:40 PM
VcC - avatar
+ 6
choose r or l for left- or righttruncated primes righttruncated: prefix must be a right truncated prime last digit must be 1,3,7,9 otherwise cant be a prime prefix *10 + last digit must be a prime build 2-digit prefix and derive valid 3-digit prefix and derive 4digit prefix... left truncated:: see other post
5th Jan 2018, 3:34 PM
Oma Falk
Oma Falk - avatar
+ 6
I've learned a lot solving this challenge . Especially how to reduce time cost. Thanks everyone.
5th Jan 2018, 5:25 PM
Viraj Singh
Viraj Singh - avatar
5th Jan 2018, 11:26 AM
István Ócsai
István Ócsai - avatar
5th Jan 2018, 4:21 PM
Aryeh - avatar
6th Jan 2018, 2:32 AM
Prabhanu Gunaweera
Prabhanu Gunaweera - avatar
7th Jan 2018, 5:12 AM
Jonathan Pizarra (JS Challenger)
Jonathan Pizarra (JS Challenger) - avatar
+ 5
Also for lefttruncatable Primes there can be done something: crossSum must never become devisible by 3 So eg the crossSum of a number is 2, next number may not be 4,7,9 So we can shorten runtime Tried it by a dynamic generator for demonstrazion
7th Jan 2018, 5:01 PM
Oma Falk
Oma Falk - avatar
+ 3
I found an interesting attribute of right-truncated primes: They may only have digits 1,3,7,9 so far... additionally: The sum of digit 7 and digit 1 must be one or two - Exactly one digit 7 and no digit 1 - One digit 7 and one digit 1 - Two digits 7 and no digit 1 - Two digits 1 and no digit 7 This comes from checking if 3 is a devisor via crosssum 9 and 3 are neutral 7 and 1 mod 3 is 2 so as soon as you have the third digit(1,7) the number is devisible by 3. With this knowledge I could shorten runtime a bit
7th Jan 2018, 12:55 PM
Oma Falk
Oma Falk - avatar
8th Jan 2018, 2:17 PM
Hiroki Masuda
Hiroki Masuda - avatar
+ 3
####UPDATED##### After all that 'Time Limit exceeds' finally it is done....
10th Jan 2018, 6:32 AM
Zoetic_Zeel - avatar