/sci/ - Science & Math » Thread #13840868

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How do you do this c++ program?

Anonymous Thu 11 Nov 08:29:23 2021 No.13840868 View Reply Original Report

Quoted By: >>13840874 >>13841045 >>13841442 >>13843412 >>13845094

A Palindromic prime is a prime number that is also a palindromic number. Write a
program that displays all the palindromic prime numbers between 100 and 999.
>pic unrelated

Anonymous

Anonymous Thu 11 Nov 2021 08:31:59 No.13840874 Report

Quoted By: >>13840883

>>13840868
cs 101 tier problem
do you know what a prime is?
do you know what a palindrome is?

Anonymous

Anonymous Thu 11 Nov 2021 08:37:19 No.13840883 Report

Quoted By:

>>13840874
i do know what a prime number is and what a palindrome is but i dont know how to write the code and my assignment is due in 20 minutes
pls help

Anonymous

Anonymous Thu 11 Nov 2021 08:46:53 No.13840901 Report

Quoted By: >>13840919 >>13840941 >>13843086

>make a list of all prime numbers between 100
and 999
>for each prime, check the first and last digits only
>if the same, print it
>if different, move on to the next prime on list
>boom, palindromic primes

Anonymous

Anonymous Thu 11 Nov 2021 08:53:49 No.13840919 Report

Quoted By: >>13840941

>>13840901
get first digit by dividing by 100
get last digit by modulus 10

Anonymous

Anonymous Thu 11 Nov 2021 09:04:30 No.13840941 Report

Quoted By:

>>13840901
>>13840919
Yes, that'll do

Anonymous

Anonymous Thu 11 Nov 2021 10:04:59 No.13841045 Report

Quoted By: >>13841442

>>13840868
>write function to check if n is prime
>write function to check if n is palindrome
>loop through all n 100 to 999 and check using both functions
There are more efficient solutions but this is the basic one. The annoying thing about this problem is one condition is really about integers (prime) and the other is really about strings/arrays (palindrome) and you have to find a way to do both.

Anonymous

Anonymous Thu 11 Nov 2021 12:46:10 No.13841442 Report

Quoted By: >>13843328

>>13840868
Are you retarded? Not doing your intro homework for you.

>>13841045
Wooow he has to call to_string(int) so hard.

Anonymous

Anonymous Thu 11 Nov 2021 23:39:26 No.13843086 Report

Quoted By: >>13843190 >>13843198 >>13843287

>>13840901
That is inefficient

Outer loop 101 .. 909 in step of 202 since we know it cannot end on an even number.
Inner loop 0 - 90 in step of 10
Sum the two numbers above and check if prime. You check 5 * 10 candidates, so this should be trivial.
Optimise by dropping 505 since we know no prime numbers end on 5.

Anonymous

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Anonymous Fri 12 Nov 2021 00:09:18 No.13843190 Report

Quoted By: >>13844232

>>13843086
premature optimization

Anonymous

Anonymous Fri 12 Nov 2021 00:11:57 No.13843198 Report

Quoted By: >>13844232

>>13843086
You sound like the kind of mouth breathing retard that progams in C++ by choice.

Anonymous

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Anonymous Fri 12 Nov 2021 00:43:51 No.13843287 Report

Quoted By: >>13843355 >>13844232

>>13843086
>filtering out the non-palindromes first rather than the non-primes
Good idea, schizophrenic implementation.

Anonymous

Anonymous Fri 12 Nov 2021 00:53:45 No.13843328 Report

Quoted By:

>>13841442
>Wooow he has to call to_string(int) so hard.
Not hard necessarily but figuring out the libraries and what's in them is part of the learning process. On the other hand, learning to use % and / in the from-scratch solution is also part of the learning process.

Anonymous

Anonymous Fri 12 Nov 2021 01:01:53 No.13843355 Report

Quoted By:

>>13843287
The other anon is a faggot but it's not a bad idea to filter out based on the digits. I am going to generalize.
The time complexity that way would be $O(N)$ where $N$ is the number you want to search palindrome primes up to.

On the other hand, a prime sieve requires $O(NlogNloglogN)$ time (actually there are a lot of bitwise optimizations for eratosthenes so I am not sure about the time) but also requires memory, which you can not always have if $N$ is big.

Anonymous

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Anonymous Fri 12 Nov 2021 01:19:09 No.13843412 Report

Quoted By: >>13843431 >>13843940 >>13844240

>>13840868
Like this. The numbers are 131,151,181,313,353,373,383,727,757,787.

Anonymous

Anonymous Fri 12 Nov 2021 01:27:42 No.13843431 Report

Quoted By: >>13845500

>>13843412
I am not sure if you already know that but since it makes me seethe when I see code that has that implementation of checking a prime number I am going to tell you anyways:
It can be proven that if a number $n$ is composite then it has at least 1 divisor that is not greater than $\sqrt{n}$ .
In that way you can change your for loop condition to i*i <= num and improve the time needed drastically (surely it is not needed for this problem but I'm just informing).

Anonymous

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Anonymous Fri 12 Nov 2021 05:32:17 No.13843940 Report

Quoted By: >>13845107 >>13845500

>>13843412
>int k=2
>k++

Anonymous

Anonymous Fri 12 Nov 2021 07:52:24 No.13844232 Report

Quoted By: >>13846090

>>13843190
No, it is simpler and solves the question.

>>13843198
Totally the opposite. I started in assembly programming for embedded platforms. We have plenty experience with people like you who try to shove in a 1 MB library on a 64 KB platform. The algorithm I propose is trivial to implement in assembly.

>>13843287
>schizophrenic implementation
Oh?

Anonymous

Anonymous Fri 12 Nov 2021 07:55:51 No.13844240 Report

Quoted By: >>13845500

>>13843412
>The numbers are 131,151,181,313,353,373,383,727,757,787
What happened to 919?

Anonymous

Anonymous Fri 12 Nov 2021 16:34:25 No.13845094 Report

Quoted By:

>>13840868
CS fags GTFO

Anonymous

Anonymous Fri 12 Nov 2021 16:39:13 No.13845107 Report

Quoted By: >>13845171

>>13843940
?

Anonymous

Anonymous Fri 12 Nov 2021 16:59:41 No.13845171 Report

Quoted By: >>13845500

>>13845107
Starting on 2 means you have not already eliminated even numbers. That should have been done in the outermost loop.
Condition [code:lit]k<num[/code:lit] is clearly sub optimal since you never have to go beyond the square root of num. It should be clear even to small children that you don't have to try to divide 919 by 918 to see if this is a prime number.
The step [code:lit]k++[/code:lit] is grossly sub optimal since every other number is an even number and you already tested for even numbers.

Nothing, absolutely nothing in this one line of code is acceptable. Whoever grunted out that code will never in his life write good code.

Anonymous

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Anonymous Fri 12 Nov 2021 18:37:57 No.13845500 Report

Quoted By: >>13846081 >>13846352

>>13843431
>>13843940
>>13844240
>>13845171
Here.

Anonymous

Anonymous Fri 12 Nov 2021 21:16:52 No.13846081 Report

Quoted By:

>>13845500
The code for this exact problem works (because i only gets odd values) but in general your "prime check" is wrong.
For example it would return that 4 is prime, but it is not. You will start at k=3 but you need to check if your number is 2 or divisible by 2 separately.

Anonymous

Anonymous Fri 12 Nov 2021 21:20:08 No.13846090 Report

Quoted By: >>13846336

>>13844232
>We have plenty experience with people like you who try to shove in a 1 MB library on a 64 KB platform.
You're a fucking retard, and I have plenty experience with "people" like you who write 1 MB of big-ridden code to "optimize" a trivial problem, which ends up being slower than my one-liner. Your entire mindset is profoundly inferior, and if you aren't LARPing, I assure you that anyone who's ever worked with you thinks you are dogshit.

Anonymous

Anonymous Fri 12 Nov 2021 22:37:03 No.13846336 Report

Quoted By: >>13846338

>>13846090
So by screaming loudly into your keyboard you really think you can shove a 1 MB library into a 64 KB platform? Got it. Your "one liner" will probably work on a simulator but never in production.

What is amusing is that people like you get angry about something but are never willing to understand that the filth you try to shovel into repository just has no chance of working, ever. And of course I don'w spew out 1 MB code, it still would not work. You are so wrong you cannot even begin to understand it and when you get the chance you are among the people that sent aircrafts into the ground.

You were put to the test and you failed. People like you should have been put to the sword before you hurt others with your filth.

Anonymous

Anonymous Fri 12 Nov 2021 22:38:12 No.13846338 Report

Quoted By:

>>13846336
>t. low-IQ NEET schizo showing off his "intelligence" by "optimizing" kindergarten exercises

Anonymous

Anonymous Fri 12 Nov 2021 22:41:36 No.13846352 Report

Quoted By:

>>13845500
Since this is C++ you can take advantage of the other for construction

for (int n : {1, 3, 7, 9})

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