Any conditions I've missed?

[tame quiver]

The purpose is to test for prime numbers. Are there any conditions that should be accounted for?

[thick perch]

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[tame quiver]

#include <iostream>
#include <cmath>
using namespace std;

int main()
{
int a,b;
cout << "Enter a natural number: ";
cin>> a;
b = sqrt(a);

if(a==1) {cout<< "Not Prime";}
else if(a==2 |\| a==3 |\| a==5 |\| a==7) {cout<< "Prime";}
else if(a%2==0 |\| a%3==0 |\| a%5==0 |\| a%7==0 |\| a%b == 0) {cout<< "Not Prime";}
else {cout<<"Prime";}

return 0;

}

[modern crown]

statistically speaking about everything

this is not the correct way to find a prime

[tame quiver]

where will it not work?

[potent pumice]

11

[tame quiver]

11 works

[potent pumice]

how about 143

[quiet shore]

143

[potent pumice]

@tame quiver

[tame quiver]

for 143 it says not prime

but i've realised my mistake tho

[potent pumice]

;compile ```cpp
#include <iostream>
#include <cmath>
using namespace std;

int main()
{
int a,b;
a=187;//cin>> a;
b = sqrt(a);

if(a==1) {cout<< "Not Prime";}
else if(a==2 || a==3 || a==5 || a==7) {cout<< "Prime";}
else if(a%2==0 || a%3==0 || a%5==0 || a%7==0 || a%b == 0) {cout<< "Not Prime";}
else {cout<<"Prime";}

return 0;

}```

[lament lantern]

Program Output

Prime

aj00#7487 | 52ms | c++ | x86-64 gcc 12.2 | godbolt.org

[tame quiver]

it doesn't work with 713

ok i think i need a complexer code

[potent pumice]

I figured out why 144 worked

*143

it's because sqrt rounded down

[quiet shore]

187

[tame quiver]

so int b always rounds down if b is decimal?

[potent pumice]

I am quite certain

[tame quiver]

i put the sqrt there because of numbers like 169

[potent pumice]

Yes but it's kinda stupid, isn't it?

[modern crown]

But say a number that is made by a multiplying 2 primes can be unique

But you need some fancier code for this

[tame quiver]

why? it deals with all the numbers that only have 3 factors: 1, itself, and its square root. And if b is a decimal then there's still no problem

[tame quiver]

yea i think this is the only problem

[potent pumice]

I kinda made a program to check if a number was unique and then i ran it for values under 1000 and there were 53 occurences where it didn't have the right answer;

[potent pumice]

This is the same as sayingif the number isn't a square it's prime

[tame quiver]

yea if that was the only condition i put in my code

[potent pumice]

Ok then

If the number is 2 3 5 7 or it's a square it's prime

[tame quiver]

else if(a%2==0 || a%3==0 || a%5==0 || a%7==0 || a%b == 0) {cout<< "Not Prime";}

[potent pumice]

I realized that

New attempt
if the number is 2 3 5 7 or alternatively if it doesn't have any of those as divisors and isn't perfect square, then its prime

[tame quiver]

yea, i've realised this generally works but its still wrong

[potent pumice]

GENERALLY WORKS?!

[tame quiver]

well at what point can you say generally

ok it doesn't matter if it works 95% of the time or 3% of the time
its still wrong, so how do i implement the condition?

[potent pumice]

Well what's the definition?

[tame quiver]

of primes?

[modern crown]

yeah

[tame quiver]

a natural number that doesn't have any divisors other than 1 and itself?

[modern crown]

yup :D

and are you checking every possible devisor? :D

[tame quiver]

no

[potent pumice]

small voice

[tame quiver]

it would be an infinite code then

[modern crown]

no ;)

whats the biggest a divisor could ever be?

[tame quiver]

itself

[modern crown]

bzt bzt

[tame quiver]

eh?

[potent pumice]

Keep your mind churning

[modern crown]

well not wrong, but for primes that is irrelevant

any number is always divisable by itself

[potent pumice]

Hint: loops exist

[tame quiver]

o

i haven't covered that yet

[potent pumice]

recursion ||probably not right||

[modern crown]

well honestly there is no way to check primes without looping of some kind

[potent pumice]

Well integers are finite sized right ...

[modern crown]

I know where this is going :D

[potent pumice]

It wouldn't work because you would almost certainly need to generate that list with code which wood use a loop

[modern crown]

@potent pumice

only works for uint16_t though

[potent pumice]

This doesn't work

[modern crown]

why not

promise it's correct

ah forgot the division by 0 and 1

need to remove those

[potent pumice]

and 4

and 6

and 8

and 10

and 12

[modern crown]

nah those need to be tested

[potent pumice]

The stupid part is that I am actually trying to do this

[modern crown]

tbh made this as a bit of a joke

[potent pumice]

And why would that stop me?

[modern crown]

this person means buisnuiss

[potent pumice]

;cpp << 0x7fffffff

[lament lantern]

Program Output

2147483647

aj00#7487 | 40ms | g101 | godbolt.org

[quiet shore]

Runtime error in iterm_3
On line 1 at position 1

log10(2147483647)
^
Failed to access variable log10

[potent pumice]

9.3 approx

[jolly moss]

it's generally easier to figure out if various numbers are not prime

like, except 2, any even number is not prime

except 5 iteself, any number divisible by 5 is not prime

this means that if the first digit (lowest order of magnitude) is 0, 2, 4, 5, 6, or 8 then the number is not prime

so you can rule out a lot of options just by looking at the first digit

or, said another way, only bother with the more expensive computation if the first digit is 1, 3, 7 or 9

[potent pumice]

Well i ended up running my program to generate a program that would work for every positivie integer and it is 1.6 gigs and vs Is really struggeling to open it

[random nimbus]

well

do you understand how prime numbers work? there are much more elegant solutions

[potent pumice]

I don't see how there could be anything more elegant than a predifined if statement

[random nimbus]

i meant something like a consteval sieve of primes approach

[potent pumice]

The problem isn't the speed of the program (A little bit) but the main problem is that thefile is way to large and vs crashes every time that try to open it and notepad does the same

[rigid forge]

268337161 is not a prime

[quiet shore]

268‚337‚161

[potent pumice]

And?

[thick perch]

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