C Programming Tutorial 74 - Algorithm to Check for Prime (Counting Prime Numbers Part 3)

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[@nadpro16]

If you run into undefined reference to `sqrt'add -lm to the end of the compile command. examp: gcc -lm

[@ColbusMaximus]

this solved my issue, but why?

[@JTCano42]

Thanks a lot, Caleb! Great tutorial.

[@ralph_d_youtuber8298]

I'll say that discrete math was really stresfull but it helps when doing some types of programming assumptions, cuz like u can mentally do the proof that yap its mathmatecally valid to do this. And ya its to loop to the sqrt of the number you want to find

[@ashokkumar845]

Hi caleb, in for loop at end of loop it is going to divide by 2 and based on this every number will be decide either prime or not, since we are not breaking loop. For example take 9, at end using for loop it will be divided by 2. So it should be prime as per coding. Bt still it is saying 9 is not prime... need your explination on this

[@diegocassinera]

Wow you continue working after you know the answer

[@kaelarellano4540]

how long does it take for you to create a program like this? it's just i' m a bit curious since you're like doing things like this like it was nothing

[@PunmasterSTP]

Prime number? More like "I wonder...how many more amazing videos you have in store for us!"

[@viviennezhang5326]

Why is isPrime an int when initialized? Shouldn't it be bool?

[@victorgodwin3027]

That's typecasting, it was typecasted to a bool

[@viviennezhang5326]

@@victorgodwin3027 I meant line 7, when "isPrime" was initialized and set to "true", the type was "int" instead of "bool". I don't think it's typecasting here.

[@victorgodwin3027]

The variable isPrime of type int was stored as a type bool, either ways the meaning is the same. (Typecasting)

[@pradyumnasrivastava2319]

It works but when I took an extremely large number it said its prime even though it had a 0 at the end

[@tiagodmota5840]

There are more details about the primitive data types... This error has to do with the interval of numbers that int supports, because it has just 4 bytes, and if you calculate 2^32 bits you'll see the largest number that can be stored in an int data type.

[@bibfocal]

The sqrt() method is incorrect. Attempt 121 as input for both versions, with and without sqrt(). With sqrt() it is stated as prime, without it is stated as not prime. 121 is not prime because it can be divided by more than two whole numbers, 1, 11, 121. A prime number can only be divided by itself and 1.

[@codebreakthrough]

😢 sorry if anything’s incorrect. It happens from time to time

[@bibfocal]

@@codebreakthrough It's all good, man. Keep up the great work your doing. I see you are teaching Python now. I do love the language, and it has so many more tools available than C. Great for getting some quick and dirty work done to keep the bosses happy.

[@christopherindrawan1316]

i changed the boolean name to notPrime and didnt assign a value to it so it will be false by default. and for the if statement, everytime the input % i is equal to 0, it will triggers the notPrime value to be true and print a statement that it is not a prime number. prime number will never triggers the notPrime value and it means the number is prime. This solves ur problem, i tried 121 and it results a not prime number. and everything else works fine.

[@tiagodmota5840]

This code of mine doesn't have that error: #include #include #include int main(void) { int numInput = 0; bool isPrime = true; printf("Enter a number and see if it is prime or not: "); scanf("%d", &numInput); if (numInput < 2) { isPrime = false; }else { for (int i = sqrt(numInput); i > 1 && isPrime == true; i--) { isPrime = ((numInput % i) != (0)) ? (true) : (false); } } printf("%d is prime? Answer: %d (0: No. 1: Yes.) ", numInput, isPrime); return 0; }

[@londonsystem6622]

The Best

[@iromakoootieno2713]

clever

[@joshuahao6991]

can some one copy the code thanks

[@z-root8955]

why don't you just check if the input % 2 == 0 then print is prime otherwise print not prime I might be wrong but I think you can do it just in few lines

[@ronpipes1988]

If you watch a couple of videos back he states that there is probably an easier way but that he's going to do it a more difficult way.