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Prime numbers

4 New Member
I'm writing a program to calculate the number of prime numbers in a sequence. There's a variable called num that is given and then the program must calculate how many prime numbers there are from 2 (the smallest prime number) to num.

It should be something like that:

Enter a number greater than 1: 7 There are 4 prime numbers between 2 and 7.

This is because 2,3,5 and 7 are prime numbers, a total of 4 numbers.

The code below shows the variable num which is equal to 7, the main function calls isPrime (which says if a number is prime or not), then the way I thought about was using a for loop from 2 to num and check if each i is prime or not.

I don't know how to proceed from that. Please, could someone help?

#include <stdio.h>
#include <math.h>

int isPrime(int num) {
int i;
for (i=2; i<=sqrt(num); i++) {
if (num%i == 0) {
return 0;
}
}
return 1;
}


int main(){

int i, num, numofprimes;

num=7;

for (i=2;i<=num;i++ ){
if (isPrime(i)){

}

printf("%d", numofprimes);
}

return 0;
}
If someone could help I would be glad.
Nov 20 '21 #1
5 21008
dev7060
644 Recognized Expert Contributor
I'm writing a program to calculate the number of prime numbers in a sequence. There's a variable called num that is given and then the program must calculate how many prime numbers there are from 2 (the smallest prime number) to num.

It should be something like that:

Enter a number greater than 1: 7 There are 4 prime numbers between 2 and 7.

This is because 2,3,5 and 7 are prime numbers, a total of 4 numbers.

The code below shows the variable num which is equal to 7, the main function calls isPrime (which says if a number is prime or not), then the way I thought about was using a for loop from 2 to num and check if each i is prime or not.

I don't know how to proceed from that. Please, could someone help?

Expand|Select|Wrap|Line Numbers
  1. #include <stdio.h>
  2. #include <math.h>
  3.  
  4. int isPrime(int num) {
  5. int i;
  6. for (i=2; i<=sqrt(num); i++) {
  7. if (num%i == 0) {
  8. return 0;
  9. }
  10. }
  11. return 1;
  12. }
  13.  
  14.  
  15. int main(){
  16.  
  17. int i, num, numofprimes;
  18.  
  19. num=7;
  20.  
  21. for (i=2;i<=num;i++){
  22. if (isPrime(i)){
  23.  
  24. }
  25.  
  26. printf("%d", numofprimes);
  27. }
  28.  
  29. return 0;
  30. }
If someone could help I would be glad.
Expand|Select|Wrap|Line Numbers
  1. int main() {
  2.  
  3.   int i, num, numofprimes = 0;
  4.  
  5.   num = 7;
  6.  
  7.   for (i = 2; i <= num; i++) {
  8.     if (isPrime(i)) {
  9.       numofprimes++;
  10.     }
  11.   }
  12.   printf("%d", numofprimes);
  13.  
  14.   return 0;
  15. }
Nov 21 '21 #2
guiromero
4 New Member
Thanks very much for your response.

I'd like go further in this code.
I 've created a function biggestPrime to tell the biggest prime number between 2 and num. The only way I could think about doing this was creating an array with numofprimes as its dimension, storing each prime number in the array and then identifying the biggest number in that array. However, I have no idea how to do that.

#include <stdio.h>
#include <math.h>

int isPrime(int num) {
int i;
for (i=2; i<=sqrt(num); i++) {
if (num%i == 0) {
return 0;
}
}
return 1;
}


int biggestPrime(in t num){

int i, num, numofprimes;

num=7;


for (i=2;i<=num;i++ ){
if (isPrime(i)){
numofprimes++;
}
}

int Arr [numofprimes] =

return 0;
}


Does anyone know thow to do that?

Thank you.
Nov 21 '21 #3
dev7060
644 Recognized Expert Contributor
Thanks very much for your response.

I'd like go further in this code.
I 've created a function biggestPrime to tell the biggest prime number between 2 and num. The only way I could think about doing this was creating an array with numofprimes as its dimension, storing each prime number in the array and then identifying the biggest number in that array. However, I have no idea how to do that.

Expand|Select|Wrap|Line Numbers
  1. #include <stdio.h>
  2. #include <math.h>
  3.  
  4. int isPrime(int num) {
  5. int i;
  6. for (i=2; i<=sqrt(num); i++) {
  7. if (num%i == 0) {
  8. return 0;
  9. }
  10. }
  11. return 1;
  12. }
  13.  
  14.  
  15. int biggestPrime(int num){
  16.  
  17. int i, num, numofprimes;
  18.  
  19. num=7;
  20.  
  21.  
  22. for (i=2;i<=num;i++){
  23. if (isPrime(i)){
  24. numofprimes++;
  25. }
  26. }
  27.  
  28. int Arr [numofprimes] =
  29.  
  30. return 0;
  31. }

Does anyone know thow to do that?

Thank you.
Expand|Select|Wrap|Line Numbers
  1. int biggestPrime(int num) {
  2.   for (int i = num; i >= 2; i--)
  3.     if (isPrime(i))
  4.       return i;
  5. }
Nov 21 '21 #4
donbock
2,426 Recognized Expert Top Contributor
Look of Sieve of Erastothenes. That may be a more efficient algorithm for finding all the prime numbers in a given range.
Jan 22 '22 #5
james121
1 New Member
Also need this, got many points from comments. Thanks.
Feb 10 '22 #6

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