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# C++ question on prime numbers

By : cgaffga
Date : October 14 2020, 01:41 PM

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## Why is this check for prime numbers returning true on some non-prime numbers?

By : shiri
Date : March 29 2020, 07:55 AM
I wish this help you Possibly it because of the rounding of the sqrt(x) as result of this function call is floating point value. So it can be a little less than rounded to the closest integer.
In this case e.g. sqrt(25) could be rounded to 4 instead of 5.
code :
`````` if(!(x&1)) return 0;   //if x is even  return false
``````
``````if(!(x%2)) return 0;   //if x is even  return false
``````

## C Programming: Sum of Prime Numbers of a Given Range Printing Both The Prime Numbers and The Sum

By : kwietstorm20
Date : March 29 2020, 07:55 AM
I wish this helpful for you The solution is to print no plus sign in front of the first number, and to print a plus sign in front of all other numbers. To achieve this I am using a boolean with false/true values.
code :
``````#include <stdio.h>
#include <stdlib.h>
int is_prime(int m)
{
int i;
for(i=2; i<=m/2; i++)
{
if (m % i==0)
{
return 0;
}

}
return 1;
}
int main()
{
int num, min, max, sum = 0;
scanf("%d", &min);
scanf("%d", &max);
bool first = true;
for(num = min; num <= max; num++)
{
if(is_prime(num))
{
sum += num;
if(!first)
{
printf("+");
}
printf("%d", num);
first = false;
}

}
printf("=%d\n", sum);
return 0;
}
``````

## Question regarding R code to do with rounding prime numbers

By : user2584502
Date : March 29 2020, 07:55 AM
this one helps. I need to have a function where if a number entered is a prime number, it must round it up to the next prime number and if it is not to round it down to the previous prime number. , Using your prime checker,
code :
``````prime <- function(x) {
if (x == 2) {
print(3)
} else if (any(x %% 2:(x-1) == 0)) {
FALSE
} else {
TRUE
}
}
``````
``````new <- function(x){
if (isTRUE(prime(x))){
x = x+1
while(prime(x) == FALSE){
x = x+1
}
return(x)
} else {
while(prime(x) == FALSE){
x = x-1
}
return(x)
}
}
``````
``````> new(7)
[1] 11
> new(10)
[1] 7
``````

## Understand what is going on with my Prime Numbers Calculation and Prime Numbers JFrame

By : Seif Tama
Date : March 29 2020, 07:55 AM
I hope this helps you . set visible(true); is out side the constructor and is not a valid method name, it should be setVisible(true);, move up so that it appears just before the } that is currently above it... private JButton instantiateJButton(String, sInCaption, Color inColor) is not a valid method signature, you've added a , after String where it doesn't need to be, it should be private JButton instantiateJButton(String sInCaption, Color inColor) JOptionPane.showMessageDialog(null, "You pressed the "" sActionCommand + " button", this.getTitle(), JOptionPane.INFORMATION_MESSAGE); is missing a + and has too many ". I think it should be JOptionPane.showMessageDialog(null, "You pressed the " + sActionCommand + " button", this.getTitle(), JOptionPane.INFORMATION_MESSAGE); ProgrammingAssignment8 = new ProgrammingAssignment8("Instantiates with a Method"); is not a valid assignment, you're not actually assigning the value to anything...something like ProgrammingAssignment8 assignment = new ProgrammingAssignment8("Instantiates with a Method"); will fix it.
The following is not a valid comment...

## Check the prime numbers in the array and return the sum of the prime numbers

By : Alfred Quiambao
Date : October 04 2020, 08:00 AM
Hope that helps The first problem is checkPrime; it should take a single value and return true if that value is prime. The algorithm below is slightly optimized; we only need to test odd values (after checking two and its' multiples). Also, none of your methods depend on class state so I would remove the class fields and make the methods static.
code :
``````static boolean checkPrime(int v) {
if (v == 1 || v == 2) {
return true;
} else if (v % 2 == 0) {
return false;
}
int sq = (int) Math.sqrt(v);
for (int i = 3; i <= sq; i += 2) {
if (v % i == 0) {
return false;
}
}
return true;
}
``````
``````static int sum(int[] arr) {
int total = 0;
for (int i = 0; i < arr.length; i++) {
if (checkPrime(arr[i])) {
total += arr[i];
}
}
return (total);
}
``````
``````public static void main(String[] args) {
int[] numbers = { 1, 2, 3 };
System.out.println(Arrays.toString(numbers));
System.out.println("The sum of primes is " + sum(numbers));
}
``````