Printing Prime Numbers from 1 Through 100

Printing prime numbers from 1 through 100

Three ways:

1.

int main () 
{
    for (int i=2; i<100; i++) 
        for (int j=2; j*j<=i; j++)
        {
            if (i % j == 0) 
                break;
            else if (j+1 > sqrt(i)) {
                cout << i << " ";

            }

        }   

    return 0;
}

2.

int main () 
{
    for (int i=2; i<100; i++) 
    {
        bool prime=true;
        for (int j=2; j*j<=i; j++)
        {
            if (i % j == 0) 
            {
                prime=false;
                break;    
            }
        }   
        if(prime) cout << i << " ";
    }
    return 0;
}

3.

#include <vector>
int main()
{
    std::vector<int> primes;
    primes.push_back(2);
    for(int i=3; i < 100; i++)
    {
        bool prime=true;
        for(int j=0;j<primes.size() && primes[j]*primes[j] <= i;j++)
        {
            if(i % primes[j] == 0)
            {
                prime=false;
                break;
            }
        }
        if(prime) 
        {
            primes.push_back(i);
            cout << i << " ";
        }
    }

    return 0;
}

Edit: In the third example, we keep track of all of our previously calculated primes. If a number is divisible by a non-prime number, there is also some prime <= that divisor which it is also divisble by. This reduces computation by a factor of primes_in_range/total_range.

Prime number from 1 to 100 in C

You have a couple of problems

First, the for(j=2;j<=i;j++) loop should be for(j=2;j<i;j++), as if j is equal to i, j % i == 0 will always be true

Second, your flag variable is not doing what you might think it is doing. As HolyBlackCat suggested this is a good opportunity to learn how to use a debugger to see what is happening. You need to reset the flag variable, because once you find a single composite number, your flag will not reset so all following numbers will be flagged as composite. Add:

for(i=3;i<=100;i++)
{
    flag = 0;
    for(j=2;j<=i;j++)
    {

C - Print all prime numbers from 1 to 100 using arrays

When you do this:

        if(i % Primes[j] != 0 && Primes[j] !=0)
        {
            Primes[j]=i;
        } 

You're saying "if the current number is not divisible by the given prime number, replace the given prime number with the current number". This is not what you want.

You need to check if the current number is not divisible by any prime number. So you need to loop though the list of primes to make sure your number isn't divisible by any of them, and if so add the number to the end of the list. You can do that as follows:

int num_primes = 0;

for (i=2;i<101;i++)
{
    int is_prime = 1;
    for(j=0; j<num_primes && is_prime; j++)
    {
        if(i % Primes[j] == 0)
        {
            is_prime = 0;
        }
    }
    if (is_prime) {
        Primes[num_primes++] = i;
    }
}

In the above code, we use num_primes to count the number of primes we have so far, and is_prime to see if we found a prime that divides the current number. As you divide each number by a prime, if the remainder is 0 you know the number is not prime and set is_prime to 0. This also causes the inner loop to exit right away. Then if is_prime is still set at the end of the inner loop, you have a prime and you add it to the end of the list.

You also have an off-by-one error in the printing loop:

for(k=0;k<51;k++)

Since Primes has size 50, the largest valid index is 49. So change it to:

for(k=0;k<50;k++)

Print All Prime Numbers Between 1 and 100 using Java-script

This is how you can get the prime numbers without individual checks:

const isPrime = n => [...Array(n).keys()].slice(2).every(divisor => n % divisor !== 0)
const primeNumbers = [...Array(101).keys()].filter(isPrime)
console.log(primeNumbers)

Program that should print the first 100 prime numbers, only prints 0 and 1 instead

You forgot to add the curly brackets to the if:

            if((i%j)==0) {
                flag=1;
                break;
            }

So only the first line was under the condition, the break was executed each time.

Then you have a logical error, start the second loop with 2:

   for(int i=1;i<100;++i){
        for(int j=2;j<i;++j){

Now your code works. To understand why, add an additionally println like

public class Programma{
    public static void main(String args[]){
        int flag=0;
        for(int i=1;i<100;++i){
            for(int j=2;j<i;++j){
                if((i%j)==0){
                    System.out.println("# " + i + "/" + j);
                    flag=1;
                    break;
                }
            }
            if (flag==0)
                System.out.println(i + " ");
            flag = 0;
        }
    }
}

and try it with / without changed start values for the for loop.

Python displays all of the prime numbers from 1 through 100

try this

def is_prime(n):
    status = True
    if n < 2:
        status = False
    else:
        for i in range(2,n):
            if n % i == 0:
                status = False
    return status

for n in range(1,101):
    if is_prime(n):
        if n==97:
            print n
        else:
            print n,",",

output is

2 , 3 , 5 , 7 , 11 , 13 , 17 , 19 , 23 , 29 , 31 , 37 , 41 , 43 , 47 , 53 , 59 , 61 , 67 , 71 , 73 , 79 , 83 , 89 , 97

Prime numbers between 1 to 100 in C Programming Language

The condition i==j+1 will not be true for i==2. This can be fixed by a couple of changes to the inner loop:

#include <stdio.h>
int main(void)
{
 for (int i=2; i<100; i++)
 {
  for (int j=2; j<=i; j++)   // Changed upper bound
  {
    if (i == j)  // Changed condition and reversed order of if:s
      printf("%d\n",i);
    else if (i%j == 0)
      break;
  }
 }
}

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