Why isn't the value of i (declared in the for loop) updating in the if condition (fmod(num, i)) with each iteration?

Question

The function is supposed to return 1 if the argument passed to it is a prime number, otherwise it should return 0.

int PrimeTest(int num)
{
if (num == 1)
cout << "One is neither a prime nor a composite number." << endl;
else if (num == 2)
return 1;
else if (num > 2)
{
for (int i = 2; i <= ceil(pow(num, 0.5)); i++)
{
if (fmod(num, i) == 0)

// num is a composite number.
return 0;
else if (i == ceil(pow(i, 0.5)))

// num is a prime number, indeed.
return 1;
}
}
}

Answer 1

1. if number is 1 you are not returning any value so an error can crawl in your program. return 2
2. i <= ceil(pow(num, 0.5)) is not good.better
i<=floor(pow(num, 0.5)) or even better
i<=sqrt(num)
3. use {} after for loop as you have two statements
4. no requirement for that last condition.
int PrimeTest(int num)
{
if (num == 1)
{ cout << "One is neither a prime nor a composite number." << endl; return 2;}
else if (num == 2)
return 1;
else if (num > 2)
{
for (int i = 2; i<=sqrt(num); i++)
{
{
if (fmod(num, i) == 0)

// num is a composite number.
return 0;
}
return 1;
}
}
}

Answer 2

How do I update i in my if else block?

Answer 3

The for loop already have the update as a third parameter of its syntax.

for(initial_value ; condition ; update)
{
statements ;
}

In your case the update of i is incrementing it by one on each iteration (i++).

I wish I understood your question.

Answer 4

If you run the function, the i in the if else block will keep the value of 2, it won't update along with the loop

Answer 5

Thanks Megatron, now the function is working as intended!