I am trying to write a program in C++ that, given a data file that includes random integers, identifies each integer as prime or not prime, and counts the integers. Here is what I have so far:
#include <iostream>
using namespace std;
int main()
{
int number; //number to be tested
int count = 0; //count of integers in file
cin >> number;
while (cin)
{
bool prime = true;
if (number <= 0)
{
prime = false;
}
else
{
for (int i=2; i<number; i++)
if (number % i == 0)
{
prime = false;
break;
}
}
if (prime = true)
cout << number << " is a prime number." << endl;
if (prime = false)
cout << number << " is not a prime number." << endl;
cin >> number;
count++;
}
cout << "The file contains " << count << " integers" << endl;
return 0;
}
The program compiles, but finds all values in a data set as not prime. Here is what I got as output from a data file:
24 is a prime number.
13 is a prime number.
18 is a prime number.
-25 is a prime number.
42 is a prime number.
0 is a prime number.
2 is a prime number.
The file contains 7 integers
Any suggestions? Am I using the bool values correctly? Thank you
You are using the assignment operator =
instead of the equality comparison operator ==
when checking if prime
is true or false. (You should also use braces for your for
loop.)
Heads up that you can check the factors until prime/2
(or sqrt(prime)
, see below) since nothing greater than half of a number can be a factor of it!
This line:
if (prime = true)
cout << number << " is a prime number." << endl;
is assigning "true" to variable "prime". As a expression, an assignment operation returns the value being assigned, so the expression inside the if evaluates as true and hence, prints that the number is prime.
You have to change = by == . An old trick to avoid these mistakes is to change the sentence as this:
if (true = prime)
cout << number << " is a prime number." << endl;
Then the compiler had warned you about the obvious error.
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