I've just had one of my exams at the college
one of the questions were to program a function that counts how many prime numbers are between 2 and a given number from the user(including the number).
I wrote this algorithm which works for me but they still deducted all of the points of the question as if it was completely wrong.
Can please someone tell me what's wrong with the code?
Thanks a lot.
#include <iostream>
using namespace std;
void main()
{
int count = 0;
int num;
cout << "Please enter a natural number " << endl;
cin >> num;
for (int i = 2; i <= num; i++)
{
if ((i == 2 || i == 3 || i == 5||i == 7) || (i % 3 != 0 & i % 5 != 0 & i % 7 != 0 & i % 2 != 0))
count++;
}
cout << "There are " << count << " prime numbers beteween 2 amd " << num << endl;
}
Your code only checks if a number is a multiple of 2, 3, 5, or 7. That's missing a lot of multiples, meaning your code likely gives many false positives.
That condition is also quite difficult to read, so you probably would have lost points for that as well.
You would have been much better off replacing that condition in the loop with another for loop, and checking every odd number from 3 to the square root of the test number.
Related
This code I wrote is supposed to subtract one from the number inputed, or divide by 2 based on whether it is a multiple of 3 or not. However, every time I try to run the code, It outputs the numbers I want but then doesn't stop running. I am new to coding and not sure how to fix this.
#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;
int main() {
int n;
cout << "Enter a positive number: " << endl;
cin >> n;
if (n < 0) {
cout << "Invalid input." << endl;
}
while (n >= 1) {
if (n % 3 == 0) {
n = n-1;
cout << n << endl;
}
else if (n % 3 != 0) {
n = n / 2;
cout << n << endl;
}
}
return 0;
}
This is a screenshot of the output I get. Instead of giving me the opportunity to run the code again it just stays like this:
I may be misunderstanding what you're asking, however, traversing through the code you can identify that nothing is being done to make the code run again. You would need add what you have inside another while loop. This new while loop would be something like while (input != 0) then run everything you have. In your input statement you could say "Please enter a positive number or enter 0 to exit". This is just an example of an approach, but the premise is that you need something to keep this loop running.
So I need my help to correct my code, which is given to my programming class assignment. My activity is to print numbers divisible by 5 for the integers from 1 to 99. So I tried to code like this:
#include <iostream>
using namespace std;
int main () {
int num, min, max;
cout << "Enter Number: ";
cin >> num;
min = 1;
max = 99;
if (num > min || num < max) {
if (num % 5 == 0) {
cout << "Divisible.";}
else {
cout << "Not Divisible";}
}
else {
if (num % 5 == 0) {
cout << "Error Input.";}
else {
cout << "Error input.";}
}
return 0;
}
So when I compile and run, I test to enter a divisible number by 5 or not. When I put 0, it says "Error input," that's correct. However, when I put 100, it says "divisible" instead of "error input." What is the correct input of my code?
The input is an integer from 1 to 99, which means that it should be >= 1 AND <= 99.
So, simply change
if (num > min || num < max)
to
if (num >= min && num <= max)
You made a mistake in the first if statement. When you are giving the OR operator, it'll return true even if only one of the conditions are satisfied.
You should use the AND operator for your code to work as expected.
Moreover, you don't have to use the min and max variables also, it is making the program unnecessarily big (only 2 lines though, but still).
I'm trying to make this code work:
#include <iostream>
using namespace std;
int main()
{
int i;
do
{
cout << ("please enter a number between 1 and 10");
cin >> i;
} while(i > 10 && i < 1)
cout << "the square of the number you have entered is " << i*i;
}
Basically, the idea is that a user enters a number between 1 and 10. While the number is not between 1 and 10, it keeps asking the user to enter a number between the values. Then, when the number is between the values, it is squared and returned to the user.
I can't see why this isn't working
Any help is appreciated
You have:
while (i > 10 && i < 1)
You want:
while (i > 10 || i < 1)
while (i > 10 && i < 1)
Your condition is logically faulty; if reinterpreted, it says:
while i is greater than 10 AND i is less than 1
Judging from your code, the || operator should be used:
} while (i > 10 || i < 1);
As others mentioned, your condition is faulty.
a number can't obviously be under 1 AND above 10 at the same time, so the while loop exits immediately after the do statement.
#include <iostream>
using namespace std;
int main()
{
int i;
do
{
cout << ("please enter a number between 1 and 10");
cin >> i;
} while (i < 1 || i > 10)
cout << "the square of the number you have entered is " << i*i;
}
You should use an Or ||, that condition with && will never be true.
The loop condition is wrong and will never loop, as i cannot be less than 1 && greater than 10 at the same time. You should use the logical OR (||) operator instead. In addition, there must be a semicolon placed after the do-while statement. And you probably want and end of line placed after the prompt. Also, you don't want to start the bad habit of polluting the global namespace, even with the awesomeness of std. So:
#include <iostream>
int main()
{
int i;
do {
std::cout << "please enter a number between 1 and 10\n";
std::cin >> i;
} while (i > 10 || i < 1);
std::cout << "the square of the number you have entered is " << i*i << std::endl;
}
My program executes just fine, but I have questions about how my while loop is set up.
I know the Boolean values for true and false are 1 and 0, respectively, but I'm not understanding why my output displays the even and odd numbers backwards (to my understanding, it's backwards). Simply put, I don't understand why if ( number % 2 == 0 ) would display that a number is even and when I change it to 1, it displays odd. I'm reading this line as, if (even number equals to false). I don't know if that's where I'm going wrong. What's the correct way to read this line?
The way I have my code set up now displays the numbers correctly, I'm just not understanding why. Can anyone help?
// Program indefinitely displays an even
// or odd number until a negative number
// is entered.
#include <iostream>
using namespace std;
int main()
{
int number;
cout << "Please enter a number: ";
cin >> number;
while ( number >= 0 )
{
if ( number % 2 == 0 )
{
cout << number << " is even \n";
cout << "Please enter a number: ";
cin >> number;
}
else
{
cout << number << " is odd \n";
cout << "Please enter a number: ";
cin >> number;
}
}
cout << "Thank you. \n";
return 0;
}
number % 2 is 0 if number divides 2 (i.e. is even), 1 if number is positive and does not divide 2 (i.e. is odd), and -1 if number is negative and does not divide 2 (i.e. is odd). (The last point must be the case from C++11 onwards).
So, since 0 == 0 is true, number % 2 == 0 is true if, and only if, number is even.
So you've written if ( number % 2 == 0 ) to trap all even cases, and the else traps the odd cases.
Testing if ( number % 2 == 1 ) is only a test for positive oddness, but older C++ standards allow this to be true for negative number.
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I am trying to print out all prime factors of any given number by the user. So far I was able to get the code to check and print the prime numbers however, I am having issues with reprinting the number multiple times. Ex: Enter a prime number: 9 , Program prints: 3 * 3
#include <iostream>
#include <cmath>
#include <stack>
using namespace std;
int main()
{
int number;
stack<int> PrimeFactor;
cout << "Please enter an integer: ";
cin >> number;
cout << endl;
cout << "The prime factors of " << number << " in descending order are: ";
if((number % 2) == 0)
PrimeFactor.push(2);
if((number % 3) == 0)
PrimeFactor.push(3);
if((number % 5) == 0)
PrimeFactor.push(5);
if((number % 7) == 0)
PrimeFactor.push(7);
if((number % 11) == 0)
PrimeFactor.push(11);
for (int i = 2; i <= sqrt(number); i++)
{
if (number % i == 0 && i % 2 != 0 && i % 3 != 0 && i % 5 !=0 && i % 7 != 0 && i % 11 != 0)
{
PrimeFactor.push(i);
}
}
while(!PrimeFactor.empty())
{
cout << PrimeFactor.top() << " * ";
PrimeFactor.pop();
}
cout << endl;
return 0;
}
It returns "3 *" because 3 is the only number ever pushed onto the stack, which means the logic you are using is wrong.
Think like this: for the number the user enters, you have to find the first prime that can divide the number, and then do the same for the result of the division, and so on.
A very simple pseudocode:
number = X;
can 2 divide X?
yes -> push 2; Y = X/2;
can 2 divide Y?
no
can 3 divide Y?
no
can <next prime> divide Y?
yes -> push <next prime>; Z = Y/<next prime>
This means your loop counter "i" should be incremented to the next prime, and not by one.
To point your error perfectly:
when i = 3, the condition i % 3 != 0 fails, and 3 doesn't get pushed onto the stack.
You shouldn't test if a number is prime dividing it by itself (which is what you are doing in the if condition), because you will always get 0 and a result that says it is not prime because of that.
If you remove "i % 3 != 0" from the if condition, you will see that you get the result that you expect: 3 * 3.
To avoid this, I recommend writing a simple isPrime(int x) function that checkes if a number is prime.