素数C ++程序

Question

I'm trying to build a program that asks to user to input a positive integer and then the prime factors of this number are outputted. I'm giving the user three attempts to enter a valid input or the program ends. So any negative integers and non integers as well as other characters such as letters will give an error message. I'm nearly there but my output won't behave as I want. It treats decimal numbers as integers and negative numbers aren't returning the error.

#include <iostream>
#include <iomanip>
#include <cmath>
#include <stdio.h>

using namespace std;

int main()
{
    int num,i,flag,n;



    //executes loop if the input fails (e.g., no characters were read)
    while (cout << "Enter a number: " && !(cin >> num)) 
    {
        cin.clear(); //clear bad input flag
        cin.ignore(numeric_limits<streamsize>::max(), '\n'); //discard input
        cout << "Invalid input, please re-enter: \n";
    }

    i=2;
    n=num;
    cout<< "\nThe Prime factors of "<< num << " are:"<< endl;
    while(i<=num)
    {
        flag=0;
        while(n%i==0)
        { 
            n=n/i;
            flag++;
        }
        if(flag>0)
        {
            cout <<i<< endl;
        }
        ++i;
     }


     system("PAUSE");
     return 0;
}

Answer 1

You are not getting an error for entering a negative number as you are not checking for that in you input validation. You could add into your while condition to check for a negative output as:

while (cout << "Enter a number: " && (!(cin >> num) || num <= 0)) 

The reason you do not catch input of a decimal number is cin successfully converts and stores the input up to the decimal point and then stops, leaving the remainder of the input in the buffer. We can see that with:

#include <iostream>

int main() 
{
    int foo;
    double bar;
    std::cin >> foo;
    std::cin >> bar;
    std::cout << foo << std::endl;
    std::cout << bar;
}

Input:

5.82

Output:

5
0.82

Live Example

You could include a check in your while loop condition to see if there is more input waiting in the stream with

while (cout << "Enter a number: " && (!(cin >> num) || num <= 0 || cin.get() != '\n'))

As for looping only three times you can add a counter to the program and increment the counter each time the body of the loop executes. Once the counter gets to 3 then you would exit the program

int counter = 0;
while (cout << "Enter a number: " && (!(cin >> num) || num <= 0 || cin.get() != '\n'))
{
    if (counter == 3)
        return 0;  // exit
    cin.clear(); //clear bad input flag
    cin.ignore(numeric_limits<streamsize>::max(), '\n'); //discard input
    cout << "Invalid input, please re-enter: \n";
    counter++;
}

Answer 2

!(cin >> num) is only true when cin fails to insert the input character data into num, which is an int . Both negative integers (like -12) and decimal quantities (like 3.14) can be stuffed into a signed int . The decimal quantity works because float s can be coerced into int s by truncation.

To do what you want, you need to first capture your console input as a string, then attempt to parse out a positive integer. Take a look at How do I check if a C++ string is an int? and also boost::lexical_cast (if boost is an option).