Write a program to calculate the sum of the factorials of the first n positive integers?

Question

I have this question for homework: Write a program to calculate the sum of the factorials of the first n positive integers. This is what I have done so far but I am not getting the output that I should get. Can someone tell me what have I done wrong.

System.out.print("Enter a number: ");
int num=input.nextInt();

for (int i=1; i<=num; i++) {
    num=num+i;          
}
System.out.print(num);

Answer 1

I suggest this, for example:

public static void main(String... args){
    int n=10; 
    BigInteger j=BigInteger.valueOf(1);
    BigInteger k=BigInteger.valueOf(0);
    for(int i=0;i<n;i++){
        j=j.multiply(BigInteger.valueOf(i+1));
        k=k.add(j); 
        //System.out.println(j + "\t" + k);
    }
    System.out.println("Sum of " + n + " first factorials equals: " + k);       
}

I hope it helps.

Answer 2

Try this:

int n, c, fact = 1;
do {
    System.out.println("Please enter a positive integer.");
    n = input.nextInt();
} while ( n < 1 );
    for ( c = 1 ; c <= n ; c++ ) {
       fact = fact*c;
    }
    System.out.println("Factorial of "+n+" is = "+fact);
}

The do-while loops asks the user for an input, and repeats, if the input is smaller then 1. Furthermore your line to calculate the factorial is not correct, I guess.

Note: This example does not prevent a "wrong" input like 12.5