writing a program that computes the factorial of a number and displays it.

[learner1234]

I'm new to c++ and want to write a program that computes the factorial of a number and displays it. A factorial of a number (written N!) is defined as N * (N-1) * (N-2) * (N-3) ... *1
In other words, 5! = 5 * 4 * 3 * 2 * 1 = 120 and 3! = 3 * 2 * 1 + 6.

Example of output 15 is 1.30767e+012

Can't get it to display error when user enters a number lower than 2 or higher 60.


// Program to calculate factorial of a number

#include <iostream>
#include <iomanip>
using std::cout;
using std::cin;
using std::endl;

int main(){

//Declare variables and constants
double factorial=1;
double number;

//Prompt the user for input
cout<<"Enter Number To Find Its Factorial (2-60): = ";
cin>>number;

while(number>=1){

factorial*=number;
number--;
}//end while
if(number < 0)
cout<<"Please Only enter Non-Negative Numbers -> "<<number<<endl;
else if(number > 60)
cout<<"Please Enter Number Between (2-60) -> "<<number<<endl;
else
cout<<"Factorial is: "<<factorial<<endl;


system("pause");
return 0;
}//end main

[keskiverto]

Please note, that this is not a homework site. We won't do your homework for you. The purpose of homework is that you learn by doing. However we are always willing to help solve problems you encountered, correct mistakes you made in your code and answer your questions.

We didn't see your attempts to solve this problem yourself and so we cannot correct mistakes you didn't made and answer questions you didn't ask. To get help you should do something yourself and get real problems with something. If your problem is "I don't understand a thing", then you should go back to basics and study again.

[learner1234]

I had forgotten to add my attempt.

[PCrumley48]

Hint:

You should always validate the input before using it anywhere else.

[learner1234]

Having a hard time figuring it out, new to c++.

[wildblue]

The instructions in the program are run sequentially - so right now, the lines to calculate the factorial precede those that check to make sure the number entered was valid. You need to swap those, so that the lines to check input come before the lines to calculate. If an invalid number is entered, does the assignment want you to allow the user to re-input a number or are you just supposed to display an error message, skip the factorial calculation and just exit the program?

[learner1234]

If it's outside of those bounds to simply show an error message, , then stop.

"You have entered an invalid number"

[learner1234]

This is how far I have got. Can't get it to show an error message when number is less than 2 or greater than 60.

Need use a loop to compute the factorial!

// Program to calculate factorial of a number

#include <iostream>
#include <iomanip>
using std::cout;
using std::cin;
using std::endl;

int main(){

//Declare variables and constants
double factorial=1;
double number;

//Prompt the user for input
cout<<"Enter Number To Find Its Factorial (2-60): = ";
cin>>number;

if(number < 0){
cout<<"Please Only enter Non-Negative Numbers -> "<<number<<endl;

}//end if

if(number<=2 && number>=60){
cout<<"You have entered an invalid number. Only 2-60 allowed -> "<<number<<endl;

}//end if

while(number>=2 && number<=60){
factorial*=number;
number--;
}//end while

cout<<"Factorial is: "<<factorial<<endl;

system("pause");
return 0;
}//end main

[learner1234]

Got it to show an error message when number is less than 2 or greater than 60 by changing (number<=2 && number>=60) to (number<=2 || number>=60).

Is there a way to get it to display the output as 5! = 5 * 4 * 3 * 2 * 1 = 120?

[wildblue]

If 2 and 60 are valid values, then you'd want < and >, not <= and >= on the error check.

Right now your output statement runs whether they've input a valid # or not. I might use an if/else structure.

if less than 0
//-> error message about negative numbers
else if less than 2 or greater than 60
//-> error message about out of range
else // number is within valid range
//->loop through to calculate factorial and output result 

You could work the descending output into the loop. Your loop right now just goes down to 2, so if you want to have 1 display as the last number displayed in the multiplied numbers, something would need to be tweaked. I might use a for loop like this for (int i = number; i >= 1; i--) and use the value of i to output the current number being multiplied.

[learner1234]

I changed <= and >= to < and > on the error check and output statement still runs whether my input is valid # or not.

// Program to calculate factorial of a number

#include <iostream>
#include <iomanip>
using std::cout;
using std::cin;
using std::endl;

int main(){

//Declare variables and constants
double factorial=1;
double number;

//Prompt the user for input
cout<<"Enter a number between 2-60: = ";
cin>>number;

if(number < 0){
cout<<"Please Only enter Non-Negative Numbers -> "<<number<<endl;

}//end if

else if(number<2 || number>60){
cout<<"Invalid number. Only numbers 2-60 allowed -> "<<number<<endl;

}//end if

else if(number>2 || number<60){
factorial*=number;
number--;
}//end while

cout<<"Factorial is: "<<factorial<<endl;

system("pause");
return 0;
}//end main

[wildblue]

I changed <= and >= to < and > on the error check and output statement still runs whether my input is valid # or not.

That was just to fix your error checking condition.

Your output statement is outside your if/else structure - it runs regardless of what the number was.

You still need the loop on the factorial calculation - the while got lost when the code moved into the else if section.

[learner1234]

Not sure how to loop on the factorial calculation.

[jemeripol]

hahaha. LOL .. Use google my friend. There are already a lot of factorial programs done through C++. :)