little question

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HelloMe 0 Junior Poster in Training

16 Years Ago

Hello everyone...

I have 2 questions and it would be great if you guys can help me on these.

I wrote a factorial program and it works already.
The problem is when it comes to higher outputs.

For example:
11 P 5 = 55440 // this works so far
but
13 P 5 = 154440 // but my output is 47917

Im using already long instead of integer.
Can someone help me out on this?

Second thing is, how do i avoid that someone will enter letters? :-)

Btw my code is:

#include <iostream>
using namespace std;

long factorial (long n)
{
  if (n > 1)
   return (n * factorial (n-1));
  else
   return (1);
}

int main ()
{

	cout<< " WELCOME TO THE 2 BASED FACTORIAL CALCULATOR " << endl << endl;

start:;

  long number;
  cout << "Please type a number: ";
  cin >> number;

  long number2;
  cout << endl << "Please type a 2nd number: ";
  cin >> number2;

  long number3;

       number3 = factorial(number) / factorial(number - number2);

	  {
  
	  if (number < number2)

	  {
		  
		  cout<< "Please let the first number be bigger than the second number." << endl << endl;
		  goto start;

	  }

	  if (number >= number2)

	  {

		  cout << endl << number << " P " << number2 << " = " << number3 << endl << endl;
	  }

  }

  goto start;

  return 0;
}

c++

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Latest Post 16 Years Ago Latest Post by MyrtleTurtle

Dani AI

Generated 10 Months Ago

A few concise, practical points that build on the thread: the wrong results for inputs above 12 come from integer overflow of the chosen type (this is what was pointing to). and raised input-checking options; the goto habit mentioned should be replaced with loops for clarity and maintainability.

Instead of computing two full factorials and dividing, compute the permutation directly as the product n(n-1)...*(n-r+1) and check for overflow at each multiplication. That keeps intermediate values smaller and makes overflow detection straightforward. Example pattern (C++):

// multiply with overflow check using unsigned long long and std::numeric_limits
// throw or switch to big-int if check fails

For safer input parsing: read a whole line and parse it, so non-numeric input is detected cleanly. Two modern options are std::stoll (throws on bad input) or std::from_chars (no exceptions, returns an error code); both let the program reject invalid text rather than letting cin fall into an error state. See and for details.

Final notes and references: integer width is platform-dependent (for example, long is 32-bit on Windows but often 64-bit on Unix); use fixed-width types like int64_t or unsigned long long when a specific range is required and consult std::numeric_limits before assuming capacity. For arbitrarily large factorials or permutations, switch to a big-integer type such as Boost.Multiprecision::cpp_int ().

All 13 Replies

MyrtleTurtle 52 Junior Poster

16 Years Ago

I am not sure about the first part but I can answer the second part. One way to ensure that the input is good (the user enters a number) is to do this:

void clearError(long int& num){ //pass number in by reference so it can be changed within the function
    cin.clear(); //clear the error state
    //ignore characters left in the buffer up to and including the new line character
    cin.ignore(numeric_limits<streamsize>::max(), '\n');      
    //allow them to reenter number
    cout<<"That is not a number. Try again: ";
    cin>>num;
}

int main ()
{

	cout<< " WELCOME TO THE 2 BASED FACTORIAL CALCULATOR " << endl << endl;

start:;

  long number;
  cout << "Please type a number: ";
  cin >> number;
  
  while(!cin.good()){
     clearError(number);    
  }

I like to put it in a function so it can be reused, but you don't have to do it that way.

Edited 16 Years Ago by MyrtleTurtle because: I usually think of a better way to code it, after posting

WaltP 2,905 Posting Sage w/ dash of thyme

16 Years Ago

1) Are you certain the program is handling 13! correctly?

2) Not easily. Do you understand how to parse strings? If so, you can try it. If not, you might want to wait a while to handle that.

3) Do not use goto . It is considered bad programming practice since a loop can easily be used instead and to better effect.

MyrtleTurtle 52 Junior Poster

16 Years Ago

Oh yeah...I was going to say that, too. Don't use goto.

And, you can't actually control what the user will input. But your program can handle it if they do not enter what you want them to, if you add the code I suggested. :)

WaltP 2,905 Posting Sage w/ dash of thyme

16 Years Ago

Oh yeah...I was going to say that, too. Don't use goto.
And, you can't actually control what the user will input. But your program can handle it if they do not enter what you want them to, if you add the code I suggested. :)

And he can fail the project for using code that was not taught and has no business using. The instructor will know that the code was given to him and he's getting others to write his code for him.

Please keep in mind who is asking and their level of knowledge before showing your prowess as a programmer and blowing their mind with higher level techniques. :icon_wink:

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HelloMe 0 Junior Poster in Training · Answer 1 · 2010-03-08T07:58:34+00:00

1) Are you certain the program is handling 13! correctly?
2) Not easily. Do you understand how to parse strings? If so, you can try it. If not, you might want to wait a while to handle that.
3) Do not use goto . It is considered bad programming practice since a loop can easily be used instead and to better effect.

Hello...

To number1:
What is the difference between 12 and 13? (except an increase of 1) :-)

2:
I know strings but parse strings sounds new to me.

3:
I know, sorry i will try to get rid of this habbit ;-)

HelloMe 0 Junior Poster in Training · Answer 2 · 2010-03-08T08:03:18+00:00

And he can fail the project for using code that was not taught and has no business using. The instructor will know that the code was given to him and he's getting others to write his code for him.
Please keep in mind who is asking and their level of knowledge before showing your prowess as a programmer and blowing their mind with higher level techniques. :icon_wink:

First thanks to both of you for your help.

Second i did the code by my own and it works somehow.
It´s not like i was asking for everything :-P
I just need little help by these 2 parts. I guess this will be okay by the teacher.

HelloMe 0 Junior Poster in Training · Answer 3 · 2010-03-08T08:09:57+00:00

I am not sure about the first part but I can answer the second part. One way to ensure that the input is good (the user enters a number) is to do this:

void clearError(long int& num){ //pass number in by reference so it can be changed within the function
    cin.clear(); //clear the error state
    //ignore characters left in the buffer up to and including the new line character
    cin.ignore(numeric_limits<streamsize>::max(), '\n');      
    //allow them to reenter number
    cout<<"That is not a number. Try again: ";
    cin>>num;
}

int main ()
{

	cout<< " WELCOME TO THE 2 BASED FACTORIAL CALCULATOR " << endl << endl;

start:;

  long number;
  cout << "Please type a number: ";
  cin >> number;
  
  while(!cin.good()){
     clearError(number);    
  }

I like to put it in a function so it can be reused, but you don't have to do it that way.

Do i have to include something in the header?
Because i add your codes into mine and i have errors on max (not a member of '`global namespace) and numeric_limits (undeclared identifier)

WaltP 2,905 Posting Sage w/ dash of thyme Team Colleague · Answer 4 · 2010-03-08T08:13:25+00:00

First thanks to both of you for your help.

Sure. Did you figure out what was wrong with 13? What was it?

Second i did the code by my own and it works somehow.
It´s not like i was asking for everything :-P
I just need little help by these 2 parts. I guess this will be okay by the teacher.

I was not suggesting you wanted someone to write your code. What I was saying is that by using someone else's code that you don't understand in your code, your instructor may believe someone else wrote it all. Keep your code at a level you understand.

HelloMe 0 Junior Poster in Training · Answer 5 · 2010-03-08T08:27:18+00:00

Sure. Did you figure out what was wrong with 13? What was it?

I was not suggesting you wanted someone to write your code. What I was saying is that by using someone else's code that you don't understand in your code, your instructor may believe someone else wrote it all. Keep your code at a level you understand.

It´s not 13 alone... every number above 13 gives the wrong answer.
Somehow i think it hast to do with my decleration that there is not enough memory. I tried to use an array instead of long but id didn´t work either. Okay maybe it will work but i had errors :-D
To 13 again i think to know its the Ascii for the enter or return key.

My teacher is cool because i show a lot of effort... and using higher level of coding makes me better in the long run i guess.

strmstn 27 Junior Poster · Answer 6 · 2010-03-08T08:59:48+00:00

There is a very easy way to parse strings using atoi().

WaltP 2,905 Posting Sage w/ dash of thyme Team Colleague · Answer 7 · 2010-03-08T09:07:00+00:00

Actually, 13! is larger than the maximum size your variable can hold. That's why it doesn't work. Nothing to do with ASCII 13.

HelloMe 0 Junior Poster in Training · Answer 8 · 2010-03-08T09:30:04+00:00

Okay Thanks to all of you...

I just pass it as it is can´t figure it out now.

Thanks again!!!

MyrtleTurtle 52 Junior Poster · Answer 9 · 2010-03-08T20:13:45+00:00

And he can fail the project for using code that was not taught and has no business using. The instructor will know that the code was given to him and he's getting others to write his code for him.
Please keep in mind who is asking and their level of knowledge before showing your prowess as a programmer and blowing their mind with higher level techniques. :icon_wink:

Oh....sorry! I was just trying to help.

I do see your point, but in my classes, I always look for other ways to do things, even if the instructor hasn't taught it yet. I try to make my programs the best they can be, regardless of what level has been taught in class. To me, it's best to learn everything you can instead of just the (very limited) amount that they teach in school.

But I don't want to get anyone in trouble, either. :(

little question

Dani AI

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