C# HELP - Factors of a number.

Hey guys,

I need help regarding this small application I have to do as an assignment in school. I have to write a program that will show all factors of a number after the input. Ex. if the user enters 8 the program will output 4 and 2.

The only way (I thought of) to go around this would be to take the user input and start dividing from 1 and up, if the result is type int then it's a factor if double then it's not. I would use arrays to run the numbers in a loop and use arrays to save them too.

My question is what command in C# actually checks a stored variable or input for type, like int, double, char, string? I don't want the answer because I have to figure this out on my own, but a little help with this command would be appreciated. Thanks in advance.

I don't know about C#- but you won't need to check the type.

VB6 has a function called "int". C++ and C have one called "Floor", what it will do is strip off the fractional portion of a number. (if the number is negative one should use ceil()).

So what you would do is compare the result for each division with the Int  (or ceil or floor) of that same number, and if they are equal the number is a factor.

Also: I'd imagine it would be in the Framework "Math" assembly.

Thanks. The idea I wanted to use is possible but is used in a different way. I was looking for a way to return an array from a method and stumbled on http://www.java2s.com/Code/CSharp/Language-Basics/Returnanarray.htm site which has the solution for my program. It uses % (division) to detect if the number after decimal is a 0 or something else. If a 0 it's a factor if not then it's not.

This is the line: if( (num%i)==0 )

The last problem was to find a way to use only two methods instead of classes to do this task and I think I got it covered. Thanks a lot though.

% is modulus, I wasn't sure the operator for C# for modulus so I didn't mention it

Not really division but remainder after integer division.

Yes,you use modulus.
 If the language does not have a modulus operator you can create something that will do . You divide an integer by a smaller integer. Next  you take the answer and multiply it to get back to the larger number. If you do not get the original large number, then the modulus was not zero. Integer arithmetic is much faster. Check to see how your language handles integer division. Some will round up a number. No No.

But, before you do that, create an array of integers with the first 3500 prime numbers. Don't bother using non-prime numbers. Wares of time. What does you professor expect? You should factor the numbers down to primes. For example, if a number is divisible by 3 and 6, you need to reduce the 6 down to 2 and 3 so the factors would be 3, 3, 2 which are primes.
Does this help any?

Actually, any processor past the Pentium with MMX will do floating point at the same speed as Integer arithmetic. Of course a speed difference occurs between small integers and large Floating point numbers regardless, since an Integer, for example, is 2 bytes whereas a Double is 8 Bytes (the Integer only cycle on a 32-bit processor, the Double taking two)...

But since this is an academic exercise, Speed is a non-issue; it's more a test of algorithm development.

An interesting exercise is developing a Modulus routine that accepts and returns a floating point result- most Modulus operators in languages only deal in integral values. As such I created a routine for those instances where I need to determine the exact remainder even for floating point division:

Code: [Select]

Function Modulo(ByVal Num As Double, ByVal divisor As Double) As Double
    Dim Temp As Double, Initial As Double
    Dim Remainder As Double, oldval As Double
    Initial = divisor
    If divisor > Num Then Modulo = 0: Exit Function
    Do Until divisor >= Num
        oldval = divisor
        divisor = divisor + Initial
    Loop
    Modulo = Int(Abs(oldval - Num))


End Function


The assignment is not to find prime factors, as the OP's first post makes clear.

I have to write a program that will show all factors of a number after the input. Ex. if the user enters 8 the program will output 4 and 2.

I didn't know 4 was prime.

Here's a method, crude I know. It mimics what you might do if asked to do this with a pencil and paper, often a good starting point for finding an algorithm. Which you can later refine or discard.

Given an input number N, start at 2 (since 1 is not desired in the results) and multiply 2 by all numbers between itself and N-1, storing in an array all those where the product is N. Then do the same with 3,4,5, etc until N-1 (since N is not desired in the results) is reached.

Here it is in BASIC, I whipped this up in a hurry... I used longints in case somebody input a big number, although you could wait a long time for the answer... 9978 took 6 seconds on my 3 GHz Pentium 4. I'm still waiting for the answer to 98763, which started 6 minutes ago... But I have to go to work... Compiled in FreeBasic.

[Edit]

S:\Test\Basic\factors>factors4
number ?98763
started at 08:02:17
ended   at 08:11:26
 3  7  21  4703  14109  32921

There are various ways you could make this more efficient...

Code: [Select]

dim N as longint
dim i as longint
dim j as longint
dim product as longint
input "number ?", N
dim array(1 to N) as longint
for element=1 to N
        array(element)=0
next element
for i = 2 to N-1
        for j = 2 to N-1
                product=j*i
                if product=N then
                        array(j)=1
                        array(i)=1
                endif
        next j
next i
for element=1 to N
        if array(element)=1 then
                print element;" ";
        endif
next element
print

Code: [Select]

S:\Test\Basic\factors>factors3
number ?12
 2  3  4  6

S:\Test\Basic\factors>factors3
number ?72
 2  3  4  6  8  9  12  18  24  36

S:\Test\Basic\factors>factors3
number ?365
 5  73

S:\Test\Basic\factors>factors3
number ?9978
 2  3  6  1663  3326  4989

S:\Test\Basic\factors>
 

I was actually thinking of getting into freeBASIC a little more, merely so I could make console apps with basic... but all my existing codebase is in VB6, and freeBASIC is a nard-crusher when it comes to using COM components. And I have of way of creating console apps in VB6, which a little LINK command line work after compilation, the subsystem is CONSOLE! hooray!

Great for QuickBASIC like programs, where you really don't want to bother with all the hassle of project files. (or if you want the extra types)

hey... guyz.. i'm new here... I'm a comsci student...
Our prof told us to do program that determine the max & min value of the 5 input numbers.. I finish it but my program quite not good because sometimes it gives a wrong answer... can someone help me..

1. Start your own thread; do not hijack somebody elses
2. Post your code that you have done already.
3. You do not say what language you are using.

hey.. guyz.. can someone know where to download.. ms visual basic..

hey.. guyz.. can someone know where to download.. ms visual basic..

1. Start your own thread; do not hijack somebody elses
2. Post your code that you have done already.
3. You do not say what language you are using.

4. read the forum rules.

a VB.NET download for the express edition is available free from MS. earlier versions (1 through 6) are still under copyright by microsoft and thus any information as to their acquisition is against forum rules.

sorry sir.... I don't mean that..
by the way I'm making a program now... I want to ask...
how can i use the do while statement in case statement...
I'll show you what I want to do...
______________________________
the output
Press A for Action Press B for Comedy
Enter your choice:
______________________________
if the user type wrong choice the program will repeat... help me plz...
                              

1. Start your own thread; do not hijack somebody elses

do not hijack somebody elses

And BC,  you never do that?