Computer Science Canada

math problems
Author:  TokenHerbz [ Mon Oct 10, 2005 12:48 am ]
Post subject:  math problems
I want to make a code that asks the user for 2 numbers, and inbetween these numbers it will print out numbers which are only able to divide by its self, and 1 evenly...

Ex, 1,2,3,5,7,11 ( i think )

code:

var num1, num2: int
var n1, n2: int

put "please enter 1 number"
get num1
put "please enter another number"
get num2

if num1 < num2 then
    n1 := num1
    n2 := num2
elsif num1 > num2 then
    n1 := num2
    n2 := num1
else
    put "Bah, your numbers are the same"
end if

for i: 1 .. n2
    if i >= n1 then
        if i / i = 1 then     %%% <--   heres where i need the math help?
            put i
        end if
    end if
end for


Help me please
Author:  zylum [ Mon Oct 10, 2005 1:14 am ]
Post subject: 
so you want to print the primes in a range of numbers?

code:
var a, b : int
put "what are the numbers"
get a, b
var primes : array 1 .. b of boolean
for i : 1 .. b
    primes (i) := true
end for

for i : 2 .. b
    for j : i * 2 .. b by i
        primes (j) := false
    end for
end for

for i : 1 .. b
    if primes (i) then
        put i, " ", primes (i)
    end if
end for
Author:  Drakain Zeil [ Mon Oct 10, 2005 9:58 am ]
Post subject: 
I ran through a for loop with i as a count.

if i was not divisable by the number,(use mod), then I would output.

You only need to run the loop from: a..(b/2)+1
And a decent range for prime check is (a/3)-1..(b/2)+1
Author:  GlobeTrotter [ Mon Oct 10, 2005 7:25 pm ]
Post subject: 
Drakain Zeil wrote:
I ran through a for loop with i as a count.

if i was not divisable by the number,(use mod), then I would output.

You only need to run the loop from: a..(b/2)+1
And a decent range for prime check is (a/3)-1..(b/2)+1



Actually to check for a given prime, you only need to check if the number is divisible by any prime between 0 and the root of the number.
Author:  jamonathin [ Mon Oct 10, 2005 7:38 pm ]
Post subject: 
Drakain Zeil wrote:

Actually to check for a given prime, you only need to check if the number is divisible by any prime between 0 and the root of the number.

But if we're looking for a prime number, how would be divide it by primes to find out if it's prime?
Author:  Cervantes [ Mon Oct 10, 2005 8:04 pm ]
Post subject: 
jamonathin wrote:

But if we're looking for a prime number, how would be divide it by primes to find out if it's prime?

Start from the bottom up. Smile

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