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How to do an "if" block in Sage?

asked 2014-07-09 16:36:24 -0600

updated 2014-07-09 16:58:01 -0600

calc314 gravatar image

I am trying to do an if block in sage. For example, I have code:

for n in range(7):
    if gcd(n,3)==3:
        n=n/3;
        n=n+1;
    print(n)

This "should" only add one when n is divisible by 3, but seems to do this all the time. Any ideas?

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Specifically, I am trying to sum up al multinomial coefficients that have two equal parts. I am using the code: for n in range(21): t=0; for i in range(n+1): t = t + Combinations(n,i).cardinality()*Combinations(n-i,i).cardinality(); t=3*t; if gcd(n,3)==3: t = t-2*Combinations(n, n/3).cardinality()*Combinations(2*n/3, n/3).cardinality(); print t However, for n=3 the answer should be 18 not 15. Ideas?

ClemFanJC07 gravatar imageClemFanJC07 ( 2014-07-09 16:43:02 -0600 )edit

Can you put the code from your comment in your original question so we can see the formatting better?

calc314 gravatar imagecalc314 ( 2014-07-09 17:01:31 -0600 )edit

For your first bit of code, if you print n before and after your "if" block, you will see that the value of n is only modified when n is divisible by 3.

calc314 gravatar imagecalc314 ( 2014-07-09 17:04:36 -0600 )edit

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answered 2014-07-09 17:17:08 -0600

Rolandb gravatar image

updated 2014-07-26 01:04:34 -0600

Maybe you get confused by the fact that gcd(0,3) equals 3. If I print each step, it works as expected.

for n in range(9):
    print "n=",n
    if gcd(n,3)==3:
        print "?"
        n=n/3
        n=n+1
    print "*",n

n= 0
?
* 1
n= 1
* 1
n= 2
* 2
n= 3
?
* 2
n= 4
* 4
n= 5
* 5
n= 6
?
* 3
n= 7
* 7
n= 8
* 8
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If you indent by 4 spaces the code and outpout that you pasted in your answer, it will be displayed nicely in a code block. When editing your answer, you can also select the relevant lines and click the "code" button which has little digits "101 010", next to the "bold", "italics" etc buttons, and this will put your code in an appropriate block so that it will be displayed nicely.

slelievre gravatar imageslelievre ( 2014-07-09 20:51:06 -0600 )edit

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Asked: 2014-07-09 16:36:24 -0600

Seen: 98 times

Last updated: Jul 26 '14