range and division : unexpected behavior

candide
81 ●2 ●6 ●13

slelievre
17779 ●22 ●161 ●352 http://carva.org/samue...

Consider the following snippet :

N=5

# code 1
for n in range(N,N+1):
    for k in range(0,n):
        print k/n

print '-'*10

# code 2
n=N
for k in range(0,n):
    print k/n

I was expecting code 1 and code 2 to print the same output. This is not the case :

0
0
0
0
0
----------
0
1/5
2/5
3/5
4/5

In the first case, k/n is Python-evaluated as an integer division, in the second case, k/n is Sage-evaluated as a fraction. Can someone elaborate please ?

I only notice that substituting srange(N,N+1) to range(N,N+1) fixes the problem.

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answered 8 years ago

ndomes

2429 ●1 ●22 ●55

There is a difference between Sage and Python (2.x) concerning the division operator / . In Python 2.x the operator / returns the floor of the result of division if the operands are integers (Python ints). In Sage the operator / returns the result as rational number (if so) if the operands are Sage integers. The Python function range returns a list of Python integers. In your nested for loops all operands are Python ints, so you get the floor of the division result. In your second code example N (and therefor n) is an Sage integer, so you get the rational numbers as result.

The Sage function srange returns a list of Sage integers. With srange(N,N+1) in your first code n becomes a Sage integer.

You can see the difference by placing some print type(k) and print type(n) commands inside the loops.

Comments

Thanks, now the benefits of using srange is more apparent.

candide ( 8 years ago )

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