How to generate exponentially increasing range in Python

Question

I want to test the performance of some code using an exponentially increasing value. So that as an extra digit is added to the numbers_size the increment is multiplied by 1

Answer 1

If you consider numpy as one of the standards ;), you may use [numpy.logspace][1] since that is what it is supposed to do.... (note: 100=10^2, 1000000000=10^9)

for n in numpy.logspace(2,9,num=9-2, endpoint=False):
    test(n)

example 2 (note: 100=10^2, 1000000000=10^9, want to go at a step 10x, it is 9-2+1 points...):

In[14]: np.logspace(2,9,num=9-2+1,base=10,dtype='int')
Out[14]: 
array([       100,       1000,      10000,     100000,    1000000,
         10000000,  100000000, 1000000000])

example 3:

In[10]: np.logspace(2,9,dtype='int')
Out[10]: 
array([       100,        138,        193,        268,        372,
              517,        719,       1000,       1389,       1930,
             2682,       3727,       5179,       7196,      10000,
            13894,      19306,      26826,      37275,      51794,
            71968,     100000,     138949,     193069,     268269,
           372759,     517947,     719685,    1000000,    1389495,
          1930697,    2682695,    3727593,    5179474,    7196856,
         10000000,   13894954,   19306977,   26826957,   37275937,
         51794746,   71968567,  100000000,  138949549,  193069772,
        268269579,  372759372,  517947467,  719685673, 1000000000])

on your case, we use endpoint=False since you want not to include the endpoint... (e.g. np.logspace(2,9,num=9-2, endpoint=False) ) [1]: http://docs.scipy.org/doc/numpy/reference/generated/numpy.logspace.html

Answer 2

OP wrote "Suggestions for improvements without introducing non-standard libraries?"

Just for completeness, here's a recipe for generating exponential ranges - each element is a fixed factor bigger than the previous:

from math import exp
from math import log

def frange(start, stop, numelements):
    """range function for floats"""
    incr = (stop - start) / numelements
    return (start + x * incr for x in range(numelements))

def exprange(start, stop, numelements):
    """exponential range - each element is a fixed factor bigger than the previous"""
    return (exp(x) for x in frange(log(start), log(stop), numelements))

Test:

print(", ".join("%.3f" % x for x in exprange(3,81,6)))

Output:

3.000, 5.196, 9.000, 15.588, 27.000, 46.765

Answer 3

Why not

for exponent in range(2, 10):
    test(10 ** exponent)

if I'm reading your intent right.

Answer 4

Using a generator expression:

max_exponent = 100
for i in (10**n for n in xrange(1, max_exponent)):
    test(i)

Answer 5

example of 'NOT reading the question properly' and 'NOT how to do it'

for i in xrange(100, 1000000000, 100):
    # timer
    test(i)
    # whatever

Is about as simple as it gets... adjust xrange accordingly

Answer 6

I like Ned Batcheldor's answer, but I would make it a bit more general:

def exp_range(start, end, mul):
    while start < end:
        yield start
        start *= mul

then your code becomes

for sz in exp_range(100, 1000000000, 10):
    t = time.time()
    test(sz)
    print sz, test(sz), time.time()-t