Is there any "pythonic" way to do this? I am pretty sure my method is not good:
sample = "This is a string"
n = 3 # I want to iterate over every third element
i = 1
for x in sample:
if i % n == 0:
# do something with x
else:
# do something else with x
i += 1
you can use step for example sample[start:stop:step]
If you want to iterate over every second element you can do :
sample = "This is a string"
for x in sample[::2]:
print(x)
output
T
i
s
a
s
r
n
If you want to do something every nth step, and something else for other cases, you could use enumerate to get the index, and use modulus:
sample = "This is a string"
n = 3 # I want to iterate over every third element
for i,x in enumerate(sample):
if i % n == 0:
print("do something with x "+x)
else:
print("do something else with x "+x)
Note that it doesn't start at 1 but 0. Add an offset to i if you want something else.
To iterate on every nth element only, the best way is to use itertools.islice to avoid creating a "hard" string just to iterate on it:
import itertools
for s in itertools.islice(sample,None,None,n):
print(s)
result:
T
s
s
r
g
islice is a lot slower than just using a regular slice, since it has to actually retrieve every character from the string and ignore the ones it's not interested in, while a regular slice can skip straight to the elements it needs.s[::n] (memory copy but fast iteration). If the string is big, maybe islice is better but you're probably right for regular-sized strings.Logic to print every nth character:
#Sample input = 'abcdefghijkl'
#n = 4
for i in range(0,4):
for x in input[i::4]:
print(x, end='')
Output:
aeibfjcgkdhl
Seeing the answers above - except the mostly voted one (which doesn't fully cover the request) - it was mainly division in a loop. Why not keep it more "programmatic", even if yet not that pythonic? Here is example based on original question (no division used):
sample = "This is a string"
n = 3 # I want to iterate over every third element
i = 1
for x in sample:
if n == i:
i = 0
# do something with x
else:
# do something else with x
i += 1
If you sill like to process with valid index:
sample = "This is a string"
n = 3 # I want to iterate over every third element
# (!) here do something else with all elements within list/substring: sample[0:n-1]
for i in range(n-1, len(sample), n):
# (!) here do something with element: sample[i]
# (!) here do something else with all elements within list/substring: sample[i+1:i+n]
And now coming back to solution with division, but as a single line:
sample = "This is a string"
n = 3 # I want to iterate over every third element
# you can call below your own correspondent functions (according to case) instead of print() ;-)
_ = [print('Do something: ' + sample[i]) if 0 == ((1+i) % n) else print('Do something else: ' + sample[i]) for i in range(len(sample))]
for i,x in enumerate(sample,1):