for example this kind of for loop in JAVA or C++:
int N = 20;
for (i = 1; i < N; i = 3 * i + 1)
i will be 1, 4, 13
I can only use while to complete it
while i < N:
i = 3 * i + 1
How can write it using another kind of style in python?
Sorry for my English.
Your while loop implementation is nothing wrong from Python's perspective. If you want a generalized loop structure to mimic the loop structure of C/C++/Java you can do something similar
>>> def loop(init, cond, incr):
i = init
while cond(i):
yield i
i=incr(i)
>>> list(loop(1, lambda e:e < 20, lambda e:3*e + 1))
[1, 4, 13]
Once you create the loop routine, you can use it to create any custom loop format as you desire
for i in loop(1, lambda e:e < 20, lambda e:3*e + 1):
print i
for', rather than C/Java for in Python.
range emulator which is what a typical person would need. I, however, disagree with the use of having cond be callable. It should be a simple integer. The incr argument should be callable though. Regardless, this isn't exactly what the OP is looking for as far as I can tell.
i < cond? That would be significantly less general than a C-style for-loop (which can have arbitrary conditions). Most obviously it wouldn't allow you to write loops that count down.This is a job for generators. Do this:
def series(i=1):
while True:
yield i
i = 3 * i + 1
from itertools import takewhile
takewhile(lambda x: x < 20, series())
Here's another way to do it. It is more specialized than Abhijit's answer.
def timesThreePlusOne(init,limit):
i = init
while i < limit:
yield i
i = (3 * i) + 1
N = 20
for i in timesThreePlusOne(1,N):
print i
3 * i + 1, I would suggest looking intorangebut the step argument it accepts expects an integer. You could write your own version though.style in python, for which that loop is not the typical style regardless of whether it is valid.