random iteration in Python

Question

When you want to iterate sequentially over a list of numbers you will write:

for i in range(1000):   # do something with i 

But what if you want to iterate over the list of numbers from the range (0..999) randomly? There is a need (in every iteration) to choose randomly the number that wasn't chosen in any previous iteration and there is a need to iterate over all of the numbers from the range (0..999).

Do you know how to do that (smart)?

Answer 1

You can use random.shuffle() to, well, shuffle a list:

import random  r = list(range(1000)) random.shuffle(r) for i in r:   # do something with i 

By the way, in many cases where you'd use a for loop over a range of integers in other programming languages, you can directly describe the "thing" you want to iterate in Python.
For example, if you want to use the values of i to access elements of a list, you should better shuffle the list directly:

lst = [1970, 1991, 2012] random.shuffle(lst) for x in lst:   print x 

NOTE: You should bear the following warning in mind when using random.shuffle() (taken from the docs:

Note that for even rather small len(x), the total number of permutations of x is larger than the period of most random number generators; this implies that most permutations of a long sequence can never be generated.

Answer 2

Here's a different approach to iterating a list in random order. This doesn't modify the original list unlike the solutions that use shuffle()

lst=['a','b','c','d','e','f'] for value in sorted(lst,key=lambda _: random.random()):     print value 

or:

for value in random.sample(lst,len(lst)):     print value