random.randint() call with -1?

Question

Can someone explain why the range in the random.randint(0, len(messages) - 1) has a minus 1 at the end? The list has 9 values:

import random

messages = ['It is certain',
            'It is decidedly so',
            'Yes definitely',
            'Reply hazy try again',
            'Ask again later',
            'Concentrate and ask again',
            'My reply is no',
            'Outlook not so good',
            'Very doubtful']

select_number = random.randint(0, len(messages) - 1)
list_select  = messages[select_number]
print(list_select)

Answer 1

random.randint() 's second value is inclusive . It'll pick a number from 10 random integers ( 0 , 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 and 9 ), where you only have 9 indices in the list. len(messages) is 9 , but there is no messages[9] index, as Python indexing starts at 0 . Instead, the only valid indices go from 0 to 8 .

So by using random.randint(0, len(messages) - 1) , the function is limited to picking a valid index for the messages list.

The code should instead use random.choice() , which picks a random element from a list:

list_select = random.choice(messages)

In other situations, random.randrange() could be an option too, as it doesn't include the endpoint in the possible values, working analogous to Python indexing, slicing and range() :

index_select = random.randrange(len(messages))

Answer 2

This is because random.randint picks the indexes of the list, from 0 to 8 at random, and selects a message.
Now, the indexes of a list goes from 0 to len(messages)-1=8 .
According to the docs: https://docs.python.org/3/library/random.html#random.randint

random.randint(a, b) . Return a random integer N such that a <= N <= b. Alias for randrange(a, b+1).

Which means an integer between 0 and 8 is picked, which are the valid indexes of the list, If we have instead put len(messages) , index 9 would have been picked too which is an invalid index for a list in Python, since Python lists are 0-indexed

Answer 3

random.randint(a, b) Return a random integer N such that a <= N <= b.

len(messages) - 1 Is a bit tricky because range of your list is indeed 9 but you have to remember that counting is starting from 0 instead of 1. So in your case len(messages) is going to return 9 but the first element is in messages[0]

It means that in your case the select_number = random.randint(0, len(messages) - 1) will give number from 0 to len(messages) - 1 which is 9 - 1.