Generating a Random Number List

In the Generate Random Number section of the Modules chapter, we see how to use the randint function to generate a random integer within a given range.

Example

import random

for turn in range(5):
   num_choice = random.randint(15, 25)
   print(num_choice)

Console Output:

22
18
15
22
17

Each time the loop runs, line 4 selects a number within the range 15-25.

In this example, we use a loop to print out one random number each iteration. Instead of printing, however, we can use the accumulator pattern and a list to collect each value.

This technique provides us with a quick and easy way to build a set of random numbers without having to pick them ourselves! If we want to use the list of numbers later in the program, we can easily refer to the list variable.

Try It!

Run the following code several times to try the following:

  1. Generate lists of different lengths.
  2. Generate lists over different ranges.

Note

Each time the program runs, it generates a new list of random numbers. If you want to save a list between runs, then you must copy it from the output and paste it into your code as a variable assignment.

When is this helpful? If you are a teacher and want to give every student the same set of “random” values, you can run this program first to populate your list and check if it meets your needs.

Generate a Set of Unique Random Numbers

If we run the program above several times, we will notice that num_list often contains repeated values. Sometimes, however, we need a list of random numbers that does NOT contain any repeats.

To generate a set of unique numbers (no repeats), we must use a while loop instead of a for loop in the accumulator pattern.

Try It!

Examine the code below to see how it differs from the for loop.

  1. Lines 1-4 work as they did before.
  2. The while condition in line 6 checks if num_list has the desired length. As long as the number of entries is less than list_length, the loop continues running. Once num_list reaches the proper size (5 in this case), the loop ends.
  3. Line 8 selects a new number from within the given range.
  4. Line 11 checks if new_num does NOT exist inside num_list. If True, then the append statement on line 14 runs, and the chosen value is added to the list.
  5. If the condition on line 11 evaluates to False, then new_num is already an element in num_list. The append statement gets skipped, and the chosen number gets thrown away.

We MUST use a while loop to generate non-repetitive numbers. Since we do not know ahead of time how many turns it will take to build the list, trying to use a for loop will give inconsistent results.

Run this program several times to prove that it builds a list with no repeated values.

Warning

Keep the length of your list SMALLER than the range of values you use in randint! If you get this wrong, you will create an infinite loop.

For example, the statement random.randint(0, 3) picks one of 4 possible values (0, 1, 2, 3). Given this range of four numbers, it is IMPOSSIBLE to build a list of 5 or more unique numbers!

Unique Dictionary Values

We can also assign random numbers to the keys of a dictionary. For example, lets say we have a collection of names and we want to give each person a raffle ticket. We can use a while loop as above to generate unique ticket numbers.

However, since dictionaries use key/value pairs, we need to be very deliberate about how we search for repeats. We need to use the .values() method to check if a particular number has already been assigned to a key.

Example

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import random

names = ['Bob', 'Ann', 'Mae', 'Syd']
ticket_holders = {}  # Create an empty dictionary.

index = 0   # Set the index value for the first entry in the names list.
while len(ticket_holders) < len(names):
   # Pick a new, random number from the given range.
   new_num = random.randint(100, 110)

   # Check for new_num in the values of the dictionary.
   if new_num not in ticket_holders.values():
      ticket_holders[names[index]] = new_num  # Add a new key/value pair
      index += 1     # Advance to the next entry in the names list.

Console Output:

{'Bob': 110, 'Ann': 102, 'Mae': 101, 'Syd': 107}

The syntax on line 12 is important! If we use:

if new_num not in ticket_holders:

instead of:

if new_num not in ticket_holders.values():

then Python searches for new_num in the key names of the dictionary. Since new_num is an integer and all of the keys are strings, the condition will ALWAYS evaluate to True. The chosen number, even if it is a repeat, will get assigned to the current key.