Does +1 solve the range function not handling the last item in a list?

Question

I'm using plotly to create a histogram of the results from rolling a 6-sided die 1,000 times.

In die_visual.py, under the "# Visualize the results" comment, I assign a list to the x_values variable.

The list contains the range (1, die.num_sides+1). What is the purpose of the '+1'? To me it seems that since die.num_sides has a default value of 6, the '+1' would increase the end value of the range to 7. I have included the contents of both files below.

die.py:

from random import randint

class Die:
    """A class representing a single die."""

    def __init__(self, num_sides=6):
        """Assume a 6-sided die."""
        self.num_sides = num_sides

    def roll(self):
        """Return a random value between 1 and number of sides."""
        return randint(1, self.num_sides)

die_visual.py:

from plotly.graph_objs import Bar, Layout
from plotly import offline
from die import Die

# Create a D6.
die = Die()

# Make some rolls, and store results in a list.
results = []
for roll_num in range(1000):
    result = die.roll()
    results.append(result)

# Analyze the results.
frequencies = []
for value in range(1, die.num_sides+1):
    frequency = results.count(value)
    frequencies.append(frequency)

# Visualize the results.
x_values = list(range(1, die.num_sides+1)) <---
data = [Bar(x=x_values, y=frequencies)]

x_axis_config = {'title': 'Result'}
y_axis_config = {'title': 'Frequency of Result'}
my_layout = Layout(title='Results of rolling one D6 1000 times',
        xaxis=x_axis_config, yaxis=y_axis_config)
offline.plot({'data': data, 'layout': my_layout}, filename='d6.html')

Any help in understanding this is greatly appreciated. Thank you.

Are you aware of why you need to pass `1` as the explicit first argument? — chepner, Sep 05 '21 at 15:24
@chepner I pass 1 as the explicit first argument to make the minimum value rolled a 1. — mrm, Sep 05 '21 at 15:27
almost everything in python is zero indexed, `list(range(6))` gives 0..5 (6 items) `list(range(1,7))` gives 1..6. which is 7-1 items to maintain full consistency — Rob Raymond, Sep 05 '21 at 15:30
Duh, because the range doesn't include the last value, which would be 6. It's only 1 - 5. The +1 adds the 6. For whatever reason it just wasn't clicking in my head. Thank you! — mrm, Sep 05 '21 at 15:30

score 1 · Accepted Answer · answered Sep 05 '21 at 15:29

1

range(a, b) gives an output of Range(a, a+1, a+2, ..., b-1) so range(a, b+1) gives Range(a to b).

One of the reasons for this is that if you want to iterate over the indexes of a list, the length of the list is 1 higher than the last index so range() only going to b-1 makes sense for this.

answered Sep 05 '21 at 15:29

Lecdi

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You should rarely be iterating over a list of indices, and when you do, you probably want to use `enumerate` instead. The real reason is that half-open intervals compose: chaining `range(a, b)` and `range(b, c)` is equivalent to `range(a, c)`, because `b` is not part of both subranges. – chepner Sep 05 '21 at 15:56

Does +1 solve the range function not handling the last item in a list?

1 Answers1