While Loop 4

Start Quiz:

# We now would like to summarize this data and make it more visually appealing
# We want to go through count_list and print a table that shows the number and its 
# corresponding count.

# The output should look like this neatly formatted table:
"""
number | occurrence
     0 | 1
     1 | 2
     2 | 3
     3 | 2
     4 | 2
     5 | 1
     6 | 1
     7 | 2
     8 | 3
     9 | 1
    10 | 2
"""
# Here is our code we have written so far:

import random

# Create random list of integers using while loop --------------------
random_list = []
list_length = 20

while len(random_list) < list_length:
    random_list.append(random.randint(0,10))

# Aggregate the data -------------------------------------------------
count_list = [0] * 11
index = 0

while index < len(random_list):
    number = random_list[index]
    count_list[number] = count_list[number] + 1
    index = index + 1
    
# Write code here to summarize count_list and print a neatly formatted table that looks
# like this:

"""
number | occurrence
     0 | 1
     1 | 2
     2 | 3
     3 | 2
     4 | 2
     5 | 1
     6 | 1
     7 | 2
     8 | 3
     9 | 1
    10 | 2
"""

# Hint: To print 10 blank spaces in a row, we can multiply a string by a number "n"
# to print this string n number of times:
print "Udacity! "*10

# BONUS!
# From your summarize code you just wrote, can you make the table even more visual by 
# replacing the count with a string of asterisks that represent the count of a number?
# The table should look like

"""
number | occurrence
     0 | *
     1 | **
     2 | ***
     3 | **
     4 | **
     5 | *
     6 | *
     7 | **
     8 | ***
     9 | *
    10 | **
"""

# Congratulations! You just created a distribution table of a list of numbers! 
# This is also known as a histogram

Solution:

Breaking Down the Problem

Let's first identify what are the inputs and what are the outputs (or results) that we want to obtain.

The inputs are:

A list of integers that represent the distribution of all numbers that occurred in a randomly generated list

The output is:

A nicely formatted table that shows the number and its corresponding count like this:

number | occurrence
     0 | 1
     1 | 2
     2 | 3
     3 | 2
     4 | 2
     5 | 1
     6 | 1
     7 | 2
     8 | 3
     9 | 1
    10 | 2

What To Do

We first need to understand that we still need to loop through all of the elements in count_list and simply print out the index and the value of the count_list in a specific format.

Let's see if we can write an outline of what to do if we were to do this manually on pen and paper:

Print the header "number | occurrence"
Loop through each element in the randomly generated list
Print the number of necessary spaces to get the right alignment we want
Get the current index and its associated value in our list
Print index and value in this format: "index | value"
Are we at the end of the loop?
If not, we loop back up and go through steps 1 to 5 again while we are still going through the list

Translation

Let's step through these steps and translate these steps into computer code.

1. Print the header "number | occurrence"

This is straight forward:

 print "number | occurrence"

2. Loop through each element in the randomly generated list. We therefore setup our loop structure.

index = 0

while index < len(random_list):
  # Put other code here
  index = index + 1

Please keep in mind how we are already adding index = index + 1 to the loop. This code is crucial to guarantee that the computer not step into an infinite loop. For most loops, we want to always clearly define a stopping point. In this case, the stopping point is when the number index is greater than the length of our list.

3. Print the number of necessary spaces to get the right alignment we want.

To do this dynamically we need to realize that we can calculate the number of white spaces needed to print before we print our index. If we want to print the string "0", we need to realize that we need to print 5 spaces before we print "0" in order to line up with the letter r in "number" in the header.

What happens if we decide to print the string "10?" We need to print 4 spaces because the characters "10" takes up 2 spaces. Regardless of the number, the total amount of spaces and characters should add up to 6, the length of the string "number".

Therefore in every loop, we dynamically calculate the number of spaces we need to print out depending on the length of the characters for each index:

num_spaces = len("number") - len(str(index))

Since len("number") never changes for each loop iteration, let's calculate and store the length of the string "number" outside the loop:

num_len = len("number")
while index < len(count_list):
  num_spaces = num_len - len(str(index))

4-5. Get the current index and its associated value in our list and print it out in the format we want. We convert our integers into string and use the "+" operator to concatenate our strings.

print " " * num_spaces + str(index) + " | " + str(count_list[index])

5: Are we at the end of loop? If not, we loop back up and go through the while loop instructions all over again.

Our loop construct already takes care of this criteria because at the top of our loop, we are always checking whether our index number is still less than the length of the random list: index < len(random_list)

We use the logic: if the index number is less than the length of our list, then we can safely say that whenever we access the list with index, we will never create an error and will be able access elements in the list with the number index.

Answer Code

print "number | occurrence"
index = 0
num_len = len("number")

while index < len(count_list):
  num_spaces = num_len - len(str(index))
  print " " * num_spaces + str(index) + " | " + str(count_list[index])
  index = index + 1

Full Answer Code

import random

random_list = []
list_length = 20

while len(random_list) < list_length:
    random_list.append(random.randint(0,10))

count_list = [0] * 11
index = 0

while index < len(random_list):
    number = random_list[index]
    count_list[number] = count_list[number] + 1
    index = index + 1

print "number | occurrence"
index = 0
num_len = len("number")

while index < len(count_list):
  num_spaces = num_len - len(str(index))
  print " " * num_spaces + str(index) + " | " + str(count_list[index])
  index = index + 1