Python One Line Pattern

5/5 - (1 vote)

Problem Formulation

You appear at an interview, and the interviewer intends to test your skills in writing concise one-line solutions. So he decides to present you with a long and complex-looking piece of code that is seemingly intimidating and asks you to come up with one-line solutions for the given code.

Given:

# data
n = 5
# Multi-line Code
for i in range(1, n+1):
    print(i, end=" ")
    if i == n:
        print()
        for i in range(1, n):
            print(i, end=" ")
            if i == n-1:
                print()
                for i in range(1, n-1):
                    print(i, end=" ")
                    if i == n-2:
                        print()
                        for i in range(1, n-2):
                            print(i, end=" ")
                            if i == n-3:
                                print()
                                for i in range(1, n-3):
                                    print(i, end=" ")

Challenges:

  • Guess the output generated by the above code.
  • Formulate a one-line solution for the given multi-line code.

So, are you up to the challenge? It is highly recommended that you try to come up with your solution before diving into the solutions given here. Now without further delay, let us dive into the solutions to the given challenge.

Output

Though the code looks daunting, it is merely a coding snippet that generates a specific pattern which is the output. Thus, the output looks like this:

1 2 3 4 5 
1 2 3 4 
1 2 3 
1 2 
1 

It’s not that confusing once you get hold of the general pattern. The for loops have been purposely serialized such that each one is responsible for printing one line of digits to confuse you. There’s just no reason to nest them like that.🤯

Well! Now that we have the output, let’s formulate the one-line solutions to this ugly-looking multi-line code.

Method 1: One Line For Loop

A straightforward and concise one-line solution to the given problem is to use a simple for loop and then generate the output with the help of the range function within a print statement. All of this can be done in a single line of code.

Code:

# data
n = 5
# one-liner
for i in range(n, 0, -1): print(*range(1, i + 1))
  • The idea is to iterate over the range 0 to n with a step size of -1 using a for loop. This ensures that the counter variable starts from n, i.e., 5 in this case and iterates backward until 1. The negative step size takes care of the backward iteration.
  • We then move inside the loop body within the same line and start printing the pattern using * with the range() function. The * allows you to “unpack” an iterable, such that each element is passed as a separate argument instead of the function receiving the entire iterable object as a single argument. Let’s visualize this with an example to understand the difference between range() and *range().
>>> print(range(1, 5))
range(1, 5)
>>> print(*range(1, 5))
1 2 3 4 5
# This is the same as printing:
>>> print(1, 2, 3, 4, 5)
1 2 3 4 5
  • You must print the values that lie within the range –> (1, i+1) where i is the counter variable used in the for loop. This essentially means that you are printing a certain range of values that start from 1 and end at i.
  • As the outer loop begins from 5 and goes on till 1. The values printed in each iteration are as demonstrated below:
    • Iteration 1: i = 5, values printed ( from 1 until i) = 1 2 3 4 5
    • Iteration 1: i = 4, values printed ( from 1 until i) = 1 2 3 4
    • Iteration 1: i = 3, values printed ( from 1 until i) = 1 2 3
    • Iteration 1: i = 2, values printed ( from 1 until i) = 1 2
    • Iteration 1: i = 1, values printed ( from 1 until i) = 1

Method 2: One Line Nested For Loop

Another way of solving the coding challenge is to use the join() function along with a nested for loop in a single line of code, as shown below.

# one-liner
print("\n".join(" ".join(str(j + 1) for j in range(5 - i)) for i in range(5)))

The above one-liner is a concise one-line way of solving the following multi-line solution which generates the same output as in the given question.

for i in range(5):
    for j in range(5-i):
        print(str(j+1), end=" ")
    print()

Method 3: One Liner with List Comprehension and Nested For Loop

There’s another way of solving the problem in a single line with the help of a nested for loop. In this case, we are going to unpack the output from a list comprehension that is supported by another for loop. The list comprehension takes care of the output generated in each line while the other for loop takes care of the outer loop by iterating over the range(6,1,-1).

Code:

# data
n = 5
# one-liner
[print(*[j for j in range(1, i)]) for i in range(6, 1, -1)]

Method 4: One Liner with Built-in Functions

If you are well versed with Python’s built-in functions, then this solution might grab your attention.

# data
n = 5
# one-liner
print('\n'.join(' '.join(map(str, range(1, i))) for i in range(n + 1, 1, -1)))
# or you can also use the following solution-
for i in range(n+1, 1, -1): print(' '.join(map(str, range(1, i))))
  • The above one-line solution once again uses a for loop along with the map and join functions. The for loop allows you to iterate backward within the range –> (n + 1, 1, -1) where -1 denotes the step size and allows you to iterate backward.
  • In each iteration, we are using the map method to convert a range of values starting from 1 and ending at i-1 to a string. Here, i is the value returned by the for loop in each iteration. The join method allows you to feed in a space between each string value.

Dive into built-in functions in Python here – Python Built-In Functions

Conclusion

We have successfully solved the given programming challenge and formulated as many as four one-line solutions. I hope this tutorial helped you. Please stay tuned and subscribe for more interesting one-liners and solutions. Happy learning!

But before we move on, I’m excited to present you my new Python book Python One-Liners (Amazon Link).

If you like one-liners, you’ll LOVE the book. It’ll teach you everything there is to know about a single line of Python code. But it’s also an introduction to computer science, data science, machine learning, and algorithms. The universe in a single line of Python!

The book was released in 2020 with the world-class programming book publisher NoStarch Press (San Francisco).

Link: https://nostarch.com/pythononeliners