by Daniel Weiner

Problem-solving with Honest Abe: let’s sum all prime numbers up to n

Follow along as Honest Abe solves an intermediate algorithm challenge using the basic software development principles

Say the challenge is this:

Sum all the prime numbers up to and including the provided number.

A prime number is defined as a number greater than one and having only two divisors, one and itself. For example, 2 is a prime number because it’s only divisible by one and two.

The provided number may not be a prime.

How would Honest Abe solve this problem?

Honest Abe thinks big picture

“Give me six hours to chop down a tree and I will spend the first four sharpening the axe.” — Honest Abe

Before diving into code, Honest Abe likes to formulate requirements and determine specifications. He learns as much as possible about the problem, and figures out exactly the problem that needs to be solved. Until he fully understands the problem, he cannot begin to solve it.

He also needs to determine exactly what the program will accomplish. He focuses on what want the program will accomplish, rather than how it will work. For simple programs, this involves describing the inputs and outputs and how they relate to one another.

For this problem, the input will be some number (n), an integer. The output will be the sum of all the prime numbers from 2 through n (there are no prime numbers less than 2). The problem explains prime numbers, and Abe feels comfortable with that definition.

Honest Abe starts with paper and pencil

Abe doesn’t just jump right into coding. The next step is to create the overall structure of the program. This is where he determines how the program accomplishes its task.

The main job here is to design the algorithm(s) that will meet the specifications. The algorithm will often be written in pseudocode, or a precise English description of what the program does. This helps Abe communicate algorithms without the extra mental overhead of getting the details right in any particular programming language.

Here’s an algorithm to sum all primes up to n:

• Input n as an integer
• Find primes up to n
• Find sum of all primes found

Abe knows that he can revisit this pseudocode as he implements the design.

Honest Abe loves Python Tutor

Abe knows he has many options where he could code, including an editor such as Sublime, or an IDE such as Visual Studio Code, or even directly in a provided coding panel (like the one provided by freeCodeCamp).

Abe really prefers pythontutor.com.

Here’s an example of how Python tutor works:

Python tutor is a very intuitive interface to enter code into an editor. It allows Abe to visualize the execution of a program, without needing to learn about a debugger or an IDE. He can even set break points simply by clicking on lines of code (the break point here is marked in red). Despite the name, Python tutor is also compatible with Java, JavaScript, Ruby, and other programming languages.

Honest Abe uses Incremental Development

Unlike many lectures, tutorials, and textbooks, code that Honest Abe writes himself doesn’t come completely assembled as a working program. Although sometimes he thinks it would be nice if it did.

Therefore, Honest Abe practices incremental development.

Rather than writing a complete function, program, or whatever he’s working on, Honest Abe will write small pieces of code first, make sure those work, then link those together into a bigger program. He is, therefore, developing his program in increments.

In this example, Honest Abe starts with a small version of the code, getting input from the user. He inserts a print statement to make sure that this is working. The final version of the code is commented out below to show how he might go from one increment into a larger block of code.

Honest Abe practices Defensive Programming

Honest Abe knows that users can’t be trusted to follow the instructions provided by his program. He has to put protections in place in case users input bad values. In this case, bad values would be anything other than a positive integer.

The try / except block seen above, encapsulated in the readInt function, catches any user input that is not a positive integer, and finally returns the user input once an integer is entered properly.

Honest Abe starts with a brute force solution

Honest Abe’s first priority is to get the program to produce a correct result. He relies on a brute force, exhaustive enumeration approach, iterating over all of the numbers from five through the user input, checking to see if each is prime.

He knows that two and three are primes, so if either of those are the user input, he adds those to sum.

He also optimizes for the inner loop by only searching from two through the square root of i.

This gives the correct result, but Honest Abe knows he can do better. This would be terribly slow on large inputs.

Honest Abe studies The Classics

Attributed to the ancient Greek mathematician, Eratosthenes, this is an efficient algorithm for finding prime numbers up to any given limit.

It iteratively marks multiples of each prime as not prime, starting with the first prime number, 2. For instance, 4, 6, 8 etc. are marked as not prime up to the limit. Then, returning to the beginning of the list, 3 is marked as prime. 6 has already been marked as not prime, so 9 is marked as not prime, followed by 12, 15, etc. until the sequence is finished.

Honest Abe starts by initializing a list with the values True of length num. He sets the first two values of the list to False because neither 0 nor 1 are prime. He then creates a variable sum with an initial value of 0, which will increase with each prime number found.

Using the enumerate function in Python, Honest Abe first checks if the value in the list a is set to True, meaning that this is a prime number. If it is, he sum increases by that amount.

He then iterates, starting from i*i (a small optimization), through num, incrementing by i, changing the value at each list index to False.

For instance, the 0 and 1 are both set to False, so those values do not enter the inner for loop.

2 is set to True, so 2 gets added to sum. Then, starting at 4, list indices get set to False, including 6, 8, 10, etc, until the loop is finished.

Then i increments to 3, which is set to True, and the process above repeats itself.

Honest Abe knows there are more efficient implementations, but the tradeoff in efficiency will be explainability, so he will leave it to the reader to further explore these algorithms.

Honest Abe tests his program

Normally, Abe would do unit testing before integration testing.

Testing, however, is a very large subject best left for other articles.

Abe, instead, just wants to make sure that his program works as intended.

He uses pytest and tests his program on a series of positive integers.

He feels confident that his program provides the correct answers.

Honest Abe’s Favorite Resources

Teach yourself programming in ten years — Peter Norvig

Stanford Programming Methodology

MIT Python

Harvard CS50