Build a simple calculator

The following is my solution to a coding challenge I’ve come across. I’ve implemented it in JavaScript since that is the language I am most comfortable with at the moment.

Problem

You are building an educational website and want to create a simple calculator for students to use. The calculator will only allow addition and subtraction of non-negative integers.

Given an expression string using the “+”, “-“ operators like “5+16-2”, write a function to parse the string and evaluate the result.

Constraints

• Only allow addition and subtraction of non-negative integers.

Solution

const calculate = (expression) => {
  const numbers = expression.split(/[+,-]/).map(n => Number(n)); // transform String into Number
  const operators = expression.split(/\d/).filter(op => op !== '');
  let result = 0;

  // perform calculation
  numbers.forEach((number, id) => {
    const operator = id === 0 ? '' : operators[id - 1]; // no operator to apply to first number
    switch (operator) {
      case '+':
        result = result + number;
        break;
      case '-':
        result = result - number;
        break;
      default:
        result = number;
    }
  });

  return result;
};

Try it

Explanation

Given the constraints we are working with, I can build my calculate function without having to check for negative numbers, and mathematical expressions that do not involve integers.

My first task is to figure out what numbers we are operating on, so I use String.split to separate the full expression into substrings of just the numbers by passing the regular expression (/[+,-]/) as the separator parameter. Since split() returns an Array of strings, I can call the Array.map method to transform my Array of strings into an array of Numbers. This will allow me to operate on the numbers.

In order to get the list of operators from the expression, I use us split again, but this time I pass the regular expression (\d) as the separator parameter. I am basically telling split to build a new array using integers as the breaking point. The result of which is passed to a Array.filter so that I can clean up all of the blank array elements.

At this point I should have everything I need to evaluate the expression passed into calculate.

I’ll use Array.forEach to perform an operation on each of the integers in the given expression. Inside of the forEach callback method I’ll grab the operator from the list of operators with one caveat. I do not want to use the first operator on the first number from the numbers array. We’ll perform the operations after the first number, so that we have an accurate result.

Since the operator is of type String I cannot use that to calculate an expression (e.g. 1 + 1), because the + will be treated as a string. I’ll use a switch statement and map the string + with the Unary plus, and will do the same for the - with the Subtraction operator. My default case assigns result to number since this will be the first number in the expression.

That is about it, once we’ve iterated over all of the numbers in the expression, we return the result.