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Algorithms and Problem Solving Approaches

What is an Algorithms?

A process or set of steps to accomplish a certain task.

How do you improve?

  • Devise a plan for solving problems - this section
  • Master common problem solving patterns - here

Problem Solving Strategies

A SIMPLE, FOOLPROOF, MAGICAL, MIRACULOUS, FAIL-SAFE APPROACH
  • NOT REALLY - BUT IT'S STILL HELPFUL

Problem Solving

1. Understand the Problem

Write a function which takes two numbers and returns their sum.

  1. Can I restate the problem in my own words?

    "Implement addition"

  2. What are the inputs that go into the problem?

    - ints?
- floats?
- what about string for large numbers?

  3. What are the outputs that should come from the solution to the problem?

    - int? float? string?

  4. Can the outputs be determined from the inputs? In other words, do I have enough information to solve the problem? (You may not be able to answer this question until you set about solving the problem. That's okay; it's still worth considering the question at this early stage.)

    yeah!

  5. How should I label the important pieces of data that are a part of the problem?

    sum

2. Explore Concrete Examples

Write a function which takes in a string and returns counts of each character in the string.

  • Start with Simple Examples

    charCount("aaa"); // {a: 3}
charCount("hello"); // {h: 1, e: 1, l: 2, o:1}

  • Progress to More Complex Examples

    What about the following examples?
- "my phone number is 2132"
- "Hello hi"

  • Explore Examples with Empty Inputs

    charCount(""); // do you want return {} or null or error # edge cases

  • Explore Examples with Invalid Inputs

    What about if user pass a number, object? # edge cases

3. Break It Down

Write a function which takes in a string and returns counts of each character in the string.

بالورقة والقلم قبل م تكتب اي سطر كود

  • Explicitly write out the steps you need to take.

    // do something
// return an object with keys that are lowercase alphanumeric chars in the string, values should be the counts of those chars
--------------------------------------------------
- make object to return at end
- loop over string for each character...
  // if the char is a number/letter AND key in object, add one to count
  // if the char is a number/letter AND not in object, add it to object and set value to 1
  // if character is something else (space, period, etc.) don't do anything
- return object at end

4. Solve/Simplify

Write a function which takes in a string and returns counts of each character in the string.

  • Find the core difficulty in what you're trying to do

  • Temporarily ignore that difficulty

  • Write a simplified solution

  • Then incorporate that difficulty back in

    function charCount(str) {
  // make object to return at end
  const result = {};

  // loop over string for each character...
  for (let i = 0; i < str.length; ++i) {
    let char = str[i].toLowerCase();

    // if character is something else (space, period, etc.) don't do anything
    if (!(char >= "a" && char <= "z") || !(char >= "0" && char <= "9"))
      continue;

    // if the char is a number/letter AND key in object, add one to count
    if (result[char] > 0) {
      result[char]++;
      // if the char is a number/letter AND not in object, add it to object and set value to 1
    } else {
      result[char] = 1;
    }
  }

  // return object at end
  return result;
}

5. Look Back and Refactor

Congrats on solving it, but you're not done!

Refactoring Questions

Write a function which takes in a string and returns counts of each character in the string.
  • Can you check the result?
  • Can you derive the result differently?
  • Can you understand it at a glance?
  • Can you use the result or method for some other problem?
  • Can you improve the performance of your solution?
  • Can you think of other ways to refactor?
  • How have other people solved this problem?
function charCount(str) {
  const result = {};

  for (let char of str) {
    if (isAlphaNumeric(char)) {
      char = char.toLowerCase();
      result[char] = ++result[char] || 1;
    }
  }

  return result;
}

function isAlphaNumeric(char) {
  const code = char.charCodeAt(0);

  if (
    !(code > 47 && code < 58) && // numeric [0-9]
    !(code > 64 && code < 91) && // upper [A-Z]
    !(code > 96 && code < 123) // lower [a-z]
  ) {
    return false;
  }
  return true;
}