🔑 Key Takeaway: Every good algorithm must be correct, finite, unambiguous, and efficient.

In the previous post we understood what is an algorithm. Now let’s go one step deeper: How does an algorithm actually look? What are its essential characteristics? And how do real algorithms appear in practice?

Properties of a Good Algorithm

Computer scientists agree that a well-defined algorithm must satisfy these five important properties:

  1. Finiteness — The algorithm must terminate after a finite number of steps.
  2. Definiteness — Each step must be precisely and unambiguously defined (no confusion allowed).
  3. Input — Zero or more well-defined inputs must be given.
  4. Output — At least one well-defined output must be produced.
  5. Effectiveness — Every step must be basic enough that it can be carried out exactly and in a finite amount of time.
Some authors also add:
  • Correctness — Produces correct output for every valid input.
  • Efficiency — Uses reasonable time and memory (very important in practice).

How Does a Real Algorithm Look Like?

An algorithm can be expressed in multiple ways:

Plain English Flowchart Pseudocode Programming Code

Example Problem: Find the Largest Number in a List

1. Plain English Description

  1. Start with the first number as the current maximum.
  2. Check each remaining number one by one.
  3. If a number is bigger than current maximum, update it.
  4. After checking all numbers, return the maximum.

2. Pseudocode (Most Popular in DSA)

Algorithm FindMaximum(A, n)
    // A is array of n numbers
    max ← A[1]
    for i ← 2 to n do
        if A[i] > max then
            max ← A[i]
        end if
    end for
    return max

3. In C-like Code

int findMax(int arr[], int n) {
    int max = arr[0];
    for(int i = 1; i < n; i++) {
        if(arr[i] > max) {
            max = arr[i];
        }
    }
    return max;
}

Another Classic Example: Check if a Number is Even or Odd

Pseudocode

Algorithm IsEven(number)
    Input: number (integer)
    if number mod 2 == 0 then
        Output "Even"
    else
        Output "Odd"
    end if

Properties Check

Property Status Reason
Finiteness ✓ Yes Only 1–2 steps
Definiteness ✓ Yes Clear instructions
Input ✓ Yes One integer
Output ✓ Yes “Even” or “Odd”
Effectiveness ✓ Yes Simple modulo operation

Bad Examples: What is NOT an Algorithm?

These are NOT valid algorithms:
  • "Sort the array somehow" → Not definite
  • "Keep trying random numbers until you find the answer" → No finiteness (may never stop)
  • "Solve the Traveling Salesman Problem optimally for 100 cities" → Effectiveness issue for large input

Understanding algorithm properties is the foundation for writing correct, reliable, and efficient code.

Up Next: Learn Time Complexity — How to measure how fast an algorithm runs.

💡 Practice: Write pseudocode for everyday tasks like “making tea” or “searching a contact”.