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Muhammad Atif Iqbal
Muhammad Atif Iqbal

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Arrays, Lists, Dicts, and Sets Across Python, C++, and JavaScript

#python #javascript #cpp #programming

Comparison of Arrays, Lists, Dicts, and Sets Across Python, C++, and JavaScript

✅ 1. Arrays vs Dicts in Each Language

Concept Python C++ JavaScript
Array / List list = [10, 20, 30] int arr[3] = {10, 20, 30}; or vector<int> arr = {10, 20, 30}; let arr = [10, 20, 30];
Dictionary / Map (Key-Value) d = {"a": 1, "b": 2} map<string, int> d = {{"a", 1}, {"b", 2}}; let d = {a: 1, b: 2}; or let d = new Map([["a", 1], ["b", 2]]);
Set (Unique elements, unordered) s = {1, 2, 3} set<int> s = {1, 2, 3}; let s = new Set([1, 2, 3]);

✅ 2. Why "Array" is called "List" in Python & JavaScript

  • In C++, array is a low-level fixed-size container. Example: 
  int arr[3] = {1, 2, 3}; // Size is 3, cannot change
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  • In Python and JavaScript, arrays are dynamic and resizable, so they are closer to C++ vector: 
  arr = [1, 2, 3]
  arr.append(4)   # can grow
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  let arr = [1, 2, 3];
  arr.push(4);   // can grow
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👉 That’s why Python calls it list, not array, because it’s flexible.
👉 JS still calls it Array, but it behaves like a Python list.

✅ 3. Why C++ Arrays are Declared with {} not []

  • Square brackets [] → specify size/index. 
  int arr[3]; // array of size 3
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  • Curly braces {} → specify values for initialization. 
  int arr[3] = {10, 20, 30}; // initialize
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So:

  • [] → size
  • {} → values

✅ 4. C++ 3D Array Example

A 3D array = array of arrays of arrays.
Example: a cube 2 × 3 × 4

#include <iostream>
using namespace std;

int main() {
    int arr[2][3][4] = {
        { {1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10, 11, 12} },
        { {13, 14, 15, 16}, {17, 18, 19, 20}, {21, 22, 23, 24} }
    };

    cout << arr[1][2][3];  // Output: 24
}
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👉 Think of it as a box with 2 layers, each having 3 rows, each row having 4 columns.

✅ 5. Sets in Each Language

  • Python 
  s = {1, 2, 3, 3}
  print(s)  # {1, 2, 3} (removes duplicates)
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  • C++ 
  #include <set>
  set<int> s = {1, 2, 3, 3};
  for (int x : s) cout << x << " ";  // 1 2 3
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  • JavaScript 
  let s = new Set([1, 2, 3, 3]);
  console.log(s);  // Set(3) {1, 2, 3}
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👉 Difference with {} in Python

  • {} in Python by default = dict (empty dictionary).
  • set() must be used for empty set. 
a = {}       # dictionary
b = set()    # empty set
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👉 In C++ {} is not a set—it’s array initializer.

✅ 6. Quick Example Comparison

Array / List

  • Python: arr = [10, 20, 30]
  • C++: int arr[3] = {10, 20, 30};
  • JS: let arr = [10, 20, 30];

Dictionary / Map

  • Python: {"x": 1, "y": 2}
  • C++: map<string, int> d = {{"x",1},{"y",2}};
  • JS: {x:1, y:2} or new Map([["x",1],["y",2]])

Set

  • Python: {1, 2, 3}
  • C++: set<int> s = {1, 2, 3};
  • JS: new Set([1, 2, 3])

✅ So summary:

  • C++ arrays = low-level, fixed size.
  • Python list / JS Array = dynamic array (like C++ vector).
  • Dict/Map = key-value store (different thing).
  • Set = unique elements in all three.

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