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Lecture 7: C++ Arrays and Vectors ​

Arrays (C-Style) ​

Definition & Purpose ​

An array is a data structure used to store multiple values of the same data type in a single variable.

Declaration and Initialization ​

There are several ways to declare and initialize an array.

1. Declare with size, then assign values
If an array is declared without initial values, specifying its size is mandatory.

cpp
string syllabus[3];
syllabus[0] = "C++";
syllabus[1] = "Java";
syllabus[2] = "Database";

NOTE

If an array is declared without initial values, specifying its size is mandatory.
Failure to do so (string syllabus[];) will result in a compilation error.

2. Declare and initialize simultaneously

cpp
int numbers[5] = {10, 20, 30, 40, 50};

3. Omit size during initialization
The compiler automatically determines the size based on the number of elements.

cpp
string cars[] = {"Volvo", "BMW", "Ford"};

Accessing and Modifying Elements ​

Elements are accessed using a zero-based index number inside square brackets [].

cpp
string syllabus[] = {"C++", "Java", "Database"};

// Accessing
cout << syllabus[0]; // Outputs "C++"

// Modifying
syllabus[1] = "SQL"; // Changes "Java" to "SQL"

Looping Through Arrays ​

Iterate through an array using a counter variable for the index.

cpp
string syllabus[] = {"C++", "Java", "Database"};

for (int i = 0; i < 3; i++) {
  cout << syllabus[i] << endl;
}

for-each Loop (C++11 and later) ​

A simpler syntax for iterating over each element without using an index counter.

cpp
int myNumbers[5] = {10, 20, 30, 40, 50};

for (int i : myNumbers) {
  cout << i << endl;
}

Array Size Management ​

Calculating Array Size programmatically ​

To make loops more robust, you can calculate the number of elements using sizeof.

cpp
int myNumbers[] = {10, 20, 30, 40, 50, 60};

// Total bytes / bytes of one element
int length = sizeof(myNumbers) / sizeof(myNumbers[0]);

for (int i = 0; i < length; i++) {
  cout << myNumbers[i] << endl;
}

Limitations of C-Style Arrays ​

  • Fixed Size: Cannot be changed after definition.
  • No Built-in Functions: Lacks utilities like push_back() or size().
  • No Bounds Checking: Accessing arr[10] on a size-5 array leads to undefined behavior and may crash the program.
  • Inflexible: When passing an array to a function, its size must typically be passed as a separate argument.
  • Difficult for Dynamic Data: Adding and removing elements requires manual shifting of all subsequent elements.

Multi-Dimensional Arrays ​

Definition and Syntax ​

A multi-dimensional array is an array of arrays.

Initialization and Access ​

cpp
// [Rows][Columns]
string letters[2][3] = {
  { "A", "B", "C" },
  { "D", "E", "F" }
};

// Accessing "C"
cout << letters[0][2];

// Modifying "A" to "Z"
letters[0][0] = "Z";

Vectors (Resizable Arrays) ​

Definition and Purpose ​

For dynamic data, C++ provides the std::vector container. Vectors are resizable arrays that can grow and shrink as needed.

Key Features ​

  • Found in the <vector> library.
  • Size is dynamic, not fixed.
  • Comes with many useful built-in functions.

Basic Usage ​

cpp
#include <iostream>
#include <vector>
using namespace std;

int main() {
  // Declare and Initialize
  vector<string> syllabus = {"C++", "Java", "Database"};

  // Add an Element
  syllabus.push_back("Python");

  // Modern way to get size
  cout << "Size: " << syllabus.size() << endl;

  return 0;
}