08.ArraysAndVectors

Computer Science I COSC 1020 89 GEORGETOWN UNIVERSITY

Chapter 8: Arrays and Vectors

• Arrays are foundational for organizing data in computer programs.

• Different programming languages may use different conventions for indexing.

• Notable figure: Donald Knuth, expert on algorithms, humorously remarks on varying indexing practices.

What's an Array?

• Definition: A data structure that holds multiple values of the same type.

• Access: Indexing allows ordering by assigning positions to elements.

• Terminology: Variables can hold one value, while arrays hold collections of related values.

Declaring an Array

• An array declaration includes:

  • Type: Specifies data type of stored values.

  • Identifier: The name used to reference the array.

  • Size: Indicates how many values the array can hold.

• Example: float studentOne_quizScores[6];

• Memory Allocation: Compilers allocate arrays in a single block based on size and type at compile time.

Accessing Array Elements

• Distinction between referring to the entire array and individual elements.

• Indices start at zero—a convention in many languages:

  • Indexing example for accessing quiz scores: studentOne_quizScores[1] = 20;

• Special property: Elements are sequentially stored in memory.

Array Bounds

• Valid indices range from 0 to size - 1.

• No automatic bounds checking in C++—invalid indices lead to memory access errors, potentially causing crashes.

• Example demonstrating unsafe access can lead to undefined behavior.

Common Errors

• The off-by-one error is common, particularly when writing loops.

  • Correct usage: for (int i = 0; i < SIZE; i++)

• Initialization: Arrays need proper initialization; uninitialized elements hold garbage values.

Initializing Arrays

• Methods of initialization include:

  • Individual assignment during operations.

  • Assignment during declaration with initialization lists.

  • Example: string months[12] = {"Jan", "Feb", ...};

Processing Arrays

• Arrays often processed with loops, typically for loops, to iterate over each element.

• Operations examples: Summation, averaging values, and finding extremes all involve iterating through the elements.

Arrays vs. Parallel Arrays

• Parallel arrays maintain separate but related data sets using the same index.

• Example: One array holds quiz one scores, another holds quiz two scores, linked by the student's index.

Two-dimensional Arrays

• Defined with two sets of brackets to represent rows and columns, for example: double scores[3][4];

• Use case: Can be visualized as tables of data where each element requires both row and column indices.

STL: std::vector

• A standard template library container that provides dynamic arrays (vectors) with additional capabilities:

  • Allows resizing during runtime, can grow as needed.

  • Provides random access like arrays but includes bounds checking through methods such as .at().

Accessing and Modifying Vectors

• Vectors can be accessed through square bracket notation or .at() which provides error handling for invalid indices.

• Vectors support increasing size with push_back() and retrieving the size using .size(), among others.

• Can also empty a vector using clear(), removing all elements efficiently.