C++ Basics: Variables, I/O, Data Types, Control Flow & Style (Chapter 2 Notes)

C++ Basics: Variables, I/O, Data Types, Control Flow & Style

2.1 Variables and Assignments

Variables

• Variables in C++ are like names for memory locations.

  • We can write a value into them.

  • We can change the value stored there.

  • Memory locations are never truly "erased"; some value is always present.

• Analogy: Small blackboards where one can write, change, or erase a number (though erasing isn't quite accurate for computer memory).

• Example (Display 2.1): A basic C++ program often uses variables like int number_of_bars; and double one_weight, total_weight; to store and manipulate numerical data.

Identifiers

• Variable names are referred to as identifiers.

• Rules for Choosing Variable Names (Identifiers):

  • Should be meaningful and represent the data they store (e.g., number_of_bars).

  • The first character must be:

    • A letter (a-z, A-Z).

    • The underscore character (_).

  • Remaining characters (after the first) must be:

    • Letters.

    • Numbers (0-9).

    • The underscore character (_).

Keywords (Reserved Words)

• These are words that are used by the C++ language itself.

• They have predefined meanings and must be used as defined by the programming language.

• Cannot be used as identifiers (variable names) because that would cause ambiguity for the compiler.

Declaring Variables

• Variables must be declared before they can be used in a program.

• Declaration tells the C++ compiler the type of data the variable will store.

• Examples of Data Types:

  • int: An abbreviation for integer. Used for whole numbers without fractional components.

    • Can store values like 3, 102, 3211, -456. (e.g., int number_of_bars;)

  • double: Represents floating-point numbers with a fractional component.

    • Can store values like 1.34, 4.0, -345.6. (e.g., double one_weight, total_weight;)

• Placement of Declaration: Variables can be declared immediately prior to their first use.

• Declaration Syntax:

  • Type_Name Variable_Name;

  • Type_Name Variable_1, Variable_2, ...; (for declaring multiple variables of the same type in one statement).

• Declaration Examples:

  • int number_of_widgets, count;

  • double size;

Assignment Statements

• An assignment statement is used to change or set the value of a variable.

• Syntax: variable = expression;

  • Assignment statements always end with a semicolon (;).

  • The single variable whose value is being changed is always on the left of the assignment operator (=).

  • The right side of the assignment operator can be:

    • A constant: age = 21;

    • Another variable: my_cost = your_cost;

    • An expression: circumference = diameter * 3.14159;

• Example: total_weight = one_weight + number_of_bars;

  • This statement calculates the sum of one_weight and number_of_bars, then stores that result into the total_weight variable.

Assignment Statements and Algebra

• The =' operator in C++ is not equivalent to the equal sign in algebra.

• In C++, variable = expression; means to evaluate the expression on the right and then store the resulting value into the variable on the left.

• Example: number_of_bars = number_of_bars + 3;

  • This statement is mathematically false in algebra.

  • In C++, it means: take the current value of number_of_bars, add 3 to it, and then store this new result back into number_of_bars.

Initializing Variables

• Declaring a variable does not automatically give it a value; its memory location might contain arbitrary "garbage" data.

• Initialization: The process of giving a variable its first value.

• Variables are commonly initialized through assignment statements.

• Combining Declaration and Initialization:

  • Method 1 (Assignment-like syntax):
    int number_of_bars = 32;

  • Method 2 (Parentheses/Constructor-like syntax):
    double mpg(26.3), area(0.0), volume;
    Note: In this method, volume is declared but not initialized. Only mpg and area receive initial values.

2.2 Input and Output

Data Streams

• A data stream is a sequence of data, typically characters or numbers.

• Input Stream: Data that a program uses, typically originating from the keyboard or a file.

• Output Stream: Data that a program produces as its output, typically destined for the monitor or a file.

Output using cout

• cout is the standard C++ output stream, by default sending data to the monitor.

• The insertion operator << is used to insert (send) data into cout.

• Example: `cout << numberofbars <<