Lecture 2

• Lecture 2 covers C basics
• C program structure
• Syntax
• Data types
• Variables

C Program Structure

• C source code programs are organized as a sequence of functions
• A function contains a logical sequence of statements
• A statement may call another function that must be declared or defined before being called
• Executable programs must contain the main() function
• Program execution will always begin from the main() function
• Executable programs are organized as a sequence of function blocks of machine code

C Program Model Structure

• The model incudes:
• preprocessor directives
• global variables
• function declarations
• main() with arguments and statements
• function definitions

C Program Example

• stdio.h is a preprocessor directive included
• int a is a declaration of a global variable
• int add (int, int) is a function declaration
• int main() is the start of block function defintion
• a=1 assigns the value of 1 to a
• printf("a+b=%d\n", add(a, b)) is a function, that when, called outputs a+b=3 to terminal
• return 0 indicates the end of block function definition
• return x+y is the function body for the add(int x, int y) function
• return x-y is the function body for the minus(int x, int y) function
• A large C program is decomposed into function header files, header implementation files, and a main function file (aka driver program file)

Basic Syntax

• C has 5 types of elements: symbol, keyword, expression, statement, function

Basic Symbols

• // lines indicate comments that compilers will ignore
• /* ... */ blocks indicate comments that compilers will ignore
• # indicates the preprocessor
• ; terminates a statement
• , separates items in a list
• Parenthesis () denotes a function's argument and parameter list, and also groups algebraic expressions
• Curly brackets {} define the scope of a program block

Keywords

• C has 32 reserved keywords
• Basic data types (9): char, int, float, double, short, long, signed, unsigned, void
• Define data types (4): typedef, struct, union, enum
• Modifiers (6): const, auto, static, extern, volatile, register
• Flow control (11): if, else, switch, case, default, goto, for, while, do, break, continue
• Function (2): return, sizeof

Expressions

• Uses infix notation, consisting of constants, variables, operators, and parenthesis
• Example (1 + 2) * 3, 1==2, (1==1) && (2!=1)

Statements

• A statement is a command or instruction to the C compiler
• Declaration, assignment, condition, function call, flow control are statement types
• Statements are organized into blocks (program blocks), a sequence of statements scoped by {}.

Function Syntax

• The function declaration/header syntax is returndatatype function_name(argument type list);
• The function defintion/implementation synthax is returndatatype function_name(argument type and name list) {}
• The function call syntax is function_name(parameter list)

Data Types

• A data type defines how a type of data value is represented in programming and computers.
• Data types also defines how many bytes and what bit pattern are used to represent the value in memory
• Data Types also defines what and how operations are applied to the data values in programming and computers.
• Data types defines how data values are represented and operated in programming and computers

Primary Data Types

• C provides primary data types (primitive, basic data types) which are defined by keywords
• Keywords are char, int, short, long, float, double, signed, unsigned
• Arithmetic operations (+, -, *, /) are defined for primary types
• The modular operation % is defined for non-floating primary datatypes
• Each of the primary data types has a corresponding bit pattern in representation and operations

Secondary Data Types

• C provides methods to build secondary data types (derived, extended) using primary data types and the keywords typedef, struct, union, and enum
• Each data type has a size, i.e., the number of bytes to stores the value of the type in memory
• Each data type has a defined valid value range
• The size of some data types is platform dependent

Data Type Size Examples

• char type has size 1, using 1 byte (or 8 bits), with a value range from -128 to 127
• int type has 2 bytes in old 16 systems but has 4 bytes in 32 or 64-bit systems
• The default size for the int type is 4 bytes

Size and Range of Datatypes

Char Data Type

• Presents characters by an integer defined by the American Standard Code for Information Interchange (ASCII)
• i.e, A is coded as 65, a is coded as 97, 0 is coded as 48.
• Each addressable memory cell holds 8 bits (1 byte)
• char type size is 1 byte and uses one addressable memory cell
• The ASCII code of character A is 65
• 65 in a binary representation is 100 0001
• A is stored in memory cell as 0100 0001

Int and Unsigned Int Variables

• int type value ranges from -2147483647 = -2^31 +1 to 2147483647 = 2^31 - 1
• int type bit pattern uses 4 bytes (32-bits)
• the leftmost bit represents the sigh: 0 for positive and 1 for negative
• The rest of the bits (31 bits) represent the absolute value in base 2 (binary format)
• unsigned int type value ranges from 0 to 4294967295 = 2^32 -1
• unsigned int type uses a bit pattern of 4 bytes or 32 bits, with 32 bites that represent the value in base 2

How Int Types are Stored

• Data types sized greater than 1 require a contiguous memory cells (called memory block) to store the value of the type
• Big-endian stores the most significant byte in the lowest addres cell
• Little-endian stores the least significant byte in the lowest address cell.
• Little-endian is commonly used
• int type size is 4, which utilizes and needs 4 memory cells

Float and Double Types

• float uses 4 bytes for single precision floating point numbers
• The bit pattern and operations are specified by IEEE 754 standard
• double uses 8 bytes for double precision floating point numbers
• The bit pattern and are specied by IEEE 754 standard

Variables

• A variable is a name identifier used in a source code program to represent a data value of a certain type
• A variable is assigned a memory block with relative address by the compiler
• Instructions are used to set and to get the values to a memory address
• A variable is instanced at runtime with an absolute address of a memory block
• A Variable is an identifier of a data value in a program that is allocated relative memory during compile time and actual memory blocks at runtime

C Variables

• The must be declared with a type and name in a scope before being used within the scope
• The variable declaration tells the compiler to assign a memory block with a relative address
• A variable assignment statment tells compilers to generate instructiongs for writing values to the memory block
• Using a variable tells compilers in an expression to generate instructions to read values from the memory block
• Variables should be initiated before used in expressions
• Variable names must start with a letter, and can be followed by letters, underscores, and numbers
• C variable names are case-sensitive

Variable Scope and Declaration

• Variables have to be desclared before a variable is used
• A scope is the area where a variable is declared
• Scopes can be nested such that one cope can be an inner scope of another
• A Variable declared before an innder scope can be used in the inner scope and vice versa.
• Types of Variablies:
• Global declared ouside ant scope, usable anywhere
• Local declared in a scope block embraced by {}, only usable in the scope block, i.e. in a function

Literals

• Refer to those constant values assigned to variables in programming
• The compiler recognizes the data types of a literal and convents to its bit patterns
• Pre-processors (#define) define a literal string via macro
• Macros can be used during preprocessing steps, being then replaced by their corresponding string

#define PI 3.1415926
float r = 4;
float area = PI*r*r;
float cf = 2*PI*r;
float f = 2.4e-5; // 2.4e-5 = 0.000024

Variable Assignments

• char c assigns 1 byte memory space for char variable c
• int a assigns 4 byte memory spaces for int variables a
• float f assigns value by generating instruction that stores 2 to a's memory at runtime
• c = 'a' compilers instruction that stores 97(base10) to memory of c
• f = 1.41 compiler convert 1.41 to a 31 bit single precision number and store at f's memory

Initializing Variables

• int a = 12; initializes int a = 12
• int a = 014; initializes a = 014 for Oct 14
• int a = 0xC; initializes a = 0xc for Hex_ number C
• char c = 'a'; Initializing a char variable with a char value
• float f = 1.41;, list separated by a comma is variables are of the same type

Sizeof Operator

• The sizeof() Operator is an unary operator used to get the sizes of data types, and can determine the memory size of a variable
• For example
• sizeof(char) returns 1
• sizeof(a) returns 4, the size of int type

Constants

• Constants are fixed data values in a program, using a constant variable to present the values
• Constant varibles are declared and initialized by the const keyword
• The compiler does not allow changing values of a constant variable that has been declared and initialized
• i.e., const float pi = 3.1415926;
• Compiler prevents assigning value to 3.14, like pi = 3.14 after declaring the value with const keyword

Explore variable declaration, scope, and literals in C. Understand the use of #define, const, and data types, including char and int. Practice examples and learn about integer representation.