Squares Function and Header Files

Functions

• A function needs implementation before invocation
• A function prototype needs inclusion before calling the function as another option
• The function prototype represents the header, minus the body
• Function prototypes let the compiler validate function calls
• Prototypes of functions are placable in a separate header file with a .h extension
• The #include "file.h" directive includes a header file into a program

Squares Function Example

• The example includes the stdio.h and Math.h in this Driver.c example
• A function named squares uses a double type parameter
• Inside the main function, the program prompts for a number and calculates squares by calling the squares function in a loop
• The squares function returns the square of a given number using the pow function

Squares Header File Example

• Driver.c has standard library headers like stdio.h, Math.h, and Windows.h included
• Driver.c has a custom header file named "Squares.h" included
• Squares.h contains only the declaration of the squares function with the void return type, it serves as a function prototype

Preprocessor - #include

• The C preprocessor executes before compilation
• Actions performed include file inclusion, definition of constants and macros, conditional compilation, and conditional execution
• All preprocessor directives start with a #

Preprocessor #include directive

• The #include preprocessor directive puts a copy of a specified file into the directive's place of the directive
• The directive comes in two forms: #include and #include "filename"
• A filename in quotes searches in the same directory as the file, used for the programmer-defined headers
• Angle brackets around the filename (< and >) specify standard library headers searched through the designated system or compiler directories

Preprocessor - #define

• The #define directive creates symbolic constants
• #define identifier replacement_text replaces all identifier occurrences before compilation
• Macros, are a type of functions symbol that can be defined with or without arguments
• Arguments in a macro are substituted in the replacement text, which will replace the identifier and argument list in the program

Preprocessor - #define macros

• The code demonstrates macro definitions for PI and calculating the area of a circle
• In the past, macros with inline code avoided function calls, but now compilers inline function calls or c inline keywords
• Another code snipper defines a MAX macro that returns the greater of two arguments

#include <stdio.h>
#define MAX(a,b) ((a)>(b)?(a):(b))
void main() {
	printf("Maximum between 10 and 20 is: %d\n", MAX(10,20));
}

Preprocessor - #undef

• The #undef preprocessor directive is used for undefining a macro
• The syntax is "#undef MACRO_NAME" where MACRO_NAME is the symbolic constant or macro that has to be undefined

Preprocessor - Conditional Compilation

• Conditional compilation directives determine whether a constant is defined already, such as #if !defined(PI)
• These directives use #define to define a constant if it is not already defined
• #ifdef checks if a constant is defined, while #ifndef checks if a constant is not defined

Arrays

• Arrays represent a data structure with containing data items of the same data type
• Arrays are static entities, that maintain the same size throughout program execution
• Arrays are a contiguous memory location
• The syntax array[index] accesses array element
• Example code shows array and element assignment, such as int c[] = {1, 5, 3, 78};
• Arrays are not automatically initialized to zero
• Initializing the array's first element ensures zero initialization for all remaining elements

Arrays Example

• The example initializes one element and ensures zero initialization for all remaining elements
• The code displays garbage values due to the array initialization state
• The size_t type helps represent the difference in RAM locations
• size_t does not rely on data structure size

Arrays Syntax

• Macros, such as _countof(array), get the number of elements inside an array
• It returns sizeof(array) / sizeof(array[0])

Arrays – Security Issue

• C lacks array bounds checking
• Executing programs may "walk off" an array without warning
• All array references should stay within array bounds

Strings

• Strings are character arrays
• char string[] = "object"; is an example of initializing string
• The array contains an extra element \0 for a null terminator called the String termination character
• scanf reads characters until a whitespace like space, tab, or newline is reached
• The statement fgets (string2, 10, stdin) will read until new line

Strings example

#include <stdio.h>
int main() {
char a[10];
fgets(a, 10, stdin);
for (int i = 0; i < 10 && a[i] != '\0'; i++)
	printf("%C - ", a[i]);
}

Multidimensional Arrays

• Arrays in C allow multiple indices
• The syntax int b[2][3] identifies 2D array by row and column indices
• Example values are int b[2][3] = {{1, 2, 3}, {4, 5, 6}};