BSIT Survival Kit

The C Preprocessor

Lines that start with # in a C file are preprocessor directives — handled before compilation begins.

`#define`

Replaces a symbolic name with a value throughout the file. By convention, constants are written in UPPERCASE.

#define LIMIT 100
#define PI 3.14159

Changing the #define in one place takes effect everywhere — makes programs easier to update.

`#include`

Inserts the contents of another file (a header file) at that point during compilation. Header files end in .h.

#include <stdio.h>   // standard input/output: printf, scanf
#include <conio.h>   // console I/O: getch, clrscr

Standard Output: printf()

printf() sends formatted text to the screen. The "f" stands for "formatted."

Syntax:

printf("format string", variable1, variable2, ...);

Conversion specifications:

Specifier	Meaning
`%c`	Character
`%d`	Decimal integer
`%f`	Floating point (float)
`%lf`	Double
`%e`	Scientific notation
`%s`	String

Field width — specified between % and the conversion character:

printf("%c %3c %5c\n", 'A', 'B', 'C');
// A printed normally, B right-justified in 3 chars, C in 5

Standard Input: scanf()

scanf() reads formatted input from the keyboard. The & operator (address-of) is required before each variable.

Syntax:

scanf("%conversion_spec", &variable);

Examples:

scanf("%d", &num);       // read an integer
scanf("%f", &x);         // read a float
scanf("%lf", &y);        // read a double
scanf("%c", &ch);        // read a character
scanf("%s", name);       // read a string (no & for arrays)

scanf() returns the number of successful conversions. When reading characters, whitespace is not skipped automatically (unlike numbers).

Escape Sequences and Console I/O

Escape Sequences

Sequence	Meaning
`\n`	Newline
`\t`	Tab
`\\`	Backslash
`\"`	Double quote
`\'`	Single quote
`\a`	Bell (sound)

Console I/O (`conio.h`)

Function	Behavior
`gets(str)`	Reads a string until Enter; no newline stored
`getchar()`	Reads one character; waits for Enter
`getch()`	Reads one character without echo; no Enter needed
`getche()`	Reads one character with echo; no Enter needed
`puts(str)`	Writes string to screen + newline
`putchar(ch)`	Writes one character
`clrscr()`	Clears the screen

General C Program Structure

global declarations;

function1() {
    local variables;
    statements;
}

main() {
    local variables;
    statements;
}

Annotated sample program:

#include <stdio.h>
#include <conio.h>

main() {
    char c1, c2, c3;
    int i;
    float x;
    double y;

    printf("\nInput three characters, an int, a float, and a double:");
    scanf("%c %c %c %d %f %lf", &c1, &c2, &c3, &i, &x, &y);

    printf("Here is the data you entered:\n");
    printf("%3c %3c %3c %5d %12f %12lf", c1, c2, c3, i, x, y);

    return 0;
}

Key parts:

#include <stdio.h> — includes standard I/O declarations
main() — entry point; every C program starts here
/* ... */ — comment; ignored by compiler
{ and } — mark start and end of a function body
; — terminates every statement
return 0; — exits main() and signals success

Practice Exercises — Lesson 3

Try It: Input/Output

Modify num1 and num2 to simulate different inputs and observe the formatted output.