Lesson 3: Input/Output and Program Structure

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

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

Syntax:

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

Conversion specifications:

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

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

Console I/O (conio.h)

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

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