Writing your first Program in C

Now that we know the elements and operators, let's look at how to structure a program that interacts with the user.

6.1 Basic I/O: printf() and scanf()

printf() - Standard Output

Used to display information on the screen. Syntax: printf("Format String", variables);

printf("The value of X is %d", x);

scanf() - Standard Input

Used to read input from the keyboard. Syntax: scanf("Format Specifier", &variable_address);

scanf("%d", &age); // The '&' is the address-of operator

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6.2 The Compilation Process (Revisited)

As we write more complex programs, it's important to understand what happens when you click "Compile":

1. Preprocessing: Lines starting with # are processed (headers included, macros replaced).
2. Compilation: C code is converted into Assembly Language.
3. Assembly: Assembly code is converted into Machine Code (binary instructions).
4. Linking: Multiple object files and libraries are linked together to create the final Executable File.

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6.3 Standard Library Functions

C provides many built-in functions via header files:

• stdio.h: Standard I/O (printf, scanf, fopen)
• math.h: Mathematical functions (sqrt, pow, sin)
• ctype.h: Character testing (isdigit, isalpha)
• string.h: String manipulation (strlen, strcpy)

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6.4 Practical Example: Simple Interest Calculator

Let's write a program that takes user input to calculate Simple Interest.

#include <stdio.h>

int main() {
    float principal, time, rate, si;

    printf("--- Simple Interest Calculator ---\n");

    printf("Enter Principal amount: ");
    scanf("%f", &principal);

    printf("Enter Time (in years): ");
    scanf("%f", &time);

    printf("Enter Rate of Interest: ");
    scanf("%f", &rate);

    // Calculation
    si = (principal * time * rate) / 100;

    printf("\nThe Simple Interest is: %.2f\n", si);

    return 0;
}

Common Errors to Avoid:

1. Missing Semicolon (;): Every statement in C must end with a semicolon.
2. Missing & in scanf(): Without the address-of operator, scanf doesn't know where to store the data.
3. Wrong Format Specifier: Using %d for a float or %f for an int will result in garbage values.

Refer to the Lecture Slides for a diagram of the I/O buffer mechanism.