CSS Computer Science (Optional) — Paper-I, Section-A, Topic II subtopic. How code is organized into reusable, callable units, and how data moves in and out of them.

Why Use Functions

• Modularity: breaks a large program into smaller, manageable pieces
• Reusability: write once, call many times
• Abstraction: caller only needs to know what a function does, not how
• Easier debugging/testing: each function can be tested in isolation

Anatomy of a Function

int add(int a, int b)   // prototype/signature: return type, name, parameters
{
    int sum = a + b;    // function body
    return sum;         // return statement -- sends a value back to the caller
}

int main()
{
    int result = add(3, 4);   // function call, with arguments 3 and 4
}

• Function prototype/declaration: tells the compiler the function's name, return type, and parameter types before it's used — int add(int, int);
• Function definition: the actual body/implementation
• Function call: invokes the function with specific arguments
• Parameters (in the definition) vs Arguments (values passed at the call site) — a common terminology mix-up
• A function with return type void returns no value; return; (with no value) is optional in that case

Standard Library vs User-Defined Functions

• Standard library functions: pre-built, reusable functions provided by the language (e.g., strlen(), sqrt())
• User-defined functions: programmer-written, promote modularity and reuse

Parameter Passing

• Pass by value: a copy of the argument is passed; changes inside the function do NOT affect the original variable

void increment(int x) { x++; }   // caller's variable unchanged after this returns

• Pass by reference: the function receives a reference/alias to the original variable; changes DO affect the original