Why Functions?

• Name a task; hide its steps.
• Reuse without copy–paste.
• Test in isolation.
• Enable teamwork via clear interfaces.

Value vs Reference

void inc(int n){ n++; }
int x=5; inc(x); // x ?

void inc(int& n){ n++; }
int x=5; inc(x); // x ?

Pass by value Pass by reference

Recursion (Factorial)

int fact(int n){
    if(n==0) return 1;
    return n*fact(n-1);
}

Step through the call stack (n=4):

Reset Step

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Mini‑Quiz (1 minute)

Check

Boss Battle: Fibonacci

long long fib_rec(int n){ return n<2? n : fib_rec(n-1)+fib_rec(n-2); }
long long fib_it (int n){ long long a=0,b=1; while(n--){ long long t=a+b; a=b; b=t;} return a; }

Time n=35

(JS simulates timing to contrast exponential vs linear ideas.)

When you replace copy–paste with a function…

Abstraction: fewer moving parts to break.

Summary

• Functions provide names, contracts, and reuse.
• Prefer `const&` for large inputs; return results, not side‑effects.
• Overload prudently; keep interfaces minimal.
• Recursion needs a base case; iterative alternatives often cheaper.
• Lambdas enable on‑the‑spot behavior.