Add a demo of functions

02_control_structures/Makefile

@@ -5,7 +5,7 @@ CXX = g++
 CFLAGS = -Wall -O3
 
 .PHONY: all
-all: loops
+all: loops functions
 
 # Here I'm using some of Make's built in variables: $@ and $<. The former gets substituted with the
 # name of the target. In this case, that's "loops". The latter gets substituted with the first
@@ -14,6 +14,9 @@ all: loops
 loops: loops.c
 	$(CC) $(CFLAGS) -o $@ $<
 
+functions: functions.c
+	$(CC) $(CFLAGS) -o $@ $<
+
 .PHONY: clean
 clean:
-	rm loops
+	rm loops functions

02_control_structures/functions.c

+#include <stdio.h>
+
+int is_even(int x) {
+    return x % 2 == 0;
+}
+
+int is_odd(int x) {
+    return x % 2 == 1;
+}
+
+int is_prime(int x) {
+    for (int i = 2; i <= x / 2; ++i) {
+        if (x % i == 0) {
+            return 0;
+        }
+    }
+    return 1;
+}
+
+int factorial(int x) {
+    int result = 1;
+    for (int i = 2; i <= x; ++i) {
+        result *= i;
+    }
+    return result;
+}
+
+int n_choose_k(int n, int k) {
+    // Note: if you actually want to compute binomial coefficients, there are better ways to do it.
+    // This method will overflow except for very small n and k.
+    return factorial(n) / (factorial(k) * factorial(n - k));
+}
+
+int main(void) {
+    int two_is_even = is_even(2);
+    int three_is_odd = is_odd(3);
+    int four_is_prime = is_prime(4);
+    int five_factorial = factorial(5);
+    int six_choose_four = n_choose_k(6, 4);
+
+    printf(two_is_even ? "two is even\n" : "two is odd\n");
+    printf(three_is_odd ? "three is odd\n" : "three is even\n");
+    printf(four_is_prime ? "four is prime\n" : "four isn't prime\n");
+    printf("5! = %i\n", five_factorial);
+    printf("6 choose 4 = %i\n", six_choose_four);
+}

02_control_structures/loops.c

@@ -28,6 +28,7 @@ int main(void) {
     while (1) {
         printf("another while loop: %i\n", i);
 
+        // This one is pretty obvious, but it's arguably the most important control structure.
         if (i == 2) {
             printf("leaving the while loop\n");
             break;