Introduction to "Fun" C (using GCC)

functions.c

/**
 * This are a collection of examples for C 201.
 * These combine concepts you may or may not be
 * familiar with and are especially useful for
 * students new to C. There is a lot of really
 * cool stuff you can do in C without any cool
 * languages.
 * 
 * This is file in particular is an introduction
 * to fun function usage in C.
 */

#include <stdio.h>

int sum(int a, int b) {
    return a + b;
}

int sub(int a, int b) {
    return a - b;
}

// This function "get_operator" takes a *char expression
// and returns a function that takes two ints and returns
// an int.
int (*get_operator(char* expression)) (int, int) {
    int i;
    // char pointers are automatically given a final character '\0'
    // to allow us to know when the char* ends.
    for (i = 0; expression[i] != '\0'; i++) {
        switch (expression[i]) {
            case '+':
                return sum;
            case '-':
                return sub;
        }
    }
}

void print_operator(char* expression) {
    // get_operator will return a function that takes two ints
    // and returns an int.
    int (*operator)(int, int) = get_operator(expression);
    // sum is automatically converted to a pointer,
    // you could also say "operator == &sum", but that is longer.
    if (operator == sum) { // comparing functions!
        printf("Expression %s is a sum.\n", expression);
    } else if (operator == sub) { // comparing functions again!!
        printf("Expression %s is a sub.\n", expression);
    } else {
        printf("Expression %s has an unknown operation.\n", expression);
    }
    // Challenge:
    // Instead of just printing out which operation it is,
    // find the two operands and perform the operation on them.
    // Then print the result of the expression instead.
}

int main() {
    char* expression1 = "2 + 2";
    char* expression2 = "5 - 3";
    char* expression3 = "9 * 7";

    print_operator(expression1);
    print_operator(expression2);
    print_operator(expression3);
}

functions2.c

/**
 * This is an example of more complex function usage in C.
 * Read structs.c before continuing.
 */

#include <stdio.h>

// Good reference:
// http://www.dirac.org/linux/programming/tutorials/function_pointers/

typedef struct {
    int legs;
    void (*sayName)(void);
} Animal;

void catSayName() {
    printf("I am a cat.\n");
}

void dogSayName() {
    printf("I am a dog.\n");
}

int sub(int a, int b) {
    return a - b;
}

int sum(int a, int b) {
    return a + b;
}

double sum_d(double a, double b) { return a + b; } // would fail if passed to operate: operate(sum_d, 1, 2)

int operate(int (*f)(int, int), int a, int b) {
    return f(a, b);
}

void do_nothing(void) {
    return;
}

int main() {
    Animal cat;
    cat.legs = 4;
    cat.sayName = catSayName;

    Animal cat1;
    cat1.legs = 4;
    cat1.sayName = catSayName;

    Animal dog;
    dog.legs = 3;
    dog.sayName = dogSayName;

    cat.sayName(); // I am a cat.
    dog.sayName(); // I am a dog.
    cat.sayLegs(cat.legs);

    int my_sum = operate(sum, 1, 2);
    printf("%d\n", my_sum); // Prints 3
    int my_sub = operate(sub, 11, 2);
    printf("%d\n", my_sub); // Prints 9

    do_nothing();
}

pointers.c

/**
 * This is a collection of pointer examples.
 * It is a fun way to test yourself. I'm
 * sorry I couldn't think of a better way
 * to show the answers.
 */

#include <stdio.h>

void print(int i) {
    printf("%d\n", i);
}

int main() {
    int* myInt;
    int myIntsValue = 3;
    myInt = &myIntsValue; // int* somePointer; *somePointer = myIntsValue; myInt = somePointer;
    print(*myInt); // prints 3
    int mySecondInt = 2;
    *myInt = mySecondInt;
    print(*myInt); // prints 2
    print(mySecondInt); // prints 2
    print(myIntsValue); // prints 2
    myInt = &mySecondInt; // int* somePointer; *somePointer = mySecondInt; myInt = somePointer;
    mySecondInt = 5;
    print(*myInt); // prints 5
    myIntsValue = 7;
    print(*myInt); // prints 5
    mySecondInt = myIntsValue;
    print(*myInt); // prints 7
}

pointers2.c

/**
 * This is a continuation of (more complex)
 * pointer examples.
 */

#include <stdio.h>

void print(int i) {
    printf("%d\n", i);
}

void swap1(int a, int b) {
    int temp = b;
    b = a;
    a = temp;
}

void swap2(int* p_a, int* p_b) {
    int temp = *p_b;
    *p_b = *p_a;
    *p_a = temp;
}

int main() {
    int a = 1;
    int b = 2;
    swap1(a, b);
    print(a); // prints 1
    print(b); // prints 2
    swap2(&a, &b);
    print(a); // prints 2
    print(b); // prints 1
    int* c;
    int* d;
    c = &a; // c = some address... but *c = 2
    d = &b; // d = some address... but *d = 1
    swap1(*c, *d);
    print(*c); // prints 2
    print(*d); // prints 1
    swap2(c, d);
    print(*c); // prints 1
    print(*d); // prints 2
}

structs.c

/**
 * This is a brief example of typedef-ing and structs in C.
 */

typedef int Int; // int or Int

struct Person {
    char* name;
    Int age;
};

typedef struct Person APerson; // struct Person or APerson

typedef struct Animal {
    int legs;
} Animal; // struct Animal or Animal

typedef struct {
    int mpg;
} Car; // cannot use struct Car, can only use Car... the struct is anonymous

int main() {
    APerson person; // could use "struct Person person"
    person.name = "Phil";
    person.age = 10;

    Animal cat; // could use "struct Animal cat;"
    cat.legs = 4;

    Car car; // could not use "struct Car car;" because the struct was anonymous
    car.mpg = 30;
}

Really nice writeup, thanks!

Wow, c is cool! thanks!

Good examples of function pointers. Saved.

thanks

Good examples! But there is no definition of 'sayLegs' function in functions2.c

Nice examples!

On line 24, it should be // and returns a pointer to a function that takes....

thanks, good example for function pointer