Pointers Practice

By: Idel Martinez, Jerrett Longworth

Let’s try some example problems using pointers!

Basic Pointer Operations

  1. What will this program print if executed?
#include <stdio.h>

int main(void)
{
  int x = 7;
  int *pointer = &x;

  x = 10;
  *pointer = 75;
  printf("x = %d\n", x);

  return 0;
}
  1. What will this program print if executed?
#include <stdio.h>

int main(void)
{
  int n = 43;

  printf("%d\n", *&*&*&*&*&*&n);

  return 0;
}

Function Scope

  1. What will this program print if executed?
#include <stdio.h>

// This function adds 1 to an integer.
void change_number(int n)
{
  n++;
}

int main(void)
{
  int x = 6, y = 9;

  printf("Before:\n");
  printf("x is %d\n", x);
  printf("y is %d\n", y);

  change_number(x);
  change_number(y);

  printf("After:\n");
  printf("x is %d\n", x);
  printf("y is %d\n", y);

  return 0;
}
  1. What will this program print if executed? What is the difference between this and the previous problem?
#include <stdio.h>

// This function adds 1 to an integer.
void change_number(int *n)
{
  *n++;
}

int main(void)
{
  int x = 6, y = 9;

  printf("Before:\n");
  printf("x is %d\n", x);
  printf("y is %d\n", y);

  change_number(&x);
  change_number(&y);

  printf("After:\n");
  printf("x is %d\n", x);
  printf("y is %d\n", y);

  return 0;
}
  1. What will this program print if executed? What is the difference between this and the previous two problems?
#include <stdio.h>

// This function adds 1 to an integer.
void change_number(int *n)
{
  (*n)++;
}

int main(void)
{
  int x = 6, y = 9;

  printf("Before:\n");
  printf("x is %d\n", x);
  printf("y is %d\n", y);

  change_number(&x);
  change_number(&y);

  printf("After:\n");
  printf("x is %d\n", x);
  printf("y is %d\n", y);

  return 0;
}

More Advanced Function Scope

  1. Fill in this swap() function to swap the values referenced by two pointers.
#include <stdio.h>

void swap(int *a, int *b)
{
  // Your code here
}

int main(void)
{
  int num, ber;

  num = 68;
  ber = 70;

  printf("%d %d\n", num, ber);
  swap(&num, &ber);
  printf("%d %d\n", num, ber);

  return 0;
}
  1. What will this program print if executed?
#include <stdio.h>

int functiomatic(int *n)
{
  *n = *n + 1;
  return *n + 1;
}

int main(void)
{
  int b;
  int *c;

  b = 17;
  b = functiomatic(&b);
  c = &b;

  printf("*c = %d\n", *c);

  return 0;
}
  1. What will this program print if executed?
#include <stdio.h>

int functiomatic(int n)
{
  int m = n + 1;
  return n;
}

int main(void)
{
  int b;
  int *c;

  b = 17;
  b = functiomatic(b);
  c = &b;

  printf("*c = %d\n", *c);

  return 0;
}

Pointers and Arrays

  1. What does array contain after line 13? What will this program print if executed? Assume that any uninitialized variables contain garbage values.
#include <stdio.h>

#define LENGTH 10

int main(void)
{
  int array[LENGTH];

  // Initialize array
  for (int i = 0; i < LENGTH; i++)
  {
    array[i] = i * 10;
  }

  // What does this array now contain?

  // Print array
  for (int i = 0; i < LENGTH; i++)
  {
    printf("%d\n", *(array + i));
  }

  return 0;
}
  1. What will array contain after line 34? What will this program print if executed? Assume that any uninitialized variables contain garbage values.
#include <stdio.h>

#define LENGTH 10

void initialize_array(int *arr, int length)
{
  for (int i = 0; i < length; i++)
  {
    arr[i] = 50 + i;
  }
}

void print_array(int *arr, int length)
{
  printf("[");

  for (int i = 0; i < length; i++)
  {
    printf("%d", arr[i]);

    if (i < length - 1)
    {
      printf(", ");
    }
  }

  printf("]\n");
}

int main(void)
{
  int array[LENGTH];

  initialize_array(array, LENGTH - 2);

  // What does this array now contain?

  print_array(array, LENGTH);

  return 0;
}

Pointers and Structs

  1. Create a declaration of a struct called car that contains some information about a car: a string (char pointer) called make that indicates its manufacturer, a string called model that indicates its model, an integer called year that indicates its year of production, and a double called mpg that indicates its fuel efficiency in miles per gallon.

  2. Rewrite the following line of code using the dereference (*) and the direct member access (.) operator.

my_struct->number = 5;
  1. What is wrong with the following print_planet() function? How can you fix it?
struct planet
{
  char name[30];
  double radius_km;
  char color[30];
};

void print_planet(struct planet p)
{
  printf("Name: %s\n", p->name);
  printf("Radius: %lf\n", p->radius_km);
  printf("Color: %s\n", p->color);
}
  1. What will the following program print if executed?
#include <stdio.h>

struct pizza
{
  int radius_inches;
  double price;
  char toppings[100];
};

void initialize_pizza(struct pizza *p)
{
  p->radius_inches = 0;
  p->price = 0;
  p->toppings[0] = '\0';
}

void print_pizza(struct pizza p)
{
  printf("Radius: %d inches\n", p.radius_inches);
  printf("Price: $%0.2lf\n", p.price);
  printf("Toppings: %s\n", p.toppings);
}

int main(void)
{
  struct pizza four_meat_pizza;

  initialize_pizza(&four_meat_pizza);
  print_pizza(four_meat_pizza);

  return 0;
}
  1. What will the following program print if executed?
#include <stdio.h>
#include <string.h>

struct pizza
{
  int radius_inches;
  double price;
  char toppings[100];
};

void initialize_pizza(struct pizza *p)
{
  p->radius_inches = 0;
  p->price = 0;
  p->toppings[0] = '\0';
}

void print_pizza(struct pizza p)
{
  printf("Radius: %d inches\n", p.radius_inches);
  printf("Price: $%0.2lf\n", p.price);
  printf("Toppings: %s\n", p.toppings);
}

int main(void)
{
  struct pizza four_meat_pizza;

  initialize_pizza(&four_meat_pizza);

  four_meat_pizza.radius_inches = 13;
  four_meat_pizza.price = 9.99;
  strcpy(four_meat_pizza.toppings, "Pepperoni, beef, sausage, and bacon");

  print_pizza(four_meat_pizza);

  return 0;
}

Pointers and Structs (Advanced)

  1. Suppose I am teaching an introduction to C programming class at a university, but for some reason this university didn’t pick a system to store course material and grades. (Something called “Webcourses” was suggested, but it didn’t seem to catch on.) That said, now I want to create my own system of storing student grades. I have some basic code to get started, but it needs some work. How can I set the names of my 3 students in class to “Alice”, “Bob”, and “Charlie”?
struct gradebook
{
  char student[128];
  int grades[10];
  double avg;
};

typedef struct gradebook gradebook;

int main(void)
{
  int n = 3;
  gradebook cop3223[n];

  // Your code goes here
}
  1. Fill in the blanks to give students names, calculate their grade average, and print it out.
struct gradebook
{
  char student[128];
  int grades[10];
  double avg;
};

typedef struct gradebook gradebook;

void calculate_student_avg(gradebook *class, _____)
{
  double sum;
  for (int i = 0; i < _____; i++)
  {
    sum = _____;
    for (int j = 0; j _____ 10; j++)
    {
      sum += class[i].grades[_____];
    }
    class[i].avg = sum / _____;
  }
}

void print_averages(_____, _____)
{
  for (int i = 0; _____; i++)
  {
    printf("Average for _____ is %lf\n", _____, _____[i].avg);
  }
}

int main(void)
{
  int n = 3;
  gradebook cop3223[n];

  // Filling in student grades with data
  for (int i = 0; i < n; i++)
  {
    for (int j = 0; j < 10; j++)
    {
      cop3223[i].grades[j] = (i + 1) * (j + i);
    }
  }

  // Add student names
  _____(cop3223[0].student, "Frances E. Allen");
  _____(__________________, "Barbara Liskov");
  _____________________________________________
  // Find average
  calculate_student_avg(_____, n);

  // Print average
  print_averages(_____, _____);
  
  return 0;
}
  1. I want to create a bank that handles a single person’s money, no one elses. This person’s balance is divided into how many dollars and cents they have. However, when I run this program (1) the person can withdraw more money than it has, (2) they can deposit a negative amount, (3) they lose all of their cents, (4) their balance is sometimes completely weird, and (5) the deposit and withdraw functions don’t work. How can I solve this?
struct balance
{
  int dollars;
  int cents;
  double total;
};

void deposit(struct balance *checking, double amount)
{
  (&checking).dollars += amount;
}

void withdraw(struct balance *checking, double amount)
{
  *checking.dollars -= amount;
}

int main(void)
{
  struct balance my_checking;

  deposit(&my_checking, -100);
  withdraw(&my_checking, 1E6);

  deposit(&my_checking, 145.54);
  withdraw(&my_checking, 0.01);

  return 0;
}