Lab 1: Introduction to C++ Solution

This is an introduction to C++ through some simple activities. You should also read the C++ Primer in your textbook, and practice as much as possible.




Your work for this must be finished and shown to your lab TA by the end of this lab session or the start of your next session.




First you'll need to login to your CS ugrad account (it will look like “r2d2” or “c3p0a”). If you don't have one yet, login with userid “getacct” (with no password) to obtain it. (And if you're reading this before your lab, webpage https://www.cs.ubc.ca/getacct/ does the same thing) Once you're logged in, at the command line enter the following to create a directory (folder) to hold your cs221 files, another subdirectory for this lab, and to navigate there (the semi-colon allows several commands on one line):




mkdir cs221; cd cs221

mkdir lab1; cd lab1




First you’ll compile and run two programs that demonstrate input and output in C++.







Q1. Terminal (keyboard) I/O: Copy and paste the lines from sample_code.cc.txt. There are many ways to do this. For this lab just open a web browser and navigate to the course Lab/Lecture Notes webpage, click on Code Samples from the Lab (it works from here too), highlight the lines and copy them to the clipboard. Now back at the command line, enter:




gedit q1.cc




Be patient, it might complain about some sub -process failing but it will invoke a fairly intuitive GUI text editor. Paste the saved lines (Ctrl -V even on a Mac) from the clipboard into the empty file. If you've been following along with the commands, your current directory should be ~/cs221/lab1 and if it is, then just save and close. If it isn't, “Save As” instead (i.e. save q1.cc in ~/cs221/lab1) and after you exit gedit, enter the following to make ~/cs221/lab1 your current directory:




cd; cd cs221/lab1




These are the lines that should be in q1.cc




#include <iostream // needed for cin and cout

float circle_area(float radius); // declare function prototype




int main(void) {

float circle_radius;

std::cout << "Enter radius:" << std::endl; std::cin circle_radius;




std::cout << "Area is: " << circle_area(circle_radius) << std::endl; return 0;

}

float circle_area(float radius) {

return 3.14159 * radius * radius; // = pi * r^2

}




You can check to see what is actually there by entering:




cat q1.cc







To compile your program, enter:




g++ -Wall q1.cc -o q1
CPSC 221, Lab 1 2/5







After you get a clean compile, run your program by entering:




./q1




That's period-slash-executable. Your sub-directory isn't in the default PATH, so you have to tell the machine where to find your executable. The period says “this sub-directory”, the slash is just the usual delimiter after subdirectory names, and “q1” is the name of the file you told g++ to put the output in (that's what the “-o q1” did in the compile command above).







Q2. File I/O: First create a file called “infile.txt” containing at least 6 lines of text,




and another file called “q2.cc” containing the following code. Compile and run as in Q1.




#include <iostream

#include <string

#include <fstream




int main(void) {

std::ifstream in ("infile.txt"); // input file-stream

std::ofstream out ("outfile.txt"); // output file-stream

std::string ss;




getline() puts next line in ss and discards any newline characters while (getline(in, ss))



out << ss << std::endl; // add each line to the new file, appending endl std::cout << "End of program" << std::endl;

return 0;

}




Notice that these files are called “streams”. In Q1, std::cin used “” to extract a value from the input stream and store it in a variable. An ofstream (or std::cout) uses the “<<” operator to add to the output stream.




Also, a string Object is used to hold each line of text. You might have heard of a C-string, which is a sequence of char that terminates in a NULL char (0x00), and might have seen them used in programs:




char* err_msg; // in some C or C++ program




A string Object and a C-string are not the same thing at all. The former has constructors and methods, while the latter is actually just the address of one char – by convention we interpret this as a C-string by including all characters up until a 0x00 (\0) is encountered.




To check that q2 copied the file correctly, compare input.txt and output.txt side-by-side by entering:




sdiff infile.txt outfile.txt







Q3. Arrays. Write another program called “q3.cc”, and in it declare a global array with 10 elements. (Compile and run as in Q1 and Q2.)




Write a function called “fill_array” (with no parameters) to fill the elements of the global array with the numbers 1 through 10. Call this function from main() and after it returns, print the contents of the array to the screen.



Modify the function (and its call) so that it accepts two integers as parameters. The first integer represents the value to be assigned to the first element, the second integer represents the
CPSC 221, Lab 1 3/5




increment between each element. As before, the contents of the array are printed to the screen after fill_array() returns to main(). Only your work for part (b) needs to be checked by your TA.




For example (Part b): fill_array(4,2) fills the array with the numbers 4, 6, 8, 10, 12, 14, 16, 18, 20, 22. fill_array(0,5) will fill it with 0, 5, 10, 15, 20, 25, 30, 35, 40, 45




Your fill_array() function prototypes should look like:




void fill_array();

void fill_array(int first_value, int increment);




// for part (a)

// for part (b)



And your calls to fill_array in main should look like:



fill_array(); fill_array(4, 2));




// for part (a)

// for part (b) - you can use other numbers if you want









Q4. Towers of Hanoi, Recursion. (from Wikipedia) The Towers of Hanoi is a mathematical game or puzzle. It consists of three pegs (called A, B and C), and a number of disks (n) of different sizes which can slide onto any peg.




The puzzle starts with the disks neatly stacked in order of size on peg A, the smallest at the top, thus making a conical shape. The objective is to move the entire stack to peg C, obeying the following rules:




Only one disk may be moved at a time.




Each move consists of taking the upper disk from one of the pegs and sliding it onto another peg, on top of the other disks that may already be present on that peg.

No disk may be placed on top of a smaller disk.




Your task is to write code to solve the Towers of Hanoi problem for n disks. HINT: Use recursion.




To move n disks from A to C:




Recursively move n − 1 disks from A to B. This leaves disk n alone on peg A.



Move disk n from A to C.
Recursively move n − 1 disks from B to C so they sit on disk n.



Don't forget to check the base case when using recursion. (What is the base case in this problem?)




Your program should take a small integer “n” as input from the command line and invoke the moveDisks() function using code like this:




#include <cstdlib // for atoi

#include <iostream

// prototype

void moveDisks(int n, const char* FROM, const char* VIA, const char* TO);




int main(int argc, char *argv[]) {

if (argc != 2) {

std::cout << "Usage: " << argv[0] << " number_of_disks" << std::endl; return -1;

}




moveDisks( atoi(argv[1]), "peg A", "peg B", "peg C" ); return 0;

}

// put your moveDisks() function here
CPSC 221, Lab 1 4/5




It should produce output to solve the problem with n disks. The output of my “hanoi 3” is:




Move disk from peg A to peg C

Move disk from peg A to peg B

Move disk from peg C to peg B

Move disk from peg A to peg C

Move disk from peg B to peg A

Move disk from peg B to peg C

Move disk from peg A to peg C







Q5. Simple Guessing Game. Write a program that simulates a guessing game. It should randomly generate a number from 1 to 100, and ask the user to input a guess. The game should keep running until the user gets the number correct, or otherwise indicates that they wish to end the game.




You will need the following functions:




http://www.cplusplus.com/reference/cstdlib/srand/




use srand(0) until your program is debugged



http://www.cplusplus.com/reference/cstdlib/rand/



http://www.cplusplus.com/reference/cstdlib/atoi/




alpha-to-integer






(see the next page for the Appendix)


CPSC 221,
Lab 1
5/5
APPENDIX:
SOME OTHER USEFUL COMMANDS AND TIPS


cd ..
- change directory to the parent of the current directory


cd ../..
- change directory to the parent of the parent of the current directory
cd
- change directory to your home directory (the same as where you are




when you first log in)


ls
- list the files in the current directory


ls -alF
- list all info about the files in the current directory, one per line
ls -1 DIR.txt - that's the number “1’’ (and “-alF” has the letter: “l”)




- this will list the names of the files in the directory, one per line


and put the output in DIR.txt (this can be useful when creating the


initial version of a README.txt file, for example)


cat DIR.txt - print the contents of DIR.txt to the screen


rm fn.ext
- remove (delete) the file named “fn.ext” from current directory


rm -f *.*
- remove all files in current directory (CAREFUL) without confirmation
rmdir dir
- remove directory named “dir” (which MUST be empty)


bye
- log off








USE UP-ARROW AND DOWN-ARROW TO CYCLE THRU THE COMMAND-LINE HISTORY


TO HANDIN AN ASSIGNMENT NAMED “SampleProg” TO “cs221”:


cd


- takes you to your home directory


cd cs221


- name of directory MUST be equal to Course Name


mkdir SampleName
- name of directory MUST be equal to Assignment Name
cp ../workDir/*.* SampleName
- where “workDir” has the files you want to handin


cd SampleProg




ls -alF


- list all the file


rm *.o


- also delete any other extra files that got copied


cd ../..


- change back to your home directory


handin cs221 SampleName










You CAN use XCode or VisualStudio or Eclipse or any other IDE, but we do NOT support these and your code MUST COMPILE AND EXECUTE on the ugrad machines (outside of any IDE).




Assuming you have an Internet connection, to remotely log in to your ugrad account:




If you a running a Mac, you can just open a Terminal window and include your CS userid such as:

ssh r2d2@remote.ugrad.cs.ubc.ca

Of course you can also compile and run your code locally (in Terminal).




If you are running Windows, you'll need “Xmanager” (available free from the Department at https://my.cs.ubc.ca/docs/free-terminal- emulation-software-xmanager, just follow the instructions on that page) to access your ugrad account remotely. To compile and run locally, you can use Cygwin, a *nix machine emulator which runs under Windows, from https://www.cygwin.com/ -- to do so, select the “Devel” category or you will not be able to compile anything (if you forget, you can install it later, without doing the entire thing again).




However (from http://www.ugrad.cs.ubc.ca/~cs221/current/computing.shtml):

We recommend editing and compiling from the server using, e.g., gedit, jedit (cross-platform, since it's in Java!), or kate or the challenging but rewarding emacs or vim. The same gedit used in this lab allows you to edit your source files remotely, and opening another X11 Application allows you to make (compile) and run your programs.