Lab 13
CSci 127: Introduction to Computer Science
Hunter College, City University of New York
Spring 2020

Today's lab will continue our introduction to C++, covering more data types and control structures.

Software tools needed: web browser and the C++ compiler, gcc and a graphical editor, such as gEdit.

Quizzes & Code Reviews

See the quiz & code review page for details of the work due this week.

Using Python, Gradescope, and Blackboard

See Lab 1 for details on using Python, Gradescope, and Blackboard.

Variables in C++

In C++ (and C, Java and many other languages), you must first state (or 'declare') a variable's name and its type before using. For example,

int num;

declares a variable, called num of type int (integer or whole number). If you do not request the space in advance in C++, you will get an error. This requirement (as well as the `typing' of variables) is to avoid errors where misspellings access the wrong variable and are never caught. It also makes the underlying design of the compiler (the program that converts your program to an executable file) simpler.

There are many types of variables availabe in C++. Here are a few that we will use:

• int: The most common type used for storing whole numbers.
• float: Similar to float in Python, this type holds real numbers.
• bool: Stands for 'Boolean' and holds a value of either true or false.
• char: To hold a single character value. Note: we won't discuss strings in C++, but see the reading if you're interested (will be covered in detail in CSci 135/156)>

(here is a chart with a few more common types).

Decisions in C++

As with Python, we can make decisions that take different branches through the code. The basic structure is:

if (condition)
{
  ...
}
else
{
  ...
}

The syntax of Python and C++ are very similar, but there's a few differences to keep in mind when translating from Python to C++.

• The test or condition must be in parenthesis (this was optional in Python).
• There is no colon (':') at the end of the lines with the if and the else.
• Instead of indenting, C++ uses the curly brackets ('{' and '}') to indicate blocks of code. If you only have a single command, you can leave off the brackets.
• Similarly, most commands need to end with a semi-colon (';'). This is a bit of annoyance, but on the plus side, we can put multiple commands on a single line (instead of worrying about spacing and indenting).
• There is no elif in C++, instead you would write else if.

Here's a sample program. Read through it first, and guess what it does:

//Name:  CSci 127 Teaching Staff
//Date:  November 2017
//A program demonstrating if-else statements in C++
#include <iostream>
using namespace std;

int main()
{
  int num;
  cout << "Hello!" << endl;
  cout << "Enter a number: ";
  cin >> num;
  if (num % 2 == 0)
  {
    cout << "Even number!\n";
  }
  else
  {
    cout << "Odd number\n";
  }
  return 0;
}

Let's look in more depth at the code:

• We start by greeting the user and reading in a number:

    int num;
    cout << "Hello!" <, endl;
    cout << "Enter a number: ";
    cin >> num;
  

  Since we declared the type to be int, when we read in the number, we do not have to cast it to an integer. Instead, it is converted automatically to be a whole number.

• Much of the arithmetic carries over from Python. The code above uses the modulo, or remainder, operator to test if the number entered is even or odd and prints a message accordingly.

    if (num % 2 == 0)
    {
      cout << "Even number!\n";
    }
    else
    {
      cout << "Odd number\n";
    }
  

• Note that C++ uses the curly brackets for blocks of code, but that it is 'good programming style' to indent your code to make it readable. As a rule of thumb, if you would be required to indent in Python, you should indent in C++ (not required for the code to run but considered 'good style').

How would you modify this program to print out a different message based on the month? For example, if the user entered the month (as a number), you should print out "Happy Winter" if it is smaller than 3 or larger than 11, "Happy Spring" for 3 through 6, "Happy Summer" for 7 and 8, and "Happy Fall" otherwise. See Programming Problem List.

Indefinite Loops in C++

Now that we have discussed conditionals and definite loops, we can introduce indefinite loops. They follow a similar format to those in Python:

while (...condition...)
{
  command1;
  command2;
  ...
}

Again, we do not have the colon (':') at the end of the line with while, but we do end individual commands with a (';').

Take the Python code from Lab 10 that asks the user to enter their age (and continues checking until they enter a non-negative number):

and translate it into C++. An easy way to approach this is to start with the C++ program above that demonstrates if statements and modify it to handle the indefinite loop for checking ages. See Programming Problem List.

More Useful Unix Commands

Here are a few more commands, especially useful when you're using a public machine, and trying to figure out where things are and what is installed:

• which: Will tell which version of the program it's using by default (introduced last lab). For example,

  which g++
  

  will show the location of the g++ program.
• man: This gives the built-in help pages for the current system. It can be quite useful if you don't remember all the options for a Unix command. For example,

  man sort
  

  will print out information about the command line sort.
• w: This command was designed for multi-user machines and tells you who is logged in and what they are doing. While you are the only one on your laptop, this command is still useful in that summarizes all the activity on the machine in a short table as well as how long the system has been running ('uptime'). So, if the machine seems sluggish or you're wondering how much else is going on, it will give a concise summary.
• more: Allow you to look at output or a file, one screenful at a time. This is incredibly useful when you are trying to read information that scrolls off the screen. Here's a sample of looking at all programs (from /usr/bin-- a common place to keep them) :

  ls /usr/bin | more
  

  The pipe ('|') is explained in Lab 12.

  Say we're trying to figure out what GNU C/C++ compilers are on the machine, and as a start want to see all the possible programs that contain "cc" (anywhere in their name):

  ls /usr/bin | grep cc | sort | more
  

  We don't see g++ in the list, so, let's also look for all that start with g:

  ls /usr/bin | grep ^g | sort | more
  

  The grep ^g asks for all names that start with "g". grep is the pattern matching command mentioned briefly in Lab 9 (for an overview see the grep wiki page).

What's Next?

If you finish the lab early, now is a great time to get a head start on the programming problems due early next week. There's instructors to help you, and you already have Python up and running. The Programming Problem List has problem descriptions, suggested reading, and due dates next to each problem.