C++ Standard Output

If you recall, we have been using the cout object to display information on to the screen. Below, I’ll describe various ways to output strings, the contents of variables, expressions, and more.

Basic output with endl

Below is a simple example of printing out the string “Hello, World!” to the terminal accompanied by a newline.

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// We first print out Hello World 
cout << "Hello, World!";
 
// We then add the following to create a new line
cout << endl;

The previous example is demonstrated above.

Basic output with variables

Now, we are going to display the contents of a variable. Notice how we can output more than one item with a single cout object.

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int num = 5;
 
cout << num << endl;

Basic output with math expressions

We can also output arithmetic expressions. This by the way, is something you will hardly ever use but its good to know anyways since you’ll always want to save the result of an expression in a variable.

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cout << 5 * 4 + 3 << endl;

Basic output with boolean expressions

Finally, we will demonstrate a simple example with a boolean expression. This will not print out true or false. It will print out either a 1 for true, or 0 for false. You also need the parenthesis, else the < operator will give you an error.

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cout << (1 < 3)  << endl;

C++ Standard Input

For standard output we used the object cout and for standard input, we will use the object cin. Before we start with examples, there is are a couple of things you must know.

• User input is usually done through the keyboard and the terminal. Your program will halt at each user input and keep executing when the user enters a value and then presses enter
• The cin object will read in the type of object you tell it to. Therefore, if you wish to have a char, it will read a char. Same with an integer, float, and a string. If you mix things up, you will receive undesired behavior
• The user might provide you with invalid values and therefore require you to do error checking. We will leave this for a later topic.

Reading in a value

Below is an example of reading in a single and multiple values. Try it by changing the object type.

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// We declare and initialize the variable num to -1
int num = -1;
 
// We prompt the user for input
cin >> num;
 
// We declare three strings
string a, b, c;
 
// We read three strings from input
cin >> a >> b >> c;

Going further with input and output

Now that you understand the basics of input and output why not do the following?

1. Prompt the user for his First Name, Last Name, Age, City, and State in the following fashion.

   Enter your first name:
   Enter your last name:
   Enter your age:
   Enter your city:
   Enter your state:

   
   
   Then, output the results in the following fashion.

   Hello firstname lastname!
   I see you are age years old and live in city, state.

2. Now try declaring a variable without initializing it. Then output it. What happened?
3. Try reading in an int value, but give it a string. Output the variable and what happens?

1. Installing through the terminal (easiest)
2. Installing through the Synaptic Package Manager

Installing Open in Terminal support to Nautilus in Ubuntu

The easiest method is to install it within the terminal window.

Method 1: Installing Open in Terminal Support through the Terminal

Simply enter this in your terminal.

sudo apt-get install nautilus-open-terminal

Enter your password if prompted, and then restart Nautlius. You can do so using the following through the terminal as well.

killall nautlius

Once this is done, you should see this option within Nautilus. See above? I simply opened the current directory within the terminal. Didn’t that save some time?

Method 2: Installing Open in Terminal Support through Synaptic Packet Manager

1. Open the Synaptic Packet Manager

System -> Administration -> Synaptic Package Manager

2. You can do a quick search for nautilus-open-terminal and mark it for installation by right clicking on the entry.

3. Apply the changes, then close Synaptic Package Manager, log off and then back on.

After logging in, you should see the option available when you right click in Nautilus.

Add Open Command Prompt Here Functionality To Nautilus In Ubuntu via Watching The Net

Format of Command Line Arguments

This is how you would write a simple program without command line arguments.

#include  
using namespace std;  

double square(int number) {
     return number*number; }  

int main() {
     int number;
     cin >> number;
     cout << square(number) << endl;
     return 0;
}

To run this program you’d do the following in the terminal. ($ denotes your input)

$./a.out
$10        //Your input
 100      //The output

Using Command Line Arguments

This is how you would use command line arguments.

#include 
 using namespace std;  

double square(int number) {
     return number*number; }  

int main(int argc, char* argv[]) {
      int number = atoi(argv[1]);      //Convert our c-string to int using the atoi function
      cout << square(number) << endl;
      return 0;
} 

To run this, you’d simply do the following in the terminal.

$./a.out 10 //One line in the terminal
100 //Output

How does command line arguments work?

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

The int argc parameter denotes the number of arguments passed in.
The char* argv[] parameter denotes a char pointer to a c-string.

So, typing the following:

$./a.out 10 45

Would produce the following:

In other words, the arguments are passed in as c-strings reason for why we converted our c-string to an int using the atoi function.

Things to do with Command Line Arguments

You may pass in any type of arguments. Try the following challenges to learn more.
Challenge 1: Create a program using command line arguments where you pass in your name, and two numbers, adds the numbers together and displays them both in the terminal.
Challenge 2: Create a program that accepts one int and returns the square of the number as shown above. The difference is that you need to error check your program. In other words, if they don’t pass in an int value or more than one int value, you should terminate the program. Hint: Use argc, and if statements.

Need more help? Go ask in the forums where our committed community will surely answer all your questions!

Below I’ll list a few examples with comments denoted by #

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#Displays contents in folder
ls 
 
# Changes Directory 
cd FOLDERNAME
 
# Go up one directory
cd ..
 
# Go to your home directory
cd
 
# Make directory
mkdir FOLDERNAME
 
# Remove a file
rm -- FILENAME
 
# Copy a file
cp ORIGINALFILE NEWFILE
 
# Move a file
mv SOURCE DESTINATION
 
# Install Program in Ubuntu and other Linux Distros
sudo apt-get install PROGRAMNAME
 
# To run a program you usually specify the name, 
# if text editor, its usually name of program and 
# text file to create, if no file is specified, the text editor still opens.
firefox
emacs hello_world.cpp
 
# Compile a program using g++ 
g++ hello_world.cpp
 
# Execute program 
./PROGRAMNAME

An example of finding help for a certain command is the following:

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man mkdir

The man command will give you the manual page for a given command.

Example using the terminal

Let’s say we want to view the contents of a folder, create a folder called Example, create a text file using emacs, compile it, run it, and go back to our home directory. To do this, we would simply do the following.

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ls
mkdir C++
cd C++
emacs hello_world.cpp
g++ hello_world.cpp
./a.out 
cd

Yes there are numerous ways to achieve this, its just a matter of how you want to navigate the terminal.

What now?

If you feel another basic command needs to be here or need more help, simply drop in a line in the comments box! I’ll be sure to put it down.

Expect advanced terminal commands in the near future.

How to write and compile Hello World in C++

First, let’s locate and open up our handy terminal. The terminal is the console on where you will initially learn how to create folders in order to create the programs that we will later compile. Depending on your OS your Terminal or Console might be opened in various ways. The image below demonstrates how to open it in Ubuntu.

Installing the appropriate tools

We might have to install the tools necessary to start writing, and compile our code. In order to do this we must specify some commands to install our tools. If you already have the programs installed, it will either upgrade them, or do nothing so either way there is no harm in trying this out.

The following commands simply give permission to install the packages we need.

sudo apt-get install g++ 
sudo apt-get install emacs

If it asks you for a password, simply provide the password you log in with. And if it asks you for what version of emacs, simply type in emacs22 for now.

Other linux distributions may have different methods for installing these tools. If so, use their Package Managers to download them.

Creating a folder and creating the Hello World program file

Now, let’s create a new folder and open a file in that folder to write our Hello World program. I’ll explain the commands shown on the screen in more detail below the image.

The following command lists the contents of the current directory.

ls

The following command makes a directory with the name FOLDERNAME.

mkdir FOLDERNAME

The following command changes your current directory to FOLDERNAME.

cd FOLDERNAME

The following command opens up the program emacs with the file hello_world.cpp. If not created it will create it for you.

emacs hello_world.cpp

Writing Hello World

With emacs open, simply click on the editor and it should turn blank. Write the following code for your Hello World program. When done, simply hit save. (It will save in the folder you called emacs hello_world.cpp)

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#include <iostream>
using namespace std;
 
int main() { 
 
     cout << "Hello World!\n";
     return 0;
}

Your editor should look like the image above.

Compiling our Hello World Program

Now that you have your program, let’s compile it so it can be written in machine language that can later be executed.

g++ hello_world.cpp

The following command will execute your program.

./a.out

Congratulations! You are done! I tried making it as simple as possible. If you still have any questions, feel free to drop a comment or suggestion to make this tutorial better!

Useful resources

C++ Guide Your guide for programming in C++