While everyone that starts CS102 has some sort of programming experience, mostly Java, we will be begin with some very basic C++ programs before we ramp up quickly. This reading will cover the basic skeleton of a C++ program, basic input and output, data types, and control flow (loops and conditional statements) compared to Java.

The Basic C++ Skeleton

While Java is a pretty verbose, even to write a simple program with a single output line, C++ slims things down. Below I’ve written an example Hello World program, and you can see that none of the public static void main mess is there, nor do you have to force main inside a class.

#include <iostream>
using namespace std;

int main() {
    cout << "Hello World" << endl;
}

To break this down…

Include Statements

#include <iostream> is similar to an import statement. Just like we have to import the Scanner for reading input in Java, we need to #include the library to use cin (reading input) and cout (printing output). You’ll hear #include out loud as “pound include” (not hashtag include). In Java, you might hear a TA or me say “don’t forget your import statements” - the equivalent informal C++ saying is “don’t forget your pound includes”.

Namespaces

using namespace std; means we are using the namespace called std, which is an abbreviation of “standard”. You might hear someone refer to this as “using the standard namespace”. I assume you might be asking, “What the heck is a namespace?”

You don’t really need to know much about this, but essentially it’s a way of organizing variable/function/class names. cin and cout are a part of the standard namespace, and if you don’t have this line in your program, you would need to clarify them in the code by writing std::cout or std::cin everywhere you would normally type cout or cin.

We won’t ever make our own namespaces and won’t use anything besides the standard namespace, but it is possible to make your own. Let’s say you make two different namespaces, n1 and n2. Within each namespace, all the variables must have unique names (like we are used to - no naming conflicts!). However, a second namespace can use those same names all over again. Let’s say both namespaces have an x variable. The x in namespace 1 would be written as n1::x and the x in namespace 2 would be written as n2::x.

Kind of silly, but there’s a lot of disagreement over whether using the standard namespace is “good practice”. You’ll find out pretty quickly in this field that people can be very opinionated on what might seem to be trivial topics (like using tab versus spaces). In this class, we will use it. The more we can reduce the visual clutter as you learn, the better.

The Main Function

This is the last part of the skeleton! int main() {} is the main function, the “starting point” where your code will begin executing, just like the main function in Java. You will put your code inside the curly braces.

One small difference between Java and C++ here: if you remember, the main function in Java was void; it didn’t have a return value. Here, main actually returns an integer, but it’s kind of special and not enforced. As you can see in the “Hello World” program, I didn’t return an integer and your computer would have no complaints. As we get further into the course though, we will start returning either a 0 or 1 from main. Traditionally, main returns 0 if it completes with no issues, and it returns a 1 if it’s terminated because of an error.

Input and Output

As shown above in the example, you will use cout to print output to the screen. cin is used to receive input. Read the C++ reference guide for input/output (stop after “Cin and Strings”). https://cplusplus.com/doc/tutorial/basic_io/

Example Programs

Minor (but Common) Differences: Java vs C++

Besides the “skeletal” differences above with main, there are a few other common hiccups between Java and C++ for beginning programmers.

The first one is that the data type string is not capitalized in C++ (it is in Java). Here’s an example of declaring some strings in C++:

string firstName, lastName;
string streetName, streetType;

Another is that printf() can be used exactly like System.out.format() in Java, but if you want to print a formatted string, you must change it to a c-style string using .c_str() (more on what this is later, refers to the C language vs C++).

For example,

System.out.format("Name: %s GPA: %.2f", firstName, gpa);

is equivalent to

printf("Name: %s GPA: %.2f", firstName.c_str(), gpa);

Any other data type (like floats above) don’t need changing.

Another Java/C++ difference is that switch statements cannot be switched on strings. See the switch notes for more information here.

Another is that final in Java is called const in C++. There are more uses of const that we will get to later in the course, but if you want to make a variable read-only, this is how you do it. Conventionally, you should have read-only variable names as screaming snake case.

You can put const before or after the type name in C++.

Example (Java):

final int SHORT_GAME = 5;

Example (C++):

const int SHORT_GAME = 5;
int const LONG_GAME = 10;

One last one: we got used to closing the Scanner when we are done receiving input. You don’t close cin - one less thing to worry about!

There are other differences when you learn more material, but it is comes to using I/O, conditional statements, and loops, these are the issues I see students run into the most. Thankfully, the syntax for all types of loops, if statements, and switch statements are exactly the same!