A First Look at C++

I'd like to see some actual C++ code!
Okay! We'll begin with a program in a (hopefully) more familiar language, and then move to C++.

Let's begin with a simple “Hello” program that

Asks someone their name and greets them
Asks their favorite number and tells them something about that number.

Python Code

Here's the program in Python:

# A function to test if a number is prime, takes an integer and returns a
# boolean.
def isPrime(n):
    if n < 2:
        return False
    # Try dividing by all numbers from 2 to sqrt(n).
    for i in range(2, int(math.sqrt(n)) + 1):
        if n % i == 0:
            return False
    # If we get here, we didn't find a factor, so n is prime.
    return True

# Main program starts here.

name = input("What is your name? ")
print("Hello, {}!".format(name))
number = int(input("What is your favorite number? "))
print("Your favorite number is {}".format(number), end='')
if isPrime(number):
    print(", and that's a prime number!")
else:
    print(". It's not a prime number, but that's okay!")

Run This Code

Click the button below to open this code in the website “Online GDB” (in a new tab).

Open Code in Online GDB

You can click the green “Run” button in the ribbon at the top of the page. The window at the bottom of the page will show the program running, and you can interact with it there.

If you want to make changes to the code, you can click the “Fork this” button in the bar at the top.

I actually remember Java better than Python.
If you prefer, you can also look at a Java version of this program in Online GDB.

C++ Code

Here is the same program in C++:

#include <iostream>
#include <string>

// This is an example of a function definition, this function takes an integer
// as input and returns a boolean.
bool isPrime(int n) {
    if (n < 2) {
        return false;
    }
    // Try dividing by all numbers from 2 to sqrt(n).
    for (int i = 2; i*i < n; ++i) {
        if (n % i == 0) {
            return false;
        }
    }
    // If we get here, we didn't find a factor, so n is prime.
    return true;
}

int main() {
    std::cout << "What is your name? (one word only please) ";
    std::string name;
    std::cin >> name;
    std::cout << "Hello, " << name << "!" << std::endl;

    std::cout << "What is your favorite number? ";
    int number;
    std::cin >> number;
    std::cout << "Your favorite number is " << number;

    if (isPrime(number)) {
        std::cout << ", and that's a prime number!" << std::endl;
    } else {
        std::cout << ". It's not a prime number, but that's okay!" << std::endl;
    }

    return 0;
}

I can sort-of see the connection but there are a bunch of weird things here.
The Online GDB version adds extra comments explaining various things.

Run This Code

Open Code in Online GDB

Run the code, but also read it over. There are various comments explaining some of the unfamiliar things.

One thing I noticed is that is someone had a name like “Jean Marie” it won't accept it, it thinks “Marie” is the number and reads zero.
That's right. The >> operator reads the next “word”. But you can comment out the line that says
```
std::cin >> name;
```
and uncomment the line that says:
```
std::getline(std::cin, name);
```
Give it a try!
Remember that you need to click the “Fork this” button at the top to make changes.

Make some change to the C++ code (e.g., the one suggested above) and rerun it.

In the next section, we'll look at the differences between variables in Python and C++.
Hmm… I wonder what this code would look like in other languages…
Just for fun (if you're curious), here's the same program in Swift, Go, Rust, C (actually C89), Haskell and even Pascal.
Meh, if it won't be on the test, I'm not clicking.

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CS 70

Warning: Missing Prerequisites

A First Look at C++

Python Code

Run This Code

C++ Code

Run This Code

Differences Between Python and C++

Question

Instructor Controls