Where we left off

int main() {
    int x = 0;
    int z = 0;
    while (x < 5) {
        z += x * x;
        ++x;
    }
    cout << z << '\n';
    return 0;
}

Today's topics

• C++ program basics
  • Format and layout
  • Compiling vs interpreting
• Simple output
• Variables and data types

Textbook Sections 1.3, 2.1-2.3

A C++ Program

Every C++ program has exactly one "main" function

int main() {
    // execution begins here
    ...
    return 0; // ends here
}

Python vs C++

def main() -> None:
    print("Hello World!")

int main() {
    cout << "Hello World!\n";
    return 0;
}

Key points:

• Indentation is only for readability, with grouping indicated by braces {}
• Return type comes before function name (and is required)
• Statements are terminated with a semicolon ;

Recall: statements vs expressions

Which of the following are true about statements? Select all that apply.

Recall: statements vs expressions

Which of the following are true about expressions? Select all that apply.

Compiling vs Interpreting

• All computers only understand machine code, a sequence of 0s and 1s
• Compiling is when C++ is translated to a binary executable
• Interpreting is when Python is translated to machine code line-by-line

• Compiled code does not need the interpreter to run, and is often faster
• Interpreted code can be easier to debug

The g++ Compiler

• g++ is the GNU C++ compiler
• We feed our source code .cpp file into the compiler, and it spits out an executable (by default named a.out)
• To change the output name, specify -o flag, for example:
  g++ hello.cpp -o hello

Recall: error types

Syntax, runtime, and logic errors

def abs_val(some_var: int) -> int:
    if some_var < 0:
        soem_var = 0

    return some_var

int abs_val(int some_var) {
    if (some_var < 0) {
        soem_var = 0;
    }

    return some_var;
}

Simple Output

• While Python provides a lot of built-in functions, C++ is more modular
• To enable output, we need the following:
  • #include <iostream>
  • using namespace std;

cout << "Hello World!\n";

Side Tangent: Preprocessor directives and namespaces

• Lines beginning with # are preprocessor directives
• The preprocessor is a program that runs before the compiler
• #include tells the compiler that the named file should be included

• using namespace std; is a C++ statement
• Optionally, you could omit this line and write std::cout every time

Variables and Types

At a high level, variables in C++ and Python are similar.

• Both allow you to refer to a value by a convenient name
• Both can only store one thing at a time, with a new value replacing the old

x = 5    # int
x = "Meep" # str

int x = 5;
x = "Meep"; // error!

• In C++ variables must be declared with a type
• The type cannot change!

Variable definition in Python

Internally, there's a whole Rube Goldberg-esque process happening when you write x = 5 in Python. You end up with:

• namespaces
• objects
• references
• memory allocation

Variable declaration and initialization in C++

It looks similar to Python, but we need to be explicit about the type:

int x = 5;
x = "Hello world!"; // error, x can only be an int!

int x; // declaration
x = 5; // initialization
int x; // error! We already declared that x was an int

Separate or combined?

• From the compiler's perspective, there is no difference for primitive types
• My preference: declare and initialize on the same line
  • Less risk of uninitialized variables
  • Define variables when they are needed
  • C requires all declarations at the top of a function, so you may prefer this
• Multiple variables can be declared on one line, but this can be confusing

  int x, y, z;
  int a, b = 5, c; // Madness! Please don't do this.
  

• We'll revisit this idea when we discuss reading input

Variable naming

More or less the same restrictions as Python:

• Must start with a letter or underscore
• Can contain letters, numbers, and underscores
• Cannot be a reserved word (e.g. int, double, return)
• Case sensitive

Primitive Data Types

str vs char

• In Python, str is a sequence of characters
  • No difference between 'a', "a", 'abc', or ''
• In C++, char is a single character, or 1 byte integer
  • 'a' is a char, but "a" means something entirely different
  • A string literal behaves the same way as in Python

    cout << "Hello, world!\n";
    

  • ... but you can't declare and manipulate them the same way

    cout << "Hello, " + "world!\n"; // error!
    

What do you think will happen?

This code compiles and runs. Predict the output from the choices below:

int main() {
    char a = 'a';
    cout << 2 * a << '\n';
    return 0;
}

The assignment operator

Much like in Python, the = operator assigns the value of the expression on the right hand side to the variable on the left hand side:

int x = 5;
int y = x + 1;
x = x - 1;
y = 5 / 2;

Arithmetic operators

Arithmetic with mixed data types

• If both operands are int, the expression evaluates to int
• If at least one operand is double, the expression evaluates to double
• If a double is assigned to an int variable, it is truncated to an int (with an associated compiler warning)
• If an int is assigned to a double, it is promoted to a double

Practice!

For each of the following expressions, specify the data type of the result given i is an int and d is a double:

5 + i * 2 
d + i * 2
d / 9.33
7 / i
7.0 / i
42 + 7 / (i * 1.2)

A few new operators

Like Python, C++ has the compound assignment operators +=, -=, *=, /=, and %=. There's a few new ones as well:

• ++ and --: increment and decrement by 1
  • Can be either ++x or x++
• Unary operators: + and -
  • Finally you can write x + -5 instead of x - 5!
• ++ and -- happen first, then unary operators, then the usual BEDMAS

Coming up next

• Lab: Hello world
• Lecture: Continuing with C++ basics

Textbook Sections 2.4-2.5