Lecture 02: C++ Basics Continued

Where we left off

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

Today’s topics

• Named Constants
• Comments
• Input/Output
• Type casting
• Debugging with gdb

Textbook Sections 2.2, 2.5

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

Constants using const

PI = 3.14159
GST = 0.05
NUM_PLANETS = 8

const double PI = 3.14159;
const double GST = 0.05;
const int NUM_PLANETS = 8;

• const is a modifier that prevents the value from being changed

Comments

• C++ has two types of comments:
  • Single line comments: // (most common)
  • Multi-line comments: /* */
  • Be consistent!
• Stylistically, comments should be used the same way as Python
  • Explain why you are doing something, not what you are doing
  • Use self-documenting variable names and code structure
  • Short comments in line with code are okay, but stick to a max of ~80 characters per line total

Displaying output

Assuming #include <iostream> and using namespace std; we can display output with:

cout << "Hello World!\n";

• cout is the standard output stream
  • A stream is a source or destination of characters of indefinite length
• << is the stream insertion operator
• We’re telling C++ to “insert "Hello World!\n" into the output stream

More output magic

Like Python’s print, C++ is happy to mix and match types:

int age = NOT_TELLING;
cout << "I am " << age << " years old.\n";

cout << "This is a very long string that I want to break over "
     << "multiple lines.\n"
     << "This is on the next line.\n";

Reading input

Like the standard output cout, C++ has a standard input cin:

cin >> variable_name;

• >> is the stream extraction operator

• cin will wait for the user to type something and press enter

• variable_name must be declared, and must match the data type of the input

  int age;
  cout << "Enter your age in years: ";
  cin >> age;
  

The cin input stream

Like cout, cin can be used to read multiple values:

char first_initial, last_initial;
int year, age;
cout << "Enter your first and last initials: ";
cin >> first_initial >> last_initial;

cout << "Enter your program year and current age: ";
cin >> year >> age;

cout << "Thanks, " << first_initial << last_initial
     << "! You were " << (age - year) << " when you started!\n";

Check-in 1/2

True or false:

Like Python, C++ will include a prompt for the user when requesting input.

Check-in 2/2

True or false:

Multiple inputs can be separated by whitespace.

Buffered input

• Typed input is read and stored in a buffer (temporary storage)
• This allows the user to backspace and make corrections before submitting
• cin follows (approximately) this process:

  if the buffer is empty
      read from the keyboard
  else
      process next value in the buffer
  

• The data type of the variable to the right of >> determines how the input is interpreted

Type-dependent input processing

• For all data types, leading whitespace is ignored and multiple whitespace characters are treated as a single delimiter
• Important: the last character (often \n) is left in the buffer

Type casting

Mixed type arithmetic can result in implicit type casting:

int i = 1;
double d = (1 + i) * 3.4; // ok
d = i; // still okay
i = d; // compiler warning!

• Best to be explicit with static_cast:

  i = static_cast<int>(d);
  

• General syntax: static_cast<type>(expression)

Type casting check-in

In the following code sample, what is the final value of pi_i?

double pi = 3.14159;
int pi_i = static_cast<int>(pi / 2);

Limitations of double

• Declaring a double allocates 8 bytes or 64 bits of memory (32 bit float shown below):

• Allows for numbers up to $1.8 \times 10^{308}$, but “only” 15-17 digits of precision

• Doubles are inexact: 3 * 0.1 == 0.3 may evaluate to false!

Limitations of int

• Declaring an int allocates 4 bytes or 32 bits of memory
• This allows for storing numbers up to $2^{31} - 1$ or $2,147,483,647$

  Why not $2^{32}$?

• Integers are exact, so can be safely used for equality comparisons
• BUT if you exceed the maximum value, you get integer overflow:

  int i = 2147483647;
  i = i + 1;
  cout << i << endl; // -2147483648
  

Debugging with gdb

• In tomorrow’s lab, you will be introduced to the GNU Debugger gdb
• gdb is a command-line tool that allows you to:
  • Run your program line-by-line
  • Inspect the values of variables
  • Set breakpoints to pause execution
  • And much more!
• To build with debug info (such as line numbers) use the -g flag:

  g++ -g hello.cpp
  

gdb demo

• After building with -g, run gdb on the executable:

  gdb ./a.out
  

• You will see a (gdb) prompt
• Type run to start the program - this will run the whole thing
• Type list to see the source code
• To add a breakpoint, type b <line number> (or break <line number>), e.g.:

  b 7
  

• Now run again, and the program will pause at line 7

Basic gdb commands

Coming up next

• Lab: C++ and gdb
• Lecture: Using and defining functions in C++

Textbook Chapter 4 and start of 5