Class 2 ssh, memory, data types, variables C++ Crash Course Class 2 ssh, memory, data types, variables

Agenda Learn to use SSH and compile code on the undergrad lab accounts Learn how to code the C++ way

Variables and Basic Types C++ is a typed language each variable has a specific data type which does not change through the course of the program Type specifies the format the data is stored in, and how much space it takes up Types say what data means and what operations can be performed on it Made up of primitive types (char, int, float) as well as our own types – compound types, variable-length types

Primitive Types Type Meaning Min Size bool boolean NA char character 8 bits wchar_t Wide character 16 bits short Short integer int integer long Long integer 32 bits float Single-precision floating point 6 significant digits double Double-precision floating point 10 significant digits Long double Extended-precision floating point To avoid mysterious bugs, most programmers typically use int, char, and double as defaults unless there’s a good reason not to. Know these exist though!

Computer Memory: Basics The data in memory is just a series of 0’s and 1’s, each of which is called a bit No inherent structure We divide it up: 1 bit: 1 or 0 1 byte: 8 bits 1 word: 4 bytes Each byte of memory has an address Data type tells us how to interpret the bytes at a certain address in memory

Binary values Binary is a number in base 2 Types are limited in how large they can get by their size Since an int is 16 bits long, it can represent 216 unique numbers

Integral Types Integers, characters, boolean values are the integral types as opposed to the floating-point data types char is big enough for the basic character sets; wchar_t is needed for extended character sets (such as Chinese / Japanese)

Signed and unsigned signed types can be either positive or negative unsigned types represent only positive numbers (including 0) int, short and long are all signed by default unsigned types have the first digit of the type represent whether it is positive or negative 1 means negative 0 means positive or 0

Signed and unsigned C++ will still allow you to assign a negative number to an unsigned type Result is negative value mode type size

Floating-point Types float, double and long-double are all floating point values floats are usually in one word (32 bits); doubles in two words (64 bits); long double in 3 or 4 words (96 or 128 bits) Size of the type determines the number of possible significant digits Most real-world programs require a double level of precision over a float

Literal Constants Hard-coded into the program 42, 3.14159, ‘b’ are all literal constants These exist only for the built-in types, not library types Literals have an associated type 42 is an int 3.14159 is a float ‘b’ is a char

Integer literals Can write a literal integer in many notations C++ supports decimal (base 10), octal (base 8) or hexadecimal (base 16) 20 for decimal 024 for octal (preface with a 0) 0x14 for hexadecimal (preface with a 0x) Conversion trick: splitting a binary number into groups of three will give you its octal representation splitting into groups of four will give you its hex representation

Boolean literals true and false are literals of type bool

Escape Sequences Last time we talked about \n, the newline character Several others: newline \n horizontal tab \t vertical tab \v backspace \b carriage return \r formfeed \f alert (bell) \a backslash \\ question mark \? single quote \’ double quote \”

Character String Literals “Hello World!” internally looks like: ‘H’ ‘e’ ‘l’ ‘l’ ‘o’ ‘ ‘ ‘W’ ‘o’ ‘r’ ‘l’ ‘d’ ‘!’ ‘\0’ The \0 is the null character, used to say that we’ve reached the end of a string.

Variables Variables provide storage space of a specific type Seemingly straightforward, but… Expressions: lvalue: an lvalue may be either the left-hand or right-hand side of an assignment rvalue: an rvalue expression may appear on the right hand side, but not the left hand side, of an assignment

Variables Variables are lvalues; numeric literals are rvalues Given: int units_sold = 0; double sales_price = 0, total_revenue = 0; You cannot say the following: units_sold * sales_price = total_revenue; or 0 = 1;

Variable Naming The name of a variable is its identifier int somename, someName, SomeName, SOMENAME; All the above are different, since identifiers are case-sensitive In C++, must begin with a letter or an underscore Must be composed of letters, digits, and the underscore character No length restrictions – easier to program if it’s shorter though

Reserved Words These can’t be used as identifiers: asm do if return auto double inline short bool dynamic_cast int signed break else long sizeof case enum mutable static catch explicit namespace static_cast char export new struct class extern operator switch const false private template const_cast float protected this continue for public throw default friend register true delete goto reinterpret_cast

Conventions Variables are usually only in lowercase Identifiers give some indication of its use – if at all possible, avoid creating “int x, y, z” Identifiers with multiple words have underscores between them or have each subsequent word capitalized

Defining objects int units_sold; double sales_price, avg_price; std::string title; Sales_item curr_book; Starts with a type specifier, with a comma-separated list of names Multiple variables can be defined in a statement

Initialization A definition specifies type and identifier, and can also provide an initial value, which initializes the object There are two ways to initialize: copy-initialization: int ival = 1024; direct-initialization: int ival(1024); This is different from assignment, unlike in other languages – we’ll get into that much later

Always initialize your variables! Though the compiler may not notice, if you don’t initialize your variables, you’ll get a runtime issue If you don’t initialize a variable, then there’s still a value – whatever was in the memory block now occupied by the variable is still there, and now being interpreted as this new type

Definitions and Declarations defining a variable allocates space, and may make an initial value declaring a variable makes it known to the program definitions are declarations; declarations are not necessarily definitions The keyword extern makes the variable available without allocating storage – declares but does not define Used when you know the variable is defined elsewhere – a variable must be defined exactly once

Scope Scope is nearly always determined by curly brackets {} If you declare a variable inside a function, then you won’t be able to access it outside of the function Similarly, if you declare a variable inside a loop, you won’t be able to access it in the rest of the function in which it appears

Using const In real-world programming, you’re often going to need to compare to specific values Three number average program: 70 was used, but to someone looking at it later, the reason is unclear Thus we have const: we can create a variable that will never change after its initialization const int pass_cutoff = 70; Trying to modify pass_cutoff will result in a compile-time error

References Alternative name for an object A reference is a compound type defined in terms of another type int ival = 1024; int &refVal = ival; refVal now refers to ival so: refVal += 2; // adds 2 to ival int ii = refVal; …results in ii having the value currently in ival

References A reference is effectively an alias for a variable References cannot be rebound to a different object We can define as many as we like: int i = 1024, i2 = 2048; int &r = I, r2 = i2; int i3 = 1024, &ri = i3; int &r3 = i3, &r4 = i2;

References A reference is effectively an alias for a variable References cannot be rebound to a different object We can define as many as we like: int i = 1024, i2 = 2048; int &r = I, r2 = i2; int i3 = 1024, &ri = i3; int &r3 = i3, &r4 = i2; What just happened to the variables?

References A reference is effectively an alias for a variable References cannot be rebound to a different object We can define as many as we like: int i = 1024, i2 = 2048; // i: 1024, i2: 2048 int &r = I, r2 = i2; int i3 = 1024, &ri = i3; int &r3 = i3, &r4 = i2; What just happened to the variables?

References A reference is effectively an alias for a variable References cannot be rebound to a different object We can define as many as we like: int i = 1024, i2 = 2048; // i: 1024, i2: 2048 int &r = i, r2 = i2; // r: 1024, r2: 2048 int i3 = 1024, &ri = i3; int &r3 = i3, &r4 = i2; What just happened to the variables?

References A reference is effectively an alias for a variable References cannot be rebound to a different object We can define as many as we like: int i = 1024, i2 = 2048; // i: 1024, i2: 2048 int &r = i, r2 = i2; // r: 1024, r2: 2048 int i3 = 1024, &ri = i3; // i3: 1024, ri: 1024 int &r3 = i3, &r4 = i2; // r3: 1024, r4: 2048 What just happened to the variables?

What does this code do? int i, &ri = i; i = 5; ri = 10; std::cout << i << “ “ << ri << std::endl;

What does this code do? int i, &ri = i; i = 5; ri = 10; std::cout << i << “ “ << ri << std::endl; 10 10

typedef names We can make synonyms for types, in order to make the types more meaningful typedef double wages; typedef int exam_score; typedef wages salary; Now we have three new types: wages, exam_score, salary wages hourly, weekly; exam_score test_result; hourly and weekly are now doubles; test_result is an int

typedefs …Hide the implementation of a given type and emphasize the purpose instead …streamline complex type definitions to make them easier to understand …allow a single type to be used for more than one purpose while making the purpose clear each time the type is used

Enumerations Could create several const variables when trying to track program states e.g.: const int input = 0; const int output = 1; const int append = 2;

Enumerations This can be more quickly and easily expressed with an enum enum open_modes {input, output, append}; …which allows us to say: open_modes var = input; Each enum defines a unique type. You cannot assign 0, 1, or 2 to an open_modes object, even though the underlying representation of input, output and append are as ints