Variables, Data Types, and Arrays

Session 2, COMP 11

Variable: What is it?

f(x) = x + 17In Mathematics (what you’re used to):

In Computer Science:

x

42

Name (label)

Value (contents)

Type (size)

Variables: “Declaration”

● Defines the name (label) and type (size) of a new variable (box).● General syntax (what the code looks like): type name;● Example:

int fahrenheit;

TypeName Semicolon

Variables: Naming Conventions

Examples of valid variable names:

● x● sum● AVERAGE● myName● todays_date● text2num● Area51● tHiSabOmiNaBLeThiNG

Examples of invalid variable names:

● 5names● money$ign● email.address● int● Suzie’s_birthday● my variable● X-mas● .sysvar

1. Names may contain letters, numbers, and underscores.2. First character must be a letter.

Our first type: int

● Integer number (no fraction/decimal)● Examples: 0, -343, 2819● Range is limited!

○ Calculator -- 8 decimal (0-9) digits○ int -- (usually) 32 binary (0-1) digits○ bit = “binary digit”

● Declaring an integer variable:

int name; /* comment */

Variables: Assignment

int number; /* a new integer */

number

?

“Uninitialized Variable”- A declared variable holding an

unknown (“garbage”) value.

/* An assignment statement */

number = (1 + 2) * 2;

number

6

“Assignment” -- putting a new value in a variable“Initialize” -- put a value in a variable for the first time

Variables: Assignment

● General syntax: name = expression ;1. “Variable ‘name’ gets the value of ‘expression’.”2. Compute the expression on the RIGHT of the = to get a value3. Assign that value to the variable named on the LEFT of the =

WARNING: The right-hand side is always evaluated first to get a value, which is then stored in the box called “name”.

Variables: Using them

int x;

int y;

x = 42;

y = x + 3;

x = x + 1;

After the last line…1. What value is stored in x?2. What value is stored in y?

x y ? N/A ? ? 42 ? 42 45 43 45

Example: temperature.cpp

Our second type: float (and double)

● “Floating-point” number (has a decimal point)● Examples: 0.0, -343.25, 0.2819● Range is still limited!

○ (usually) 32 bits

● double: 64-bit float ○ More Precision: 3.1415927 vs. 3.1415927410125732○ Important for physical simulations

Our third type: char

● Strings like “Hello World” are comprised of characters○ Character: “a symbol representing a letter or number” (Google’s 3rd definition)

● The char type: a value representing a single character● Some char constants: 'a' 'A' '0' ' ' '\n' '\''● WARNING: Use single quotes to denote a char constant● Range is STILL limited: 1 byte (8 bits)

H e l l o01001000 01100101 01101100 01101100 01101111

<space>00100000

W o r l d01010111 01101111 01110010 01101100 01100100

ASCII Encodings (subset of “Unicode”)

From “ASCII” Wikipedia article http://cs.wellesley.edu/~cs110/reading/information-representation.html

Our fourth type: bool

● Boolean → True or False● Only 2 bool constants: true and false

● Named after George Boole → invented “boolean algebra”

Strings

● A “string” is a sequence of characters in double quotes● Some string constants: "hello" "h" " " "***l33t h4x0rz***"● Different from other types:

○ Not primitive → defined in a library (like <iostream>)○ Has observable properties → size, emptiness○ Individual characters are accessible

strings.cpp

Array: General Concept

● Data structure: organize collections of data efficiently● Array: collection of elements of the same type● Fixed size

27 42 222 -382 0

● Access/modify elements via “array index”

0 1 2 3 4WARNING: Arrays are always 0-indexed.

The last element of an array of size n is at index n - 1.

“Elements”

Arrays in C++

int seats[20];

Declaration1. Type of elements2. Name of array3. Size of array in [ ]

float fractions[3];

char alphabet[26];

int nums[3] = {1,2,3};

float decimals[2];

decimals[0] = -38.253;

decimals[1] = 0.25;

int sum = nums[0] + nums[1] + nums[2];

nums[0] = sum;

nums[sum - 6] = 1;

InitializationSpecify 1 or more values in { }.

Accessingarr[index] (index can be any integer expression)

Array Dimensions

int nums[5];

int nums2D[3][5];

int rubiks[3][3][3];

1-Dimensional (1D)

2-Dimensional (2D)

3-Dimensional (3D)

Summary

● What is a variable?● Difference between “assigning”, “initializing”, and “declaring” a variable?● What are the different types we’ve learned about?

○ What are their ranges?

● What’s weird about division in C++?● What is an array? What are some warnings about how to use them?