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At the lowest level, computers are entirely number-based, with everything represented by streams of numbers. A language such as Ruby insulates you from the internal workings of the computer, and numbers in Ruby are used for mostly the same things that you use numbers for in real life, such as counting, logical comparisons, arithmetic, and so on. Let s look at how you can use numbers in these ways in Ruby and how to do something with them.
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Basic Expressions
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When programming, an expression is a combination of numbers, operators (such as + or -), and variables that, when understood by the computer, result in an answer of some form. For example, these are all expressions:
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5 1 + "a" 100 x +
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2 + "b" + "c" - 5 * (2 - 1) y
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CHAPTER 3 RUBY S BUILDING BLOCKS: DATA, EXPRESSIONS, AND FLOW CONTROL
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The top four expressions all work right away with irb (try them out now!) and get the answers you d expect from such basic operations (1 + 2 results in 3, "a" + "b" + "c" results in abc, and so on). Brackets (parentheses) work the same way as with regular arithmetic. Anything inside brackets is calculated first (or, more technically, given higher precedence).
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Note You can work through all the topics in this chapter using irb, the immediate Ruby interpreter. If you
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get stuck at any point, simply leave irb by typing exit at any time, and start irb again as demonstrated in 1.
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Expressions are used regularly throughout all computer programs, and not just with numbers. However, an understanding of how expressions and operations work with numbers immediately translates into a basic knowledge of how they work with text, lists, and other items too.
Variables
In 2 we ran through a multitude of concepts, including variables. Variables are placeholders or references for objects, including numbers, text, or any objects you ve chosen to create. For example:
x = 10 puts x
Here you assign the numeric value of 10 to a variable called x. You can name variables however you like, with only a few limitations. Variable names must be a single unit (no spaces!); must start with either a letter or an underscore; must contain only letters, numbers, or underscores; and are case sensitive. Table 3-1 demonstrates variable names that are valid and invalid.
CHAPTER 3 RUBY S BUILDING BLOCKS: DATA, EXPRESSIONS, AND FLOW CONTROL
Table 3-1. Valid and Invalid Variable Names Variable Name
x y2 _x 7x this_is_a_test this is a test this'is@a'test! this-is-a-test
Valid Or Invalid
Valid Valid Valid Invalid (starts with a digit) Valid Invalid (not a single word) Invalid (contains invalid characters: ', @, and !) Invalid (looks like subtraction)
Variables are important because they allow you to write and use programs that work upon varying data. For example, consider a small program that has the sole job of subtracting two numbers:
x = 100 y = 10 puts x - y
If the code was written simply as puts 100 - 10, you d get the same result, but it s not as flexible. Using variables, you can get the values for x and y from the user, a file, or some other source. The only logic is the subtraction. As variables are placeholders for values and data, they can also be assigned the results of an expression (such as x = 2 - 1) and be used in expressions themselves (such as x - y + 2). Here s a more complex example:
x = 50 y = x * 100 x += y puts x
CHAPTER 3 RUBY S BUILDING BLOCKS: DATA, EXPRESSIONS, AND FLOW CONTROL
Step through the example line by line. First you set x to equal 50. You then set y to the value of x * 100 (50 * 100 or 5000). Next you add y to x before printing the result, 5050, to the screen. It makes sense, but the third line isn t obvious at first. Adding y to x looks more logical if you say x = x + y rather than x += y. This is another Ruby shortcut. Because the act of a variable performing an operation upon itself is so common in programming, you can shorten x = x + y to x += y. The same works for other operations too, such as multiplication and division, with x *= y and x /= y being valid too. A common way to increase a variable s value by 1 is x += 1, which is shorthand for x = x + 1.
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