2019-04-26

Variables and User Input in Ruby

So far, we've been seeing the utility of the data types like Strings, Numbers, and Booleans. However, we wrote all of those data types directly to our IRB, which means something. When you are creating programs, you'll want to store or change information in some way during the execution of the program. Let's make a simple example:

* Almost all of your operations need to be created with 50.
* Why 50? Because this is the number of employees that works in the company, so you have to calculate costs, monthly salaries, and another kind of management stuff with the total number of employees
* Let's say that you want to give a gift to them because it is near to December. The cost of each one is $49. Simple operation
* Then it is the end of the month, and you have to calculate the salary for this month. So let's say that everybody earns the same $10.000 (hypothetically), so you multiply that number for 50 employees again
* Later you have to provide a t-shirt for each employee because you'll have a Saturday retreat (in a nice place with a river and a forest), which is made in the company every 6 months and all 50 employees will be there. The cost of the t-shirt is $20
* Finally, the company is growing, and we'll be hiring 5 new employees this month, so we have to calculate the salary again

We can detect something here; we'll have to save some values in Variables.

Variables

Variables are used to store information referenced and manipulated in a computer program. They also provide a way of labeling data with a descriptive name, so the reader and ourselves can understand our programs more clearly. It is helpful to think of variables as containers that hold information. Their sole purpose is to label and store data in memory. This data can then be used throughout your program.

In our example, what are the approximate flow of the program and the variables assignment? Let's do some code here, open your IRB, and begin.

2.3.3 :001 > number_employees = 50
 => 50 
2.3.3 :002 > gift_cost = 49
 => 49 
2.3.3 :003 > total_gift_cost = number_employees * gift_cost
 => 2450 
2.3.3 :004 > # $2.450 is the total cost of the gifts for the 50 employees
2.3.3 :005 >   individual_salary = 10000
 => 10000 
2.3.3 :006 > total_cost_salaries = number_employees * individual_salary
 => 500000 
2.3.3 :007 > tshirt_cost = 20
 => 20 
2.3.3 :008 > total_cost_tshirts = number_employees * tshirt_cost
 => 1000 
2.3.3 :009 > number_employees = 55
 => 55 
2.3.3 :010 > total_cost_salaries = number_employees * individual_salary
 => 550000

As we mentioned before, a variable is a label that helps us store data in memory. In this case, the labels are "number_employees", "gift_cost", "total_gift_cost", etc. Each variable is followed by the symbol "=." This symbol means assignation because in the case we're assigning the value of "50" to the "number_employees" variable

One interesting thing to note here is that we aren't doing math with numbers directly, like in the past blog post. Instead, we're doing the math between variables! So, for instance, if you see the variable named "total_gift_cost," it contains the result of multiplication between "number_employees" and "gift_cost."

total_gifit_cost = 50 * 49
total_gifit_cost = number_employees * gift_cost

The two previous lines are quite different. First, we're doing math directly; Second, we're re-using a previously defined variable. This is the most exciting part because later in the program, I can use any of these 3 variable names to do other operations. These same variables can be assigned to different values in the future. In our case, for instance, the variable named "number_employees" changed from 50 to 55, and then we re-calculate the salary, and it goes from $500.000 to $550.000 after the math (with different values)

Also, we used the same variable called "number_employees" to do the math with the other variables, which is the intention of the variables itself; remember: "Variables are used to store information to be referenced and manipulated in a computer program. They also provide a way of labeling data with a descriptive name, so the reader and ourselves can understand our programs more clearly. It is helpful to think of variables as containers that hold information. Their sole purpose is to label and store data in memory. This data can then be used throughout your program."

Camel Case and Snake Case

The naming of the variables is critical because you cannot name different things with the same name; if so, you'll re-assign the original value, which can cause some problems for you. So it's better to think a bit deeper about the name you want to give to that variable.

Because of this, you'll find that it is probably better to name variables as a composed word, which means something like this: "Number of employees" or "Total cost of salaries" or "Gift cost." As you can see, we have to decide about the variable's name and encode the full name, deleting upper case words and blank spaces. Actually, if you name a variable with an "empty space," you'll get an error.

2.3.3 :011 > number employees = 55

As you can see, we have a space between "number employees." So what the program does is to try to find the first variable name "number," and because it doesn't exist, they give us a "NoMethodError" in the console.

Here is where we have other options:

* numberEmployees: camelCase
* NumberEmployees: PascalCase
* number_employees: snake_case
* number-employees: kebab-case

With this, we eliminate the blank space, the variable is easy to understand, and we won't have any errors. The technical name for this is "Case Styles."

It seems that the differences are not worth enough, but it is. Choosing what to use is a good idea because it is a good practice to use the same conventions over all of your programs. However, sometimes the decision depends on the good rules of the language and what other people seem to do. For instance

- * Ruby and Python: in both languages are pretty standard to use the snake_case
- * Javascript: it is common to see programmers utilizing a lot of camelCase
For more info, please refer to this link.

Reserved words

One last thing you have to consider when naming variables is the name itself. This is because Ruby uses some classes and methods with a given name, and you cannot provide the same name to your own variables.

This seems a bit awkward but is not; just follow the list of reserved words, and at some point, you'll figure out what name convention you should use. The technical name for those words is "keywords," Here is the list of Ruby Keywords: https://docs.ruby-lang.org/en/2.2.0/keywords_rdoc.html.

As you can see, it is not an extensive list, and these words are reserved for doing specific tasks like declaring functions, class, conditionals, and others. For this reason, composed words like "number_employees" are safest to use than simple words.

Let's use the "unless" keyword as a variable and see what could happen.

2.3.3 :018 > unless = "Programmer"

unless variable assignment

As we can see, we get a syntax error because the "unless" keyword is reserved for doing conditionals, so it's waiting for a block of code, not for an equal sign "=" so. As a result, we get a syntax error. The same thing will happen with other keywords.

User Input

So far, we've been assigning values to the variables directly and in some way "statically." The programs will usually need the input of the user. For instance, we can create a program that asks for the city where they live (even you can personalize the greeting according to the town, using concatenation). Here you can imagine the amount of info we can ask the user and create our programs.

Let's go to our IRB terminal again and test the user input.

2.3.3 :014 > city = gets
Berlin, Germany
 => "Berlin, Germany\n" 
2.3.3 :015 > city
 => "Berlin, Germany\n" 
2.3.3 :016 > city = gets.chomp
Medellín, Colombia
 => "Medellín, Colombia" 
2.3.3 :017 > city
 => "Medellín, Colombia"

We can see two new things here:

* gets
* gets.chomp

"gets," as the name suggests, gets the user input, and then that value is assigned to the variable named "city." When you type the first line of code, the IRB waits for you to order something with your keyboard & press the enter key. Once you give the value and hit enter, we get the output. However, this output seems a little weird because it places this symbol "\n" at the end of my input, which represents the enter key. But we don't want that as part of our string. So we'll use chomp chained to get rid of that - you can put .chomp after any string to remove the carriage return characters at the end.

"gets.chomp" is the only way to get rid of that character by adding .chomp at the end. Chomp is a String class method in Ruby, which returns a new String with the given record separator removed from the end of str (if present). chomp method will also remove carriage return characters (that is, it will remove \n, \r, and \r\n) if $/ has not been changed from the default Ruby record separator, t

With this, we have a first visual about variables and user input.

I hope you learned a lot.

Thanks for reading

Daniel Morales