data-types

Question 1

what’s a sigil?

Answer 1

a sign or symbol

e.g. in Ruby the @ symbol, as in @my_var means that ‘my_var’ is the name of an instance-variable (vs. a global variable $my_global_var or a local variable my_local_var)

sigils make code look cryptic, until you know what they mean, then you appreciate them because they make it easier to type something that might otherwise be cumbersome.

Question 2

what sort of data does the computer expect when it sees “…” (a double-quote(s))?

Answer 2

typically wrap strings of characters, e.g. “this is a string of characters”

Question 3

what sort of data does the computer expect when it sees ‘…’ (a single-quote(s))?

Answer 3

in the (kinda-old) “c” programming language single-characters* (see the ascii chart) get wrapped in single-quotes.

However, in most modern** languages (javascript, ruby, python, bash-shell, etc) single- & double-quotes are both used to wrap strings of characters:

‘this is a string’

*Note: a - z aren’t the only single-characters, there’s also stuff like “\n” which although it looks like two characters is treated like a single-character (the return character).

**Note: that said, most modern languages interpret single-quoted strings exactly. Whereas double-quoted strings allow for interpolation of variables…

Question 4

what is it called when you make the computer put the value of your variable into the middle of a string?

Answer 4

string-interpolation (i.e. of the variable)

Question 5

what sort of data does the computer expect when it sees a: { (an opening-brace)
and
} (a closing-brace)

Answer 5

in between the matching braces you are likely looking at what is sometimes called: a dictionary (in python), an associative-array (in perl), a hash (in ruby), or (in javascript) an object.

This data-structure associates a key with a value:
{
"this key": "this value",
"that key": "that value"
}

notice the “:” (colon) is the separator between the key and the value.
also, notice the “,” (comma) that separates one key-value pair from another.

Question 6

How do you read a value from a hash?

Answer 6

given:
x = {
"key1": "value1",
"key2": "value2"
}

you read the value “value2” by supplying the key “key2”
e.g.
x[“key2”] is equal to “value2”

Question 7

what sort of data does the computer expect when it sees a: [ (an opening-bracket)
and
] (a closing-bracket)

Answer 7

in between the matching square-brackets you are likely looking at what is sometimes called an array.

This data-structure is an ordered list of the elements that you add to it. Like a hash it has a key, but the key is just the ordinal-position of each value:
[
“value 1”, “value 2”, “value 3”
]

notice the “,” (comma) that separates each value, one from the next…

Question 8

how do you read a value from an array?

Answer 8

given:
x = [“value 1”, “value 2”, “value 3”]

you read the value “value 2” by suppling it’s position:
x[1] is equal to “value 2”
(why not x[2]?)

Question 9

what is meant by a zero-based array?

Answer 9

in most programming languages the values in arrays are stored, starting with position “0” then “1”, then “2” and so on…

e.g.
a = [1,2,3] 
a[0] = 1
a[1] = 2
a[2] = 3

Question 10

how do you access the second to last value in an array?

Answer 10

a[-2]

e.g. given:
a = [1,2,3,4]
a[-2] == 3

*the double-equals means “is equivalent to”
whereas the single-equals means “is assigned to”

Question 11

What kind of things can you store in a variable?

Answer 11

numbers, characters, strings, arrays, and objects (aka: hashes or associative-arrays of key-value pairs)

e.g.
var foo = “this variable stores a string”;
var bar = 42;
var baz = {
“this string is the key used to lookup the answer”: bar,
“key2”: 9
};

in some languages, variables are “typed” meaning you have to specify the “type” of number, or that you want to store a string…

e.g. (golang):
var x uint16;
var y uint64;
var isHungry bool;
var myDecimal float32;
as well as
 string
(whereas in `c` a `string` is essentially an array of char[] )