Alice Arrays: Intro

• A primitive data type is one built into the language

– They hold a single value, like integers, Booleans, and doubles

• An array is a data structure

– A variable represents multiple values

– Components of an array are called elements

– Elements are numbered starting with zero

– Individual elements can be accessed by an integer index

– Size indicates how many elements are in the array

– In an array, all elements must be the same data type

– The value stored in an array variable (memory) is a pointer (reference) tovalues it represents

• Arrays are useful in situations where you need to store many values of the sametype that are related in some way

– E.g., grades, days in months, team scores

– Easier than using a variable for each individual value

– Easier processing because can use a loop to process each element of an array

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Alice Arrays: Creating and Using Arrays

– To create an array, use the same steps used to create any other kind ofvariable, but check the array box

– Give it a name and type as usual

– To add elements to the array, select Custom Array

∗ You then choose values to add to the array

∗ The first value goes in slot (position) zero, the second in slot 2, etc.

∗ The size/length of the array is determined by how many elements youadd

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Alice Arrays: Creating and Using Arrays (2)

– The actual code for the resulting declaration is

∗ The square brackets indicate an array

∗ The braces indicate the initial values to be stored in the array

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Alice Arrays: Creating and Using Arrays (3)

– Creation works the same for other data types:

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Alice Arrays: Programming with Arrays - Alice 3’s Special Control Structures

• A common activity with arrays is processing each element in the array, oneafter the other

– This is done with a loop

– Alice3 provides a special loop for doing this: for each in array

– To use this control structure, you need to

1. Specify the data type of the array elements

2. Name the LCV

3. Provide the array to be iterated thru

– The syntax for this statement is

for (Type LCV: array_name) {

statements

}

– The semantics: Execute statements for element 0, then for element 1, etc.

– NOTE: Use the LCV variable name in statements to have the code affectthe array elements

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Alice Arrays: Programming with Arrays - Alice 3’s Special Control Structures(2)

• On the other hand, you might want to process all elements in the array at thesame time

– Alice3 provides a special control structure for doing this: for each in arraytogether

– To use this control structure, you specify the same things as for for each inarray

– The syntax for this statement is

ThreadUtilities.eachInTogether ()(Type = LCV) -> {

statements

}, array_name)

– The semantics: Execute statements for elements 0 thru n all at once, likedo together

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Alice Arrays: Programming with Arrays - ”Regular” Processing

• Array operators/properties

– The subscript operator: [ ]

∗ Syntax: array name[integer]

∗ Semantics: Refers to the element in array name at slot integer

· Note: Each element in an array is a variable in its own right

∗ The text states there are two versions: read and write

· Which is meant depends on which side of an assignment the referenceis on

· v = a[2] is a read version: read the value in slot 2 of array a

· a[2] = 5 is a write version: write the value 5 into slot 2 of array a

– The length property

∗ Syntax: array name.length

∗ Semantics: Refers to the number of elements in array name

∗ This is useful for creating your own loop structures for processing arrays

• A for loop that does the same thing as for each in array

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Alice Arrays: Programming with Arrays - ”Regular” Processing (2)

• Parallel arrays

– Sometimes you have different kinds of data that need to be processed intandem

– For example, names (Strings) and grades (doubles)

– These can’t be stored in the same array because an array only holds a singledata type

– In this case, use two arrays: a String array for the names, a double arrayfor the grades

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Alice Random Number Generation

• Sometimes you need to generate random values in a program

– For example, you want to simulate rolling dice for a game, for studyingprobability, etc.

– For a six-sided die, you’d want to generate a number between one and sixin an unpredictable (usually) pattern

• A random number generator (more properly a pseudo random number genera-tor) is a function that does just this

• To generate random values in Alice:

1. Create a variable in the usual manner

– Select a data type (integer, double, or boolean)

– Give it a name

– Assign an initial value

2. Once the variable has been created, it can be assigned a random value

– Click on the arrow next to the assigned value

– One of the choices will be Random

– Clicking on Random will give one or more options for generating therandom value

∗ Integer

(a) Random number x where 0 ≤ x < N

(b) Random numbers x where A ≤ x < B

(c) Random numbers x where A ≤ x ≤ B

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Alice Random Number Generation (2)

∗ Double

(a) Random number x where 0.0 ≤ x < 1.0

(b) Random numbers x where A ≤ x ≤ B

∗ Boolean

· This provides no options - it generates either true or false

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