Making Changes to Arrays

An array's contents will often have to be changed, like when needing to remove an item from a player's inventory. Lua has prebuilt functions for use with tables to make this easier.

This article will outline prebuilt functions for tables like add and remove while describing how to search for a value.

To apply these skills, you'll create a script that helps to manage a player's inventory in a game.

Adding Values to Arrays

To add a new value to an array, use table.insert(array, valueToInsert). The second parameter can be any value such as a string, number, or entire object, like Player or IntValue.

To practice, you'll create a script that stores player items in a table, then add to it. Since this example is just a script, it can be used with any file or project. Testing the code will be done using the Output Window.

1. Create an empty array named playerItems.

   playerItems = {}

2. Add inventory items to the array by typing table.insert(). In the parenthesis, type playerItems, the table to add to, followed by a string for each item. Insert at least three values.

   playerItems = {}
   
   table.insert(playerItems, "Potion")
   table.insert(playerItems, "Bread")
   table.insert(playerItems, "Sleeping Bag")
   
   print(playerItems)

3. Run the project. In Output, expand the three dots ... to see the printed table.

Removing Values from Arrays

To remove a value, like if a player used an item or someone in a list of active players leaves an experience, use table.remove(). Depending on the parameters provided, the function can either remove the last value of a table, or at a specific index.

Removing the Last Value

Sometimes a script needs to remove a specific item. For instance, removing the first item in a player's inventory, or picking the first player in a list. To remove the last value in an array, use table.remove(arrayName). In this use case, the only parameter needed is the table itself.

1. Use the array of items from the previous example. Then, to remove the last value from the table, in this case "Sleeping Bag", type table.remove(playerItems)

   playerItems = {}
   
   table.insert(playerItems, "Potion")
   table.insert(playerItems, "Bread")
   table.insert(playerItems, "Sleeping bag")
   
   table.remove(playerItems)
   
   print(playerItems)

2. Run the project. In the Output window, the last value, "Sleeping Bag", shouldn't print.

Removing by Index

To remove a value at a specific point in the array, input in the second parameter the index to remove, such as table.remove(arrayName, 1).

1. Using the same table, type table.remove(). In the parameters, input the name of the table and 1, the value to remove.

   playerItems = {}
   
   table.insert(playerItems, "Potion")
   table.insert(playerItems, "Bread")
   table.insert(playerItems, "Sleeping bag")
   
   table.remove(playerItems, 1)

   The second parameter for table.remove() only accepts a numerical index. Typing something like "Potion" will create an error.

2. Run the project. Check that the first value, "Potion" was removed.

Searching for Values in an Array

To find specific values in arrays, like the name of a winning player, use the table.find() function. Alternatively, you can code your own search function using for loops and if statements.

Finding and Returning a Single Value

To find a value in an array, create a function named findValue() that goes through an array and stops the first time it finds a matching value.

Once it finds the value, the function will use the return keyword to return that value's index. With an index, you can remove it from the array.

1. Copy the array named playerItems below.

   local playerItems = {
   	"Potion",
   	"Bread",
   	"Bread",
   	"Sleeping Bag"
   }

2. Code a new function named findValue() with two parameters:

   • whichArray - the array to search through.

   • itemName - the specific string to check for.

     local function findValue(whichArray, itemName)
     
     end

3. In findValue(), use a for loop to check if any value in the array matches itemName. If the value matches, use the keyword return to return the value found in currentIndex.

   local function findValue(whichArray, itemName)
   	for currentIndex = 1, #whichArray do
   		if whichArray[currentIndex] == itemName then
   			return currentIndex
   		end
   	end
   end

4. Test the search function by creating a variable named valueFound and calling findValue(). Pass in the array to search, and what value it should search for. Run the code to confirm that the expected output is index 2.

   local function findValue(whichArray, itemName)
   	for currentIndex = 1, #whichArray do
   		if whichArray[currentIndex] == itemName then
   			--Sends the value of currentIndex back
   			return currentIndex
   		end
   	end
   end
   
   local valueFound = findValue(playerItems, "Bread")
   
   print("The value is at index " .. valueFound)

Removing a Value

If a value was found using the find function, it can be removed. Check if a value was found with an if statement.

1. Check if a value is inside valueFound; if so, remove the value using table.remove().

   if valueFound then
   	table.remove(playerItems, valueFound)
   end

2. Print out the array using the code below.

   for index = 1, #playerItems do
   	local itemString = playerItems[index]
   	print("Index " .. index .. ": " .. itemString)
   end

3. Playtest and check that the first "Bread" value was removed from the array. Try removing other values by changing the second parameter in findValue().

   Notice that because this function was called once, only the first instance of "Bread" was removed. The following section will cover how to find and remove all instances.

Finding and Removing All of a Specific Value

While the previous code could only remove the first instance of a value found, this code snippet will find and remove all occurrences from an array. For example, if, say, a player wanted to sell all their bread at an in-game store.

1. Use an array named playerItems with at least four values and a set of duplicates.

   local playerItems = {
   	"Potion",
   	"Bread",
   	"Bread",
   	"Sleeping Bag"
   }

2. To go through the array, create a for loop that goes backward through playerItems, starting at #playerItems, ending at 1, and incrementing by -1.

   for index = #playerItems, 1, -1 do
   
   end

   Remember, removing items causes later indexes to shift. By starting at the end, you avoid skipping a value that may have shifted if you started at the beginning.

3. In the loop, use an if statement to check if the value inside playerItems[index] is equal to "Bread", and if so, remove the item.

   for index = #playerItems, 1, -1 do
   	if playerItems[index] == "Bread" then
   		table.remove(playerItems, index)
   	end
   end

4. Use the code below to add a second for loop that prints the array.

   for index = 1, #playerItems do
   	local itemString = playerItems[index]
   	print("Index " .. index .. ": " .. itemString)
   end

5. Run the script and check that all values named "Bread" are removed.

Summary

Values can be removed or added to an array, but be careful of indexes shifting doing so. Use loops to iterate through a table to remove all instances of a value, or just the first instance found.

The keyword return can be used to stop a loop, and send information back where needed.

A finished version of the script can be referenced below.

The following project includes all scripts in this tutorial. Download here.

Note all scripts are in ServerScriptService and disabled. To use a script, in its properties, uncheck the Disabled field and run Studio.