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pairs, ipairs and #tablename iteration
Tables and the table library. example from a Well House Consultants training course
More on Tables and the table library. [link]

This example is described in the following article(s):
   • Lua tables - they are everything - [link]

Source code: ph Module: U105

Tables in Lua may contain both ordered (numbered) and unordered
(named) members. Most of the time, you'll use just named
elements, or just numbered elements, but there are surprisingly
frequent occasions when it's useful to use both. You then need
to remember if you iterate through members that ...

* #tablename will only go through numbered elements, and will
give you nil if there's a gap in the numbering

* pairs will take you through all elements with non-nil values,
but won't give you any members which are explicitly or implicitly
set to nil

* ipairs will start at the element numbered 1, and go up through
numbered elements until it reaches a nil, at which point it will
go no further.


print ("Set up a table, indexed, with a hole")
print ("Iterate through it using #tablename")
vals = {"one","two","three",nil,"five"}
for k=1,#vals do
        print (k,vals[k])
print ("------------- ")

print ("Add a numbered value after a gap. Works fine")
vals[8] = "eight"
for k=1,#vals do
        print (k,vals[k])
print ("------------- ")

print ("Add a named value - but not seen by the loop")
vals["nine"] = "It's 9"
for k=1,#vals do
        print (k,vals[k])
print ("------------- ")

print ("Pairs goes through all the elements with values")
for k,v in pairs(vals) do
        print (k,v)
print ("------------- ")

print ("Ipairs starts at element 1, goes up 1 at a time and stops when it finds a nil")
for k,v in ipairs(vals) do
        print (k,v)
print ("------------- ")

Learn about this subject
This module and example are covered on the following public courses:
 * Learning to program in Lua
 * Lua Programming
 * Lua Programming
Also available on on site courses for larger groups

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This example comes from our "Tables and the table library." training module. You'll find a description of the topic and some other closely related examples on the "Tables and the table library." module index page.

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