While trying to completely understand the solution to Lua - generate sequence of numbers, the section 4.3.4 of Programming in Lua is unclear:
for i=1,f(x) do print(i) end
for i=10,1,-1 do print(i) end
The for loop has some subtleties that you should learn in order to
make good use of it. First, all three expressions are evaluated once,
before the loop starts. For instance, in the first example, f(x) is
called only once. Second, the control variable is a local variable
automatically declared by the for statement and is visible only inside
the loop. [...]
The first line of code doesn't work of course.
What is f(x) and where is it defined?
Unfortunately the documentation isn't available as a single page, making it a huge effort to search for the first occurrence. Searching for "lua f(x)" doesn't bear fruit either.
Explanation: now that I have received answers, I realize the problem was a misunderstanding. I incorrectly interpreted "f(x) is called only once" as "the line containing f(x) - for i=1,f(x) do print(i) end - will only return one value" and didn't pay enough attention to "all three expressions are evaluated once, before the loop starts".
This sentence clarifies it: expressions are evaluated once, before the loop starts.
Thus, f(x) is called only once is merely stating that the expressions will not be affected by potential changes in the loop.
For example, the following code (expressions are i=1 and x in the second line):
x=5
for i=1,x do
x = x - 1
print(i, x)
end
print(x)
will produce the following output:
1 4
2 3
3 2
4 1
5 0
0
and will not produce the following output:
1 4
2 3
3 2
2
f(x) is just a function which takes the argument x and returns a value that is used as the upper bound for the loop.
So for example, if the function f(x) calculates x² and you call it as f(3), it would return the value of 9. The resulting for loop would look like this:
for i=1, f(3) do print(i) end
which is exactly the same as
for i=1, 9 do print(i) end
Related
I have a string which delimited by \n, and I'm using for to do something with that. But I don't want the for loop to the end of the string, just want the for loop to any times like this code below but it doesn't work.
for w=1,10 in webdata:gmatch("(.-)\n") do
--something
end
There are two different for loops in Lua. You are mixing them together.
Numerical for:
for i=1,10 do -- or e.g. i=10,1,-1
-- do something
end
Generic for:
for k,v in pairs(t) do -- or ipairs, or completely custom functions
-- do something
end
For more information please refer to:
PIL - 4.3.4 – Numeric for
PIL - 4.3.5 – Generic for
Lua Reference Manual - 3.3.5 – For Statement
Your problem
To achieve your goal you could wrap gmatch with another iterator or... just go with the most straight-forward and simple solution: count the lines you have processed:
local n = 1
for l in webdata:gmatch("(.-)\n") do
-- do something
n = n + 1
if n > 10 then
break
end
end
It's not the most elegant one, but it works.
I am stuck trying to make the contese of a table (all integers) add together to form one sum. I am working on a project where the end goal is a percentage. I am putting the various quantities and storing them in one table. I want to then add all of those integers in the table together to get a sum. I haven't been able to find anything in the standard Library, so I have been tyring to use this:
function sum(t)
local sum = 0
for k,v in pairs(t) do
sum = sum + v
end
return sum
However, its not giving me anything after return sum.... Any and all help would be greatly appreciated.
A more generic solution to this problem of reducing the contents of a table (in this case by summing the elements) is outlined in this answer (warning: no type checking in code sketch).
If your function is not returning at all, it is probably because you are missing an end statement in the function definition.
If your function is returning zero, it is possible that there is a problem with the table you are passing as an argument. In other words, the parameter t may be nil or an empty table. In that case, the function would return zero, the value to which your local sum is initialized.
If you add print (k,v) in the loop for debugging, you can determine whether the function has anything to add. So I would try:
local function sum ( t ) do
print( "t", t ) -- for debugging: should not be nil
local s = 0
for k,v in pairs( t ) do
print(k,v) --for debugging
s = s + v
end
return s
end
local myTestData = { 1, 2, 4, 9 }
print( sum( myTestData) )
The expected output when running this code is
t table: [some index]
1 1
2 2
3 4
4 9
16
Notice that I've changed the variable name inside the function from sum to s. It's preferable not to use the function name sum as the variable holding the sum in the function definition. The local sum in the function overrides the global one, so for example, you couldn't call sum() recursively (i.e. call sum() in the definition of sum()).
How do I generate a sequence of integer numbers based on first and last number for for to loop over?
The following pseudocode
for i in sequence(4,9) do
print(i)
end
should produce the following output
4
5
6
7
8
9
Please include a short explanation what the solution does in the background and what terminology would have allowed one to find the solution.
Search attempts lead to unsearchable huge pages of documentation.
You can use numeric for loop to do that. You will find details in Programming in Lua section I referenced or the Lua manual section on For statement.
Just for the full record, there are basically 3 ways you could do this loop, one with a slightly different syntax, and 2 with the exact syntax as your pseudocode. Links point to relevant chapters in Programming in Lua (which is a great book to read, by the way).
1) Using a simple numeric for loop - in this case you won't use sequence:
for i=4,9 do
print(i)
end
2) Implement sequence as a closure:
function sequence(from,to)
local i = from - 1
return function()
if i < to then
i = i + 1
return i
end
end
end
for i in sequence(4,9) do print(i) end
3) Implement sequence as a coroutine:
function sequence(from, to)
return coroutine.wrap(function()
for i=from,to do
coroutine.yield(i)
end
end)
end
for i in sequence(4,9) do print(i) end
This question already has answers here:
Lua for loop reduce i? Weird behavior [duplicate]
(3 answers)
Closed 7 years ago.
im trying this in lua:
for i = 1, 10,1 do
print(i)
i = i+2
end
I would expect the following output:
1,4,7,10
However, it seems like i is getting not affected, so it gives me:
1,2,3,4,5,6,7,8,9,10
Can someone tell my a bit about the background concept and what is the right way to modify the counter variable?
As Colonel Thirty Two said, there is no way to modify a loop variable in Lua. Or rather more to the point, the loop counter in Lua is hidden from you. The variable i in your case is merely a copy of the counter's current value. So changing it does nothing; it will be overwritten by the actual hidden counter every time the loop cycles.
When you write a for loop in Lua, it always means exactly what it says. This is good, since it makes it abundantly clear when you're doing looping over a fixed sequence (whether a count or a set of data) and when you're doing something more complicated.
for is for fixed loops; if you want dynamic looping, you must use a while loop. That way, the reader of the code is aware that looping is not fixed; that it's under your control.
When using a Numeric for loop, you can change the increment by the third value, in your example you set it to 1.
To see what I mean:
for i = 1,10,3 do
print(i)
end
However this isn't always a practical solution, because often times you'll only want to modify the loop variable under specific conditions. When you wish to do this, you can use a while loop (or if you want your code to run at least once, a repeat loop):
local i = 1
while i < 10 do
print(i)
i = i + 1
end
Using a while loop you have full control over the condition, and any variables (be they global or upvalues).
All answers / comments so far only suggested while loops; here's two more ways of working around this problem:
If you always have the same step size, which just isn't 1, you can explicitly give the step size as in for i =start,end,stepdo … end, e.g. for i = 1, 10, 3 do … or for i = 10, 1, -1 do …. If you need varying step sizes, that won't work.
A "problem" with while-loops is that you always have to manually increment your counter and forgetting this in a sub-branch easily leads to infinite loops. I've seen the following pattern a few times:
local diff = 0
for i = 1, n do
i = i+diff
if i > n then break end
-- code here
-- and to change i for the next round, do something like
if some_condition then
diff = diff + 1 -- skip 1 forward
end
end
This way, you cannot forget incrementing i, and you still have the adjusted i available in your code. The deltas are also kept in a separate variable, so scanning this for bugs is relatively easy. (i autoincrements so must work, any assignment to i below the loop body's first line is an error, check whether you are/n't assigning diff, check branches, …)
I would like to evaluate a math string in my corona app. Right now I'm focusing on the trig functions, so let's let the example be the most difficult we're likely to face:
local expr = "2sin(4pi+2)+7"
My goal is for this to somehow be (either) evaluated as is with maybe a pi --> math.pi switch, or to even break it up. The breaking up would be much more difficult, however, since it COULD be as complicated a above, but could also just be sin(1).
So I would prefer to stay as close to the python eval(expr) function as possible, but if that can't happen, I am flexible.
The simplest way would be to replace sin with math.sin (pi with math.pi and so on), add missing multiplications signs, and run it through loadstring, but loadstring is not available in Corona environment.
This means you will need to write your own parser for these expressions. I found a discussion on Corona forums that may help you as a starting point: here, with some details and a demo here
This should do the trick, it is able to use the lua math functions without putting 'math.function' so just sqrt(100) works fine. I threw this together because I have seen this question asked way too many times. Hopes this helps :)
If you have any questions feel free to contact me at rayaman99#gmail.com
function evaluate(cmd,v) -- this uses recursion to solve math equations
--[[ We break it into pieces and solve tiny pieces at a time then put them back together
Example of whats going on
Lets say we have "5+5+5+5+5"
First we get this:
5+5+5+5 + 5
5+5+5 + 5
5+5 + 5
5 + 5
Take all the single 5's and do their opperation which is addition in this case and get 25 as our answer
if you want to visually see this with a custom expression, uncomment the code below that says '--print(l,o,r)'
]]
v=v or 0
local count=0
local function helper(o,v,r)-- a local helper function to speed things up and keep the code smaller
if type(v)=="string" then
if v:find("%D") then
v=tonumber(math[v]) or tonumber(_G[v]) -- This section allows global variables and variables from math to be used feel free to add your own enviroments
end
end
if type(r)=="string" then
if r:find("%D") then
r=tonumber(math[r]) or tonumber(_G[r]) -- A mirror from above but this affects the other side of the equation
-- Think about it as '5+a' and 'a+5' This mirror allows me to tackle both sides of the expression
end
end
local r=tonumber(r) or 0
if o=="+" then -- where we handle different math opperators
return r+v
elseif o=="-" then
return r-v
elseif o=="/" then
return r/v
elseif o=="*" then
return r*v
elseif o=="^" then
return r^v
end
end
for i,v in pairs(math) do
cmd=cmd:gsub(i.."(%b())",function(a)
a=a:sub(2,-2)
if a:sub(1,1)=="-" then
a="0"..a
end
return v(evaluate(a))
end)
end
cmd=cmd:gsub("%b()",function(a)
return evaluate(a:sub(2,-2))
end)
for l,o,r in cmd:gmatch("(.*)([%+%^%-%*/])(.*)") do -- iteration this breaks the expression into managable parts, when adding pieces into
--print(":",l,o,r) -- uncomment this to see how it does its thing
count=count+1 -- keep track for certain conditions
if l:find("[%+%^%-%*/]") then -- if I find that the lefthand side of the expression contains lets keep breaking it apart
v=helper(o,r,evaluate(l,v))-- evaluate again and do the helper function
else
if count==1 then
v=helper(o,r,l) -- Case where an expression contains one mathematical opperator
end
end
end
if count==0 then return (tonumber(cmd) or tonumber(math[cmd]) or tonumber(_G[cmd])) end
-- you can add your own enviroments as well... I use math and _G
return v
end
a=5
print(evaluate("2+2+2*2")) -- This still has work when it comes to pemdas; however, the use parentheses can order things!
print(evaluate("2+2+(2*2)"))-- <-- As seen here
print(evaluate("sqrt(100)"))
print(evaluate("sqrt(100)+abs(-100)"))
print(evaluate("sqrt(100+44)"))
print(evaluate("sqrt(100+44)/2"))
print(evaluate("5^2"))
print(evaluate("a")) -- that we stored above
print(evaluate("pi")) -- math.pi
print(evaluate("pi*2")) -- math.pi