in a ruby learning book .I face this code :
f = File.new("a.txt", "r")
while a = f.getc
puts a.chr
f.seek(5, IO::SEEK_CUR)
end
author writes that this code produce every fifth character in a file, but I don't understand why? please explain me line by line.
thanks.
f = File.new("a.txt", "r")
This line opens the file in read mode and keeps the file object(as an I/O(Input/Output) stream) in variable f. (See File#new)
while a = f.getc
getc is a method of class IO which gets one character of the I/O stream at a time and it will give nil, when it meets the end of the I/O stream. So while a = f.getc will loop until the end of file. (See IO#getc)
puts a.chr
f.getc will give the ASCII value of the character and inorder to get the character from the ASCII value, we apply a.chr(See Integer#chr). I think in Ruby 1.9, we will get the character itself as the output of getc, but for earlier versions, we get the ASCII value as the output. The first getc command reads the first character and moves the position of I/O stream after the first character.
f.seek(5, IO::SEEK_CUR)
seek is a method of I/O stream which changes the position of the I/O stream by an offset of the first parameter from the second parameter. IO::SEEK_CUR is a constant which gives the current position of the I/O stream. So f.seek(5, IO::SEEK_CUR) moves the position to 5 places from the current position. (See IO#seek)
This will continue till a = f.getc becomes a false condition(here at the end of file, f.getc becomes nil, which is a falsey value in Ruby(false and nil are the only falsy values in Ruby, all others are truth values))
Use IRB to study and experiment with Ruby.
a = f.getc; puts a.chr outputs a single character; f.seek(5, IO::SEEK_CUR) moves forward by 5 characters.
Related
I'm new to this platform and I'm still learning to
program in Lua, so, if any newbie errors appear, forgive me.
The following code is from one of the functions in my project that reads the insert
of the user and validates whether or not it is a data of type "Number". If,
the loop will be broken and the function will return the user input, otherwise, the
program will ask the user to enter the data again:
function bin.readnum(text)
local insertion
if text == nil then text = "Text: " end
while (insertion == nil) do
insertion = nil
print(text)
insertion = io.read("number")
if insertion ~= nil then break end
end
return insertion
end
But, if the user enters a wrong data (string) the function prints the text
madly instead of asking the user to re-enter the data.
When io.read fails to parse the data it got into a number, it doesn't discard it, but instead leaves it in the buffer for the next call to it. That means that in your code, instead of letting the user enter something else, it'll just keep trying to parse the same non-number forever. To fix it, in your if insertion ~= nil then block, do io.read() right before break, to read and discard the whole invalid line.
In addition to what Joseph Sible said:
io.read("number") is wrong: 5.1 docs demand "*n" and 5.4 docs demand just "n" for reading numbers. It probably works nevertheless due to Lua just searching for the chars in the string.
I recommend just replacing insertion = io.read("number") withinsertion = tonumber(assert(io.read(), "EOF")) - this will read a line and try to parse it as a number; the assert gracefully deals with nil being returned by io.read for EOF.
You don't need to set insertion to nil, the later assignment will do that already if what was read is not a valid number.
Style: Consider replacing your explicit nil checks with truthiness checks and removing the parentheses around the while-condition. You don't need a break, you can immediately return the read number; finally, you can even replace the entire loop with tail recursion.
All in all I'd rewrite it as follows:
function bin.readnum(text)
print(text or "Text: ")
local num = tonumber(assert(io.read(), "EOF"))
if num then return num end
return bin.readnum(text)
end
or alternatively using a repeat-until loop:
function bin.readnum(text)
local num
repeat
print(text or "Text: ")
num = tonumber(assert(io.read(), "EOF"))
until num
return num
end
I'm attempting to write a script in LUA for the Minecraft mod ComputerCraft. It's supposed to send a turtle down, mine a hole, and place ladders before returning to the surface. I'm trying to make an error display when the turtle doesn't have enough ladders, but I'm receiving an error that prevents it from running. "mineDown :18: attempt to compare string with number expected, got string."
-- This gets the user to tell the turtle how far to dig down
print('How far down should I go?')
distDown = io.read()
distMoved = 0
ladders = turtle.getItemCount(13)
-- Check if the number of ladders is less than the distance needed to move. If so, returns error.
turtle.select(13)
if ladders < distDown then
error('Not enough ladders!')
end
The error means that ladders is number and distDown is a string. You need to convert them to the same type. For example to convert ladders to a string use tostring or distDown to a number use tonumber:
if ladders < tonumber(distDown) then
I have an binary file with shows glibberish infos if i open it in Notepad.
I am working on an plugin to use with wireshark.
So my problem is that I need help. I am reading in an File and need to find 'V' '0' '0' '1' (0x56 0x30 0x30 0x31) in the File, because its the start of an Header, with means there is an packet inside. And I need to do this for the whole file, like parsing. Also should start the Frame with V 0 0 1 and not end with it.
I currently have an Code where I am searching for 0x7E and parse it. What I need is the length of the frame. For example V 0 0 1 is found, so the Length from V to the Position before the next V 0 0 1 in the File. So that I can work with the length and add it to an captured length to get the positions, that wireshark can work with.
For example my unperfect Code for working with 0x7E:
local line = file:read()
local len = 0
for c in (line or ''):gmatch ('.') do
len = len + 1
if c:byte() == 0x7E then
break
end
end
if not line then
return false
end
frame.captured_length = len
Here is also the Problem that the Frame ends with 7E which is wrong. I need something that works perfectly for 'V' '0' '0' '1'. Maybe I need to use string.find?
Please help me!
Thats an example how my file looks like if i use the HEX-Editor in Visual Studio Code.
Lua has some neat pattern tools. Here's a summary:
(...) Exports all captured text within () and gives it to us.
-, +, *, ?, "Optional match as little as possible", "Mandatory match as much as possible", "optional match as much as possible", "Optional match only once", respectively.
^ and $: Root to start or end of file, respectively.
We'll be using this universal input and output to test with:
local output = {}
local input = "V001Packet1V001Packet2oooV001aaandweredonehere"
The easiest way to do this is probably to recursively split the string, with one ending at the character before "V", and the other starting at the character after "1". We'll use a pattern which exports the part before and after V001:
local this, next = string.match(input, "(.-)V001(.*)")
print(this,next) --> "", "Packet1V001Packet2..."
Simple enough. Now we need to do it again, and we also need to eliminate the first empty packet, because it's a quirk of the pattern. We can probably just say that any empty this string should not be added:
if this ~= "" then
table.insert(output, this)
end
Now, the last packet will return nil for both this and next, because there will not be another V001 at the end. We can prepare for that by simply adding the last part of the string when the pattern does not match.
All put together:
local function doStep(str)
local this, next = string.match(str, "(.-)V001(.*)")
print(this,next)
if this then
-- There is still more packets left
if this ~= "" then
-- This is an empty packet
table.insert(output, this)
end
if next ~= "" then
-- There is more out there!
doStep(next)
end
else
-- We are the last survivor.
table.insert(output, str)
end
end
Of course, this can be improved, but it should be a good starting point. To prove it works, this script:
doStep(input)
print(table.concat(output, "; "))
prints this:
Packet1; Packet2ooo; aaandweredonehere
game:GetService("Players").PlayerAdded:connect(function()
for _, Player in pairs(game:GetService("Players"):GetPlayers()) do
Player.Chatted:connect(function(msg)
if string.sub(msg,1,5) == "oofergang" then
Player:Kick("no no no cringe baby")
end
end)
end
end)
return ''
How do I fix this? It doesn't do anything, no errors nothing.
Your issue seems to be in your string.sub() usage. (I'm assuming this is Roblox, which I don't know much about).
The string.sub(a, b, c) method takes the substring of the string a, starting from index b and going to index c.
Your problem is that you're trying to get the substring from characters 1-5. Character 1 is the first character and character 5 is the 5th character in the string. Your if block is checking the first 5 characters of the player's message. The issue is that the string you're comparing it to, "oofergang", is longer than 5 characters.
If the player does correctly type oofergang, the string.sub() that you're using will output oofer, which is the first 5 characters of the message. Essentially, this is what the program will see when running:
if "oofer" == "oofergang" then
oofer is never going to equal oofergang.
If you want to check if the player starts their message with oofergang then you should use the following if block instead:
if (string.sub(msg, 1, 9) == "oofergang") then
--Whatever you want to do here, in your case kick the player
end
EDIT: As suggested, the following code allows you to find a string within another string ANYwhere, not just a the start:
if (string.find(msg, "oofergang")) then
--Whatever you want to do here, in your case kick the player
end
I have many text files of this format
....
<snip>
'FOP' 0.19 1 24 1 25 7 8 /
'FOP' 0.18 1 24 1 25 9 11 /
/
TURX
560231
300244
70029
200250
645257
800191
900333
600334
770291
300335
220287
110262 /
SUBTRACT
'TURX' 'TURY'/
</snip>
......
where the portions I snipped off contain other various data in various formats. The file format is inconsistent (machine generated), the only thing one is assured of is the keyword TURX which may appear more than once. If it appears alone on one line, then the next few lines will contain numbers that I need to fetch into an array. The last number will have a space then a forward slash (/). I can then use this array in other operations afterwards.
How do I "search" or parse a file of unknown format in fortran, and how do I get a loop to fetch the rest of the data, please? I am really new to this and I HAVE to use fortran. Thanks.
Fortran 95 / 2003 have a lot of string and file handling features that make this easier.
For example, this code fragment to process a file of unknown length:
use iso_fortran_env
character (len=100) :: line
integer :: ReadCode
ReadLoop: do
read (75, '(A)', iostat=ReadCode ) line
if ( ReadCode /= 0 ) then
if ( ReadCode == iostat_end ) then
exit ReadLoop
else
write ( *, '( / "Error reading file: ", I0 )' ) ReadCode
stop
end if
end if
! code to process the line ....
end do ReadLoop
Then the "process the line" code can contain several sections depending on a logical variable "Have_TURX". If Have_TRUX is false you are "seeking" ... test whether the line contains "TURX". You could use a plain "==" if TURX is always at the start of the string, or for more generality you could use the intrinsic function "index" to test whether the string "line" contains TURX.
Once the program is in the mode Have_TRUX is true, then you use "internal I/O" to read the numeric value from the string. Since the integers have varying lengths and are left-justified, the easiest way is to use "list-directed I/O": combining these:
read (line, *) integer_variable
Then you could use the intrinsic function "index" again to test whether the string also contains a slash, in which case you change Have_TRUX to false and end reading mode.
If you need to put the numbers into an array, it might be necessary to read the file twice, or to backspace the file, because you will have to allocate the array, and you can't do that until you know the size of the array. Or you could pop the numbers into a linked list, then when you hit the slash allocate the array and fill it from the linked list. Or if there is a known maximum number of values you could use a temporary array, then transfer the numbers to an allocatable output array. This is assuming that you want the output argument of the subroutine be an allocatable array of the correct length, and the it returns one group of numbers per call:
integer, dimension (:), allocatable, intent (out) :: numbers
allocate (numbers (1: HowMany) )
P.S. There is a brief summary of the language features at http://en.wikipedia.org/wiki/Fortran_95_language_features and the gfortran manual has a summary of the intrinsic procedures, from which you can see what built in functions are available for string handling.
I'll give you a nudge in the right direction so that you can finish your project.
Some basics:
Do/While as you'll need some sort of loop
structure to loop through the file
and then over the numbers. There's
no for loop in Fortran, so use this
type.
Read
to read the strings.
To start you need something like this:
program readlines
implicit none
character (len=30) :: rdline
integer,dimension(1000) :: array
! This sets up a character array with 30 positions and an integer array with 1000
!
open(18,file='fileread.txt')
do
read(18,*) rdline
if (trim(rdline).eq.'TURX') exit !loop until the trimmed off portion matches TURX
end do
See this thread for way to turn your strings into integers.
Final edit: Looks like MSB has got most of what I just found out. The iostat argument of the read is the key to it. See this site for a sample program.
Here was my final way around it.
PROGRAM fetchnumbers
implicit none
character (len=50) ::line, numdata
logical ::is_numeric
integer ::I,iost,iost2,counter=0,number
integer, parameter :: long = selected_int_kind(10)
integer, dimension(1000)::numbers !Can the number of numbers be up to 1000?
open(20,file='inputfile.txt') !assuming file is in the same location as program
ReadLoop: do
read(20,*,iostat=iost) line !read data line by line
if (iost .LT. 0) exit !end of file reached before TURX was found
if (len_trim(line)==0) cycle ReadLoop !ignore empty lines
if (index(line, 'TURX').EQ.1) then !prepare to begin capturing
GetNumbers: do
read(20, *,iostat=iost2)numdata !read in the numbers one by one
if (.NOT.is_numeric(numdata)) exit !no more numbers to read
if (iost2 .LT. 0) exit !end of file reached while fetching numbers
read (numdata,*) number !read string value into a number
counter = counter + 1
Storeloop: do I =1,counter
if (I<counter) cycle StoreLoop
numbers(counter)=number !storing data into array
end do StoreLoop
end do GetNumbers
end if
end do ReadLoop
write(*,*) "Numbers are:"
do I=1,counter
write(*,'(I14)') numbers(I)
end do
END PROGRAM fetchnumbers
FUNCTION is_numeric(string)
IMPLICIT NONE
CHARACTER(len=*), INTENT(IN) :: string
LOGICAL :: is_numeric
REAL :: x
INTEGER :: e
is_numeric = .FALSE.
READ(string,*,IOSTAT=e) x
IF (e == 0) is_numeric = .TRUE.
END FUNCTION is_numeric