Problem updating a variable in a loop

Gupta

How i acn upadate the value of the variable “square=0” as mention in my code so that after every loop it will update it by addind + 1 in “square=0”

this is my code :

def findcontours(path) do
    #squares=0
    {:ok, imgGrey} = OpenCV.imread(path, flags: OpenCV.cv_IMREAD_GRAYSCALE())
    {:ok, {_, thrash}} = OpenCV.threshold(imgGrey, 240, 255, OpenCV.cv_THRESH_BINARY())
    {:ok, {contours, _}} =OpenCV.findContours(thrash, OpenCV.cv_RETR_TREE(), OpenCV.cv_CHAIN_APPROX_NONE())

    square = 0
    for contour <- contours do
      {:ok,peri} = OpenCV.arcLength(contour,true)
      {:ok,approx} = OpenCV.approxPolyDP(contour, 0.01 * peri,true)
      ap=OpenCV.Nx.to_nx(approx)
      {len,_,_}=Nx.shape(ap)


      if len==4 do #to bound squares from the countor
        {:ok,{x1, y1, w, h}}= OpenCV.boundingRect(approx)
        aspectRatio = (w/1)/h
        if aspectRatio >= 0.95 and aspectRatio <= 1.05 and (y1+h)*(x1+w) < 985000 do
          square=square +1
        end
      end
    end
  end

pls help me to update my value of square as i know there i a big problem of scope in elixir and also it is immutable so pls help me for this.

or suggest me another way so that i can do the whole process by another way if possible.

13 comments

#opencv #evision

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2022-10-25 19:59:03 UTC

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Aetherus

In Elixir, variables are actually constants. You can’t reassign values to an existing variable. You can only create a new variable with the same name and shadow the old one.

This line square = square + 1 reads the value of square outside the for-loop, adds 1 to it, and creates a new variable also named square, which is not used at all, because this new square variable is inside a sub-sub scope of the for-loop, and the old square is outside the for-loop. They are in different scopes.

How to fix this?

square = for contour <- contours, reduce: 0 do
  acc ->
    {:ok,peri} = OpenCV.arcLength(contour,true)
    {:ok,approx} = OpenCV.approxPolyDP(contour, 0.01 * peri,true)
    ap=OpenCV.Nx.to_nx(approx)
    {len,_,_}=Nx.shape(ap)

    if len==4 do #to bound squares from the countor
      {:ok,{x1, y1, w, h}}= OpenCV.boundingRect(approx)
      aspectRatio = (w/1)/h
      if aspectRatio >= 0.95 and aspectRatio <= 1.05 and (y1+h)*(x1+w) < 985000 do
        acc + 1
      else
        acc
      end
    else
      acc
    end
end

Post #1

Gupta

can u explain me briefly what u have done in this

square = for contour <- contours, reduce: 0 do
  acc ->
    {:ok,peri} = OpenCV.arcLength(contour,true)
    {:ok,approx} = OpenCV.approxPolyDP(contour, 0.01 * peri,true)
    ap=OpenCV.Nx.to_nx(approx)
    {len,_,_}=Nx.shape(ap)

    if len==4 do #to bound squares from the countor
      {:ok,{x1, y1, w, h}}= OpenCV.boundingRect(approx)
      aspectRatio = (w/1)/h
      if aspectRatio >= 0.95 and aspectRatio <= 1.05 and (y1+h)*(x1+w) < 985000 do
        acc + 1
      else
        acc
      end
    end
end

so that in further code if i will face problem regarding this i can rectify it

Post #2

Aetherus

Here’s a simple example of the for comprehension with the option :reduce:

for n <- 1..10, reduce: 0 do
  acc -> acc + n
end

# returns 55

It’s equivalent to this code (I’m using the same variable names as in the snippet above):

Enum.reduce(1..10, 0, fn n, acc ->
  acc + n
end)

The for comprehension first passes 0, the value of the option :reduce, to acc, the first element in 1..10 to n, and apply the anonymous function to acc and n. Then it passes the return value of the anonymous function to acc, the second element in 1..10 to n, and apply the anonymous function again. So on and so forth, until there’s no element to take from 1..10, then it returns the return value of the last call of that anonymous function.

Post #3

Aetherus

And note that both for ... do ... end and if ... else ... end are expressions, and they have return values.

The if without else returns whatever inside the if block returns, if the condition is met, otherwise, it returns nil, which usually is not what you want, so be careful.

Post #4

Gupta

I was trying to use tesseract-Ocr in elixir
but i am getting error pls :

iex(4)> import TesseractOcr.Utils
TesseractOcr.Utils
iex(5)> path="lib/grid_1.png"    
"lib/grid_1.png"
iex(6)> TesseractOcr.read(path)  
** (ErlangError) Erlang error: :enoent
    (elixir 1.13.4) lib/system.ex:1044: System.cmd("tesseract", ["lib/grid_1.png", "stdout"], [])
    (tesseract_ocr 0.1.5) lib/tesseract_ocr.ex:19: TesseractOcr.read/2
iex(6)>

pls help me to rectify it

Post #5

dimitarvp

It means the file path you specified does not exist.

Use File.exists? inside iex to find the right file path.

Post #6

Gupta

I am getting error again :

iex(6)> import TesseractOcr.Utils                                     
TesseractOcr.Utils
iex(7)> path="/home/erts/Downloads/task1b_evision_resource/grid_1.png"
"/home/erts/Downloads/task1b_evision_resource/grid_1.png"
iex(8)> File.exists?(path)                                            
true
iex(9)> TesseractOcr.read(path)
** (ErlangError) Erlang error: :enoent
    (elixir 1.13.4) lib/system.ex:1044: System.cmd("tesseract", ["/home/erts/Downloads/task1b_evision_resource/grid_1.png", "stdout"], [])
    (tesseract_ocr 0.1.5) lib/tesseract_ocr.ex:19: TesseractOcr.read/2
iex(9)> 
nil

why it is showing again this .

also i think i have the tesseract dependencies in mix.exs

  defp deps do
    [
      {:evision, "~> 0.1.0-dev", github: "cocoa-xu/evision", branch: "main"},
      {:nx, "~> 0.2"},
      {:tesseract_ocr, "~> 0.1.5"}
    ]
  end
end

but still it is showing me the error

Post #7

dimitarvp

It’s expecting you to have tesseract installed and put in the system PATH.

What does which tesseract show you when you run it in a terminal?

Post #8

Gupta

This the path i have added in my system.

erts@eyrc:~/Downloads/task1b_evision_resource$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/home/erts/Downloads/task1b_evision_resource/deps/tesseract_ocr
erts@eyrc:~/Downloads/task1b_evision_resource$ ^C

and this is my file location and location of my image grid_1.png is also there:

and this is my code with error i have written :

erts@eyrc:~/Downloads/task1b_evision_resource$ iex -S mix
Erlang/OTP 25 [erts-13.0.4] [source] [64-bit] [smp:1:1] [ds:1:1:10] [async-threads:1] [jit:ns]

Compiling 1 file (.ex)
Interactive Elixir (1.13.4) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> import TesseractOcr.Utils
TesseractOcr.Utils
iex(2)> path="/home/erts/Downloads/task1b_evision_resource/grid_1.png"
"/home/erts/Downloads/task1b_evision_resource/grid_1.png"
iex(3)> TesseractOcr.read(path)                                       
** (ErlangError) Erlang error: :enoent
    (elixir 1.13.4) lib/system.ex:1044: System.cmd("tesseract", ["/home/erts/Downloads/task1b_evision_resource/grid_1.png", "stdout"], [])
    (tesseract_ocr 0.1.5) lib/tesseract_ocr.ex:19: TesseractOcr.read/2
iex(3)>

Now I am not getting where is the error , i am not able to read a single image.