# Multidimensional arrays

**URL:** <https://discourse.processing.org/t/multidimensional-arrays/37053>\
**Category:** Processing.py\
**Created:** [May 24, 2022, 4:04am UTC](https://discourse.processing.org/t/multidimensional-arrays/37053 "2022-05-24T04:04:03Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![povijarrro](https://avatars.discourse-cdn.com/v4/letter/p/c68b51/32.png) [@povijarrro](https://discourse.processing.org/u/povijarrro)\
**Post date:** [May 24, 2022, 4:04am UTC](https://discourse.processing.org/t/multidimensional-arrays/37053/1 "2022-05-24T04:04:03Z")

</div>

hello. I am just wondering what is difference between m\*[n\*[1]] and [n\*[1] for k in range(m)]  
Because this code:

```auto
m,n = 50,50
grid = [n*[1] for k in range(m)]
screen_size =(600,600)
w=float(screen_size[0])/n
h=float(screen_size[1])/m
def setup():
    size(screen_size[0],screen_size[1])
    noLoop()
    
def draw():
    draw_grid()
    
            
def draw_grid():
    x,y=0,0
    for row in grid:
        for ele in row:
            if ele == -1:
                fill(0)
            else:
                fill(255)    
            rect(x,y,w,h)
            x+=w
        y+=h
        x=0    
            

def get_grid_coordinates(x,y):
    return int(y/h), int(x/w)                    

def mousePressed():
        coords = get_grid_coordinates(mouseX,mouseY)
        pcoords = get_grid_coordinates(pmouseX,pmouseY)
        grid[coords[0]][coords[1]],grid[pcoords[0]][pcoords[1]] = -grid[coords[0]][coords[1]],-grid[pcoords[0]][pcoords[1]]
        redraw()

```

works fine (change color of clicked tile in grid) but this code:

```auto
m,n = 50,50
grid = m*[n*[1]]
screen_size =(600,600)
w=float(screen_size[0])/n
h=float(screen_size[1])/m
def setup():
    size(screen_size[0],screen_size[1])
    noLoop()
    
def draw():
    draw_grid()
    
            
def draw_grid():
    x,y=0,0
    for row in grid:
        for ele in row:
            if ele == -1:
                fill(0)
            else:
                fill(255)    
            rect(x,y,w,h)
            x+=w
        y+=h
        x=0    
            

def get_grid_coordinates(x,y):
    return int(y/h), int(x/w)                    

def mousePressed():
        coords = get_grid_coordinates(mouseX,mouseY)
        pcoords = get_grid_coordinates(pmouseX,pmouseY)
        grid[coords[0]][coords[1]],grid[pcoords[0]][pcoords[1]] = -grid[coords[0]][coords[1]],-grid[pcoords[0]][pcoords[1]]
        redraw()

```

changes whole columns.  
The strangest is that print([n\*[1]] for k in range(m) == m\*[n\*[1]]) writes True.  
I am using processing 4.0b8

---

<div class="post-metadata">

**Author:** ![NumericPrime](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/numericprime/32/16318_2.png) [@NumericPrime](https://discourse.processing.org/u/NumericPrime)\
**Post date:** [May 24, 2022, 6:43am UTC](https://discourse.processing.org/t/multidimensional-arrays/37053/2 "2022-05-24T06:43:51Z")

</div>

I think you should move this to  
Processing.py

the Processing thread is for java mode.  
And format your code with \</\> trying to copy this would be a nightmare because the spaces actually are important in python…

---

<div class="post-metadata">

**Author:** ![NumericPrime](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/numericprime/32/16318_2.png) [@NumericPrime](https://discourse.processing.org/u/NumericPrime)\
**Post date:** [May 24, 2022, 6:50am UTC](https://discourse.processing.org/t/multidimensional-arrays/37053/3 "2022-05-24T06:50:08Z")

</div>

I looked it up and python seems to have the same “problem” than Java.  
Have you ever tried copying an array with:

```auto
array1=array2

```

You should notice that they are “linked” so any action you do with one you also do to the other this is called a deep copy.  
Types like numbers or string don’t have deep copies so you can savely use:

```auto
number1=number2

```

What python does it duplicates the line-array however it uses a deep copy so any action you perform on one is also performed on the other.

What you are looking for is a shallow copy wich only copies the data there is actually a library for this so you can use:

```auto
import copy
m=10
n=10
grid=n*[copy.copy(m*[1])]

```

I don’t know if python mode of Processing has a built in function for this.

---

<div class="post-metadata">

**Author:** ![povijarrro](https://avatars.discourse-cdn.com/v4/letter/p/c68b51/32.png) [@povijarrro](https://discourse.processing.org/u/povijarrro)\
**Post date:** [May 25, 2022, 3:45am UTC](https://discourse.processing.org/t/multidimensional-arrays/37053/4 "2022-05-25T03:45:38Z")

</div>

Thanks for reply and moving to python mode section.  
If I am right in case `m*[n*[1]]` python m times append the same physical list so if one change one of them, the others will be changed too, but in case `[n*[1] for k in range(m)]` for each k, new list of ones is generated.  
Am I right?

---

<div class="post-metadata">

**Author:** ![GoToLoop](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/gotoloop/32/86_2.png) [@GoToLoop](https://discourse.processing.org/u/GoToLoop)\
**Post date:** [May 25, 2022, 4:22am UTC](https://discourse.processing.org/t/multidimensional-arrays/37053/5 "2022-05-25T04:22:28Z")

</div>

> [@povijarrro](#):
>
> , but in case `[n*[1] for k in range(m)]` for each k, new list of ones is generated. Am I right?

Yup! Alternatively you can turn the outer dimension a [Tuple](https://py.processing.org/reference/tuple.html) container:  
`tuple(n*[0] for _ in range(m))`

But notice the literal `1` you’re using is immutable.  
That’s why it’s a good candidate for filling up the entire container.

Otherwise, something mutable like a [PVector](https://py.processing.org/reference/PVector.html) would cause the same shallow-copy mirror-change bug:  
`tuple(n*[PVector()] for _ in range(m))`

---

<div class="post-metadata">

**Author:** ![GoToLoop](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/gotoloop/32/86_2.png) [@GoToLoop](https://discourse.processing.org/u/GoToLoop)\
**Post date:** [May 25, 2022, 4:53am UTC](https://discourse.processing.org/t/multidimensional-arrays/37053/6 "2022-05-25T04:53:20Z")

</div>

BtW, creating a 2d grid container is worth making a custom function for it: #️⃣

```auto
def setup():
    global grid
    grid = createGrid(5) # 5x5 = 0
    print grid
    exit()

def createGrid(rows, cols = -1, fillValue = 0):
    if cols < 0: cols = rows
    return tuple([fillValue] * cols for _ in range(rows))

```