1-D array into a 2-D array results in wrong array values

Question

My 1-D array is importing correctly and is displayed correctly, my logic also works when I do it by hand so I'm not sure what is wrong. When I copy each value in the 1-D array into the 2-D it is doing an odd pattern of copying as well as putting the wrong values in.

This is the 1-D array:

 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]

This is the following output:

[[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
row:  0  col:  0
0
In loop... [[1, 0, 0, 0], [1, 0, 0, 0], [1, 0, 0, 0], [1, 0, 0, 0]]
row:  0  col:  1
1
In loop... [[2, 0, 0, 0], [2, 0, 0, 0], [2, 0, 0, 0], [2, 0, 0, 0]]
row:  0  col:  2
2
In loop... [[3, 0, 0, 0], [3, 0, 0, 0], [3, 0, 0, 0], [3, 0, 0, 0]]
row:  0  col:  3
3
In loop... [[4, 0, 0, 0], [4, 0, 0, 0], [4, 0, 0, 0], [4, 0, 0, 0]]
row:  1  col:  0
4
In loop... [[4, 5, 0, 0], [4, 5, 0, 0], [4, 5, 0, 0], [4, 5, 0, 0]]
row:  1  col:  1
5
In loop... [[4, 6, 0, 0], [4, 6, 0, 0], [4, 6, 0, 0], [4, 6, 0, 0]]
row:  1  col:  2
6
In loop... [[4, 7, 0, 0], [4, 7, 0, 0], [4, 7, 0, 0], [4, 7, 0, 0]]
row:  1  col:  3
7
In loop... [[4, 8, 0, 0], [4, 8, 0, 0], [4, 8, 0, 0], [4, 8, 0, 0]]
row:  2  col:  0
8
In loop... [[4, 8, 9, 0], [4, 8, 9, 0], [4, 8, 9, 0], [4, 8, 9, 0]]
row:  2  col:  1
9
In loop... [[4, 8, 10, 0], [4, 8, 10, 0], [4, 8, 10, 0], [4, 8, 10, 0]]
row:  2  col:  2
10
In loop... [[4, 8, 11, 0], [4, 8, 11, 0], [4, 8, 11, 0], [4, 8, 11, 0]]
row:  2  col:  3
11
In loop... [[4, 8, 12, 0], [4, 8, 12, 0], [4, 8, 12, 0], [4, 8, 12, 0]]
row:  3  col:  0
12
In loop... [[4, 8, 12, 13], [4, 8, 12, 13], [4, 8, 12, 13], [4, 8, 12, 13]]
row:  3  col:  1
13
In loop... [[4, 8, 12, 14], [4, 8, 12, 14], [4, 8, 12, 14], [4, 8, 12, 14]]
row:  3  col:  2
14
In loop... [[4, 8, 12, 15], [4, 8, 12, 15], [4, 8, 12, 15], [4, 8, 12, 15]]
row:  3  col:  3
15
In loop... [[4, 8, 12, 16], [4, 8, 12, 16], [4, 8, 12, 16], [4, 8, 12, 16]]
Before return... [[4, 8, 12, 16], [4, 8, 12, 16], [4, 8, 12, 16], [4, 8, 12, 16]]

Here is the function code:

def makeTwoArr(array, height, width):
    print(array)
    newArray=[]
    line=[0]*width
    for i in range(height):
        newArray.append(line)
    location=0
    print(newArray)
    for row in range(height):
        for col in range(width):
            print("row: ",row," col: ",col);
            print(location)
##            #print(array[location])
            newArray[col][row]=array[location]
##            print(newArray)
            location+=1
            print("In loop...",newArray)

    print("Before return...",newArray)
    return newArray

Both me and my computer science professor can't figure out why the values are wrong, or why its filling certain spots at the wrong iteration.

Try swapping col and row in newArray[col][row]=array[location]. I think this should work. — Vivek Jain
– Vivek Jain, Commented Apr 16, 2016 at 4:36
The swap is needed but doesn't solve the mystery.... The assignment somehow applies to all rows and the correct column. — roadrunner66
– roadrunner66, Commented Apr 16, 2016 at 4:44
A matrix [[1,2,3,4],[5,6,7,8],[9... which is easy to do in numpy (reshaping) but the original poster wants to do it manually, which should work. — roadrunner66
– roadrunner66, Commented Apr 16, 2016 at 4:49

miradulo · Accepted Answer · 2016-04-16 05:02:17Z

3

When you use line=[0]*width and then append(line) you actually create multiple references to the same line, hence when you modify one line in your loops you actually modify all lines. You need to actually create different lines such that values aren't modified everywhere by reference in the same list during assignment. It is always important to remember that lists are mutable, and appending the same one creates references to the same list.

As an aside, I'm guessing for your (educational) purposes you need to do assignment in this looping fashion, but if you were trying to do so more idiomatically with the standard library you could use iter and zip with unpacking to create lists of lists, although this would only work were you to be assured that your original list has the correct number of elements. This works by passing the same iterator to zip your desired number of times in order to unpack your entire sequence.

new_arr = list(map(list,zip(*[iter(arr)]*4)))

Outputs:

[[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12], [13, 14, 15, 16]]

edited Apr 16, 2016 at 5:02

answered Apr 16, 2016 at 4:49

miradulo

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3 Comments

Brandon Williams Over a year ago

Your catch made it past me and my professor! A big thank you!

Vivek Jain Over a year ago

Nice, more Pythonic way!

miradulo Over a year ago

@VivekJain Thanks :)

Vivek Jain · Accepted Answer · 2016-04-16 04:57:57Z

2

As Donkey Kong rightly mentioned, you were creating reference to the same line. Also, the loop construct a bit incorrect.

def makeTwoArr(array, height, width):
    print(array)
    newArray=[[0 for x in range(width)] for x in range(height)] 
    print(newArray)
    row=0
    col=0

    # Use single loop where possible. Loop in a loop may hamper performance.
    for location in range( 0, len(array)):
        print("row: ",row," col: ",col);
        print(location)
        newArray[row][col]=array[location]
        col+=1
        if col == width:
            row+=1
            col=0
        print("In loop...",newArray)

    print("Before return...",newArray)
    return newArray

oldarr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]

newarr = makeTwoArr(oldarr, 4, 4)
print "############"
print ("New Array is", newarr)

I tried a different approach to loop the source array instead of the destination. The reason was to have single loop instead of double loops. Though both could be deemed as correct.

Please take this solution as a starting point and not a copy paste solution.

edited Apr 16, 2016 at 4:57

answered Apr 16, 2016 at 4:52

Vivek Jain

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1 Comment

Brandon Williams Over a year ago

I agree with you and Donkey Kong, thank you for posting it like this. I agree with both but this helps me visually see what you mean. And no, I will definitely not just copy and paste. The initial array I gave you guys is a super simple form of the one I need to use this for. Your answer will just help me understand where I went wrong. Thank you!

SQLnoob · Accepted Answer · 2016-04-16 04:53:39Z

I believe this is happening because every row in new_array is actually pointing to the same exact list, line.

Change your code like this and it should work:

def makeTwoArr(array, height, width):
    print(array)
    newArray=[]
    #line=[0]*width                    # DON'T DO THIS
    for i in range(height):
        newArray.append([0]*width)     # DO THIS INSTEAD
    location=0
    print(newArray)
    for row in range(height):
        for col in range(width):
            print("row: ",row," col: ",col);
            print(location)
            newArray[col][row]=array[location]
            print(newArray)
            location+=1
            print("In loop...",newArray)

    print("Before return...",newArray)
    return newArray

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