Creating a Matrix in Python without numpy

Question

I'm trying to create and initialize a matrix. Where I'm having an issue is that each row of my matrix I create is the same, rather than moving through the data set. I've tried to correct it by checking if the value was already in the matrix and that didn't solve my problem.

def createMatrix(rowCount, colCount, dataList):   
    mat = []
    for i in range (rowCount):
        rowList = []
        for j in range (colCount):
            if dataList[j] not in mat:
                rowList.append(dataList[j])
        mat.append(rowList)

    return mat 

def main():  
    alpha = ['a','b','c','d','e','f','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z']
    mat = createMatrix(5,5,alpha)
    print (mat)

The output should be like this: ['a','b','c','d','e'] , ['f','h','i','j','k'], ['l','m','n','o','p'] , ['q','r','s','t','u'], ['v','w','x','y','z']

My issue is I just keep getting the first a,b,c,d,e list for all 5 lists returned

enderland · Accepted Answer · 2016-10-19T02:15:38.930

You need to keep track of the current index in your loop.

Essentially you want to turn a list like 0,1,2,3,4,....24 (these are the indices of your initial array, alpha) into:

R1C1, R1C2, R1C3, R1C4, R1C5 R2C1, R2C2... etc

I added the logic to do this the way you are currently doing it:

def createMatrix(rowCount, colCount, dataList):
    mat = []
    for i in range(rowCount):
        rowList = []
        for j in range(colCount):
            # you need to increment through dataList here, like this:
            rowList.append(dataList[rowCount * i + j])
        mat.append(rowList)

    return mat

def main():
    alpha = ['a','b','c','d','e','f','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z']
    mat = createMatrix(5,5,alpha)
    print (mat)

main()

which then prints out:

[['a', 'b', 'c', 'd', 'e'], ['f', 'h', 'i', 'j', 'k'], ['l', 'm', 'n', 'o', 'p'], ['q', 'r', 's', 't', 'u'], ['v', 'w', 'x', 'y', 'z']]

The reason you were always receiving a,b,c,d,e is because when you write this:

        rowList.append(dataList[j])

what it is effectively doing is it is iterating 0-4 for every row. So basically:

i = 0
rowList.append(dataList[0])
rowList.append(dataList[1])
rowList.append(dataList[2])
rowList.append(dataList[3])
rowList.append(dataList[4])
i = 1
rowList.append(dataList[0]) # should be 5
rowList.append(dataList[1]) # should be 6
rowList.append(dataList[2]) # should be 7
rowList.append(dataList[3]) # should be 8
rowList.append(dataList[4]) # should be 9

etc.

That should be `rowList.append(dataList[colCount * i + j])` in `createMatrix`. — Nathan Pierson, Aug 15 '20 at 05:51

dawg · Answer 2 · 2016-10-19T04:18:53.910

You can use a list comprehension:

>>> li= ['a','b','c','d','e','f','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z']
>>> [li[i:i+5] for i in range(0,len(li),5)]
[['a', 'b', 'c', 'd', 'e'], ['f', 'h', 'i', 'j', 'k'], ['l', 'm', 'n', 'o', 'p'], ['q', 'r', 's', 't', 'u'], ['v', 'w', 'x', 'y', 'z']]

Or, if you don't mind tuples, use zip:

>>> zip(*[iter(li)]*5)
[('a', 'b', 'c', 'd', 'e'), ('f', 'h', 'i', 'j', 'k'), ('l', 'm', 'n', 'o', 'p'), ('q', 'r', 's', 't', 'u'), ('v', 'w', 'x', 'y', 'z')]

Or apply list to the tuples:

>>> map(list, zip(*[iter(li)]*5))
[['a', 'b', 'c', 'd', 'e'], ['f', 'h', 'i', 'j', 'k'], ['l', 'm', 'n', 'o', 'p'], ['q', 'r', 's', 't', 'u'], ['v', 'w', 'x', 'y', 'z']]

Creating a Matrix in Python without numpy

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