Creating a matrix using lists Python

Question

In Python, matrices can be created using nested lists. For example, [[1, 2], [3, 4]]. Below I have written a function that prompts the user for the dimensions of a square matrix and then prompts the user for the values in the for loop. I have a tempArray variable that temporarily stores one row of values, and then is deleted after it is appended to the matrix array. For some reason, when I print the matrix at the end, this is what I get: [ [ ], [ ] ]. What is going wrong?

def proj11_1_a():
    n = eval(input("Enter the size of the square matrix: "))
    matrix = []
    tempArray = []   

    for i in range(1, (n**2) + 1):
        val = eval(input("Enter a value to go into the matrix: "))

        if i % n == 0:
            tempArray.append(val)
            matrix.append(tempArray)
            del tempArray[:]
        else:
            tempArray.append(val)
    print(matrix)
proj11_1_a()

the array is only deleted after it has one row of values. The reason why it is deleted is so it can then store the next row of values and then append it to the matrix array. — Rehaan Advani
– Rehaan Advani, Commented Dec 5, 2015 at 23:25

lejlot · Accepted Answer · 2015-12-05 23:58:57Z

2

You simply delete the array elements del tempArray[:] and as lists are mutable it also clears part of matrix

def proj11_1_a():
    n = eval(input("Enter the size of the square matrix: "))
    matrix = []
    tempArray = []   

    for i in range(1, (n**2) + 1):
        val = eval(input("Enter a value to go into the matrix: "))

        if i % n == 0:
            tempArray.append(val)
            matrix.append(tempArray)
            tempArray = [] #del tempArray[:]
        else:
            tempArray.append(val)
    print(matrix)
proj11_1_a()

Which could be further simplified/cleared to

def proj11_1_a():
    # Using eval in such place does not seem a good idea
    # unless you want to accept things like "2*4-2"
    # You might also consider putting try: here to check for correctness

    n = int(input("Enter the size of the square matrix: "))
    matrix = []

    for _ in range(n): 
        row = []   

        for _ in range(n): 
            # same situation as with n
            value = float(input("Enter a value to go into the matrix: "))
            row.append(value)

        matrix.append(row)

    return matrix

edited Dec 5, 2015 at 23:58

answered Dec 5, 2015 at 23:24

lejlot

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3 Comments

user1040495 Over a year ago

you will be doing the append(val) anyways so it is better to get it out from the if/else blocks

lejlot Over a year ago

@Jjpx - it is OP code, only fragment causing the problem was changed, there are many things here which could be improved

Budo Zindovic Over a year ago

As @Jjpx suggested, you could provide optimal version as well. I think it would be beneficial to the OP to have both versions.

Budo Zindovic · Accepted Answer · 2015-12-05 23:32:40Z

1

Another solution is to change the following line:

matrix.append(tempArray)

to:

matrix.append(tempArray.copy())

answered Dec 5, 2015 at 23:32

Budo Zindovic

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3 Comments

lejlot Over a year ago

which is suboptimal as you need to ... copy the object (thus if it is large, it takes linear time).

Budo Zindovic Over a year ago

As the original intention was to prompt user for the input, overhead is minimal compared to the speed of the user input.

lejlot Over a year ago

Sure, but it does not mean one should write insufficient code elsewhere. Neither should we suggest it to people learning python (like OP)

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