[Tutor] I need help with my homework. No, really....

[Steven D'Aprano]

On Wed, Jul 29, 2015 at 04:16:58AM -0500, Lissa Hopson wrote:
> I'm taking a beginning Python course at Austin Community College. I'm also
> taking two other project-based web programming courses. It's summer
> semester, meaning we have eight weeks instead of the usual 16 to finish all
> the requirements.
> The semester ends Friday, July 131st.

July 131st? Whew, you've got over 100 days to complete this! 

*wink*

But seriously... more comments (hopefully useful comments this time) 
follow below, interleaved with your code. Grab a coffee, this may be a 
bit long. Oh, and I'm going to split my reply over a couple of emails.


> Yes, I am aware that I'm a teensy bit screwed.
> 
> I have to complete eight programs ("complete" meaning "functioning"). I'm
> having a really tough time with this one. It's matrix arithmetic using 2d
> arrays.

[...]
> Given x as an array of [5,3] and y as an array of [3,7] perform the
> following:
> 
> 1. Load array x column-wise and array y row-wise

I'm not sure that I understand what this means. I think what they mean 
is that if the data looks like this:

    10, 20, 30, 40, 50, 60

and x and y are both 3x2 arrays, we end up with these:

# read data down the columns first
x = [ [10, 40],   
      [20, 50], 
      [30, 60] ]

# read data across the rows first
y = [ [10, 20],   
      [30, 40], 
      [50, 60] ]



> 2. Multiply x by y to compute array z
> 3. Compute the sum of all elements in column 2 of array x and add it to the
> sum of all elements in row 2 of y (the first row/column is 0, the second is
> 1, etc. That got me at first)
> 4. Compute the smallest element in row 1 of y
> ---using appropriate headings:
> 5. Print out matrices x, y, and z (display on screen, but y'all probably
> get that)
> 6. Print out sum and smallest element
> 
> The data with which array x is loaded:
> 1, 3, 5, 7, 9, 0, 2, 4, 6, 8, 0, 1, 2, 3, 4
> 
> The data with which array y is loaded:
> 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 3, 5, 7, 9, 0, 2, 4, 6, 8, 0, 1
> 
> Must use functions named as follows:
> LOADX, LOADY, COMPUTEZ, SMALLEST, SUMMATION, OUTDATA
> 
> lab5.dat is simply a dat file with the data with which the arrays are
> loaded in one long line, each separated by commas.

Below, you have lab5x.dat and lab5y.dat. Are there two files, or just 
one? That's going to make a big difference to the way you read the 
input.


> Thanks- in advance- no more comments after the program.
> 
> This is what I have thus far:
> 
> #Lab #5
> #COSC 1336-31493
> #SUM 2015 NRG
> #Tu/Th 1:15-4:25pm
> 
> def main():
>     #matrix initialization
>     x=[[0,0,0],[0,0,0],[0,0,0],[0,0,0],[0,0,0]]
>     y=[[0,0,0,0,0,0,0],[0,0,0,0,0,0,0],[0,0,0,0,0,0,0]]

You can simplify the matrix initialization a little bit by using list 
multiplication:

    x = [ [0]*3, [0]*3, [0]*3 ]

and similarly for y, and z. What they do should be quite obvious:

    [0]*2 --> [0, 0]
    ['hello']*3 --> ['hello', 'hello', 'hello']

Now, if you're paying attention, you might think "Wait, why don't I 
multiply each row as well?"

    [ [0]*3 ]*5  # Don't do this!

I don't want to spend to much time on this, but in a nutshell, the above 
looks like it should work, but it doesn't work as you would expect 
because it doesn't copy the inner list. Instead of getting five 
different rows of [0, 0, 0], you get the same row repeated five times.

If my explanation doesn't make sense to you, feel free to ask, or feel 
free to just accept it on faith that [ [0]*3 ]*5 will not work the way 
you want. You can always come back to discuss this later.


> z=[[0,0,0,0,0,0,0],[0,0,0,0,0,0,0],[0,0,0,0,0,0,0],[0,0,0,0,0,0,0],[0,0,0,0,0,0,0]]

Your indentation here got messed up. Unfortunately sometimes email 
doesn't work well with indentation, which is sad. To fix this, you need 
to indent the line z = ... so that it in aligned with the other lines 
inside the main function.

    z = [ [0]*7, [0]*7, etc. ]


>     #file declaration
>     infile = open('lab5x.dat','r')
>     infile = open('lab5y.dat','r')
>     outfile = open('lab5.out', 'w')

You have two variables both called "infile", that isn't going to work. 
You need to give them separate names, say, infileX and infileY.


>     #variables
>     sumx = 0
>     sumy = 0
>     small = 0
>     A = 0
>     B = 0
>     C = 0

I'm not sure that you need these A B C variables. I think you actually 
want to use x, y, z, the three matrices you already initialized.


>     #call functions
>     LOADX(infile, A)
>     LOADY(infile, B)
>     COMPUTEZ(A, B, C)
>     SUMMATION(A, B)
>     SMALLEST(A)
>     OUTDATA(file, A, B, C)

That will become:

    LOADX(infileX, x)
    LOADY(infileY, y)
    COMPUTEZ(x, y, z)
    SUMMATION(x, y)
    SMALLEST(x)
    OUTDATA(outfile, x, y, z)


>     #close files
>     infile.close()
>     infile.close()
>     outfile.close()
>     dummy = input('Press any key to continue.')

Don't forget to change the names of those infiles.

More to follow in my next email.




-- 
Steve