[course]05 —— 二维列表

1. Creating 2d Lists

Static Allocation

# create a 2d list with fixed values (static allocation)
a = [ [ 2, 3, 4 ] , [ 5, 6, 7 ] ]
print(a)

Dynamic (Variable-Length) Allocation

Wrong: Cannot use * (Shallow Copy)

# Try, and FAIL, to create a variable-sized 2d list
rows = 3
cols = 2

a = [ [0] * cols ] * rows # Error: creates shallow copy
                          # Creates one unique row, the rest are aliases!

print("This SEEMS ok.  At first:")
print("   a =", a)

a[0][0] = 42
print("But see what happens after a[0][0]=42")
print("   a =", a)

Right: Append Each Row

# Create a variable-sized 2d list
rows = 3
cols = 2

a = []
for row in range(rows):
    a += [[0]*cols]

print("This IS ok.  First:")
print("   a =", a)

a[0][0] = 42
print("And now see what happens after a[0][0]=42")
print("   a =", a)

Another good option: use a list comprehension

rows = 3
cols = 2

#This is what's called a "list comprehension"
a = [ ([0] * cols) for row in range(rows) ]

print("This IS ok.  First:")
print("   a =", a)

a[0][0] = 42
print("And now see what happens after a[0][0]=42")
print("   a =", a)

Best option: make2dList()

def make2dList(rows, cols):
    return [ ([0] * cols) for row in range(rows) ]

rows = 3
cols = 2

a = make2dList(rows, cols)

print("This IS ok.  At first:")
print("   a =", a)

a[0][0] = 42
print("And now see what happens after a[0][0]=42")
print("   a =", a)

2. Getting 2d List Dimensions

# Create an "arbitrary" 2d List
a = [ [ 2, 3, 5] , [ 1, 4, 7 ] ]
print("a = ", a)

# Now find its dimensions
rows = len(a)
cols = len(a[0])
print("rows =", rows)
print("cols =", cols)

3. Copying and Aliasing 2d Lists

Wrong: Cannot use copy.copy (shallow copy)

import copy

# Create a 2d list
a = [ [ 1, 2, 3 ] , [ 4, 5, 6 ] ]

# Try to copy it
b = copy.copy(a) # Error:  creates shallow copy

# At first, things seem ok
print("At first...")
print("   a =", a)
print("   b =", b)

# Now modify a[0][0]
a[0][0] = 9
print("But after a[0][0] = 9")
print("   a =", a)
print("   b =", b)

Right: use copy.deepcopy

import copy

# Create a 2d list
a = [ [ 1, 2, 3 ] , [ 4, 5, 6 ] ]

# Try to copy it
b = copy.deepcopy(a) # Correct!

# At first, things seem ok
print("At first...")
print("   a =", a)
print("   b =", b)

# Now modify a[0][0]
a[0][0] = 9
print("And after a[0][0] = 9")
print("   a =", a)
print("   b =", b)

Limitations of copy.deepcopy

a = [[0]*2]*3 # makes 3 shallow copies of (aliases of) the same row
a[0][0] = 42  # appears to modify all 3 rows
print(a)      # prints [[42, 0], [42, 0], [42, 0]]

# now do it again with a deepcopy

import copy
a = [[0]*2]*3        # makes 3 shallow copies of the same row
a = copy.deepcopy(a) # meant to make each row distinct
a[0][0] = 42         # so we hope this only modifies first row
print(a)             # STILL prints [[42, 0], [42, 0], [42, 0]]

# deepcopy preserves any already-existing aliases perfectly!
# best answer: don't create aliases in the first place, unless you want them.

Advanced: alias-breaking deepcopy

# Advanced: now one more time with a simple deepcopy alternative that does
# what we thought deepcopy did...
# NOTE: this uses recursion. We'll go over how that works in the future.

import copy

def myDeepCopy(a):
    if (isinstance(a, list) or isinstance(a, tuple)):
        return [myDeepCopy(element) for element in a]
    else:
        return copy.copy(a)

a = [[0]*2]*3     # makes 3 shallow copies of the same row
a = myDeepCopy(a) # once again, meant to make each row distinct
a[0][0] = 42      # so we hope this only modifies first row
print(a)          # finally, prints [[42, 0], [0, 0], [0, 0]]

# now all the aliases are gone!

4. Printing 2d Lists

# Helper function for print2dList.
# This finds the maximum length of the string
# representation of any item in the 2d list
def maxItemLength(a):
    maxLen = 0
    rows = len(a)
    cols = len(a[0])
    for row in range(rows):
        for col in range(cols):
            maxLen = max(maxLen, len(str(a[row][col])))
    return maxLen

# Because Python prints 2d lists on one row,
# we might want to write our own function
# that prints 2d lists a bit nicer.
def print2dList(a):
    if (a == []):
        # So we don't crash accessing a[0]
        print([])
        return
    rows = len(a)
    cols = len(a[0])
    fieldWidth = maxItemLength(a)
    print("[ ", end="")
    for row in range(rows):
        if (row > 0): print("\n  ", end="")
        print("[ ", end="")
        for col in range(cols):
            if (col > 0): print(", ", end="")
            # The next 2 lines print a[row][col] with the given fieldWidth
            formatSpec = "%" + str(fieldWidth) + "s"
            print(formatSpec % str(a[row][col]), end="")
        print(" ]", end="")
    print("]")

# Let's give the new function a try!
a = [ [ 1, 2, 3 ] , [ 4, 5, 67 ] ]
print2dList(a)

5. Nested Looping over 2d Lists

# Create an "arbitrary" 2d List
a = [ [ 2, 3, 5] , [ 1, 4, 7 ] ]
print("Before: a =", a)

# Now find its dimensions
rows = len(a)
cols = len(a[0])

# And now loop over every element
# Here, we'll add one to each element,
# just to make a change we can easily see
for row in range(rows):
    for col in range(cols):
        # This code will be run rows*cols times, once for each
        # element in the 2d list
        a[row][col] += 1

# Finally, print the results
print("After:  a =", a)

 Copy Visualize Run

6. Accessing 2d Lists by Row or Column

Accessing a whole row

# alias (not a copy! no new list created)
a = [ [ 1, 2, 3 ] , [ 4, 5, 6 ] ]
row = 1
rowList = a[row]
print(rowList)

Accessing a whole column

# copy (not an alias! new list created)
a = [ [ 1, 2, 3 ] , [ 4, 5, 6 ] ]
col = 1
colList = [ ]
for i in range(len(a)):
    colList += [ a[i][col] ]
print(colList)

Accessing a whole column with a list comprehension

# still a copy, but cleaner with a list comprehension!
a = [ [ 1, 2, 3 ] , [ 4, 5, 6 ] ]
col = 1
colList = [ a[i][col] for i in range(len(a)) ]
print(colList)

7. Non-Rectangular ("Ragged") 2d Lists

# 2d lists do not have to be rectangular
a = [ [ 1, 2, 3 ] ,
      [ 4, 5 ],
      [ 6 ],
      [ 7, 8, 9, 10 ] ]

rows = len(a)
for row in range(rows):
    cols = len(a[row]) # now cols depends on each row
    print("Row", row, "has", cols, "columns: ", end="")
    for col in range(cols):
        print(a[row][col], " ", end="")
    print()

8. 3d Lists

# 2d lists do not really exist in Python.
# They are just lists that happen to contain other lists as elements.
# And so this can be done for "3d lists", or even "4d" or higher-dimensional lists.
# And these can also be non-rectangular, of course!

a = [ [ [ 1, 2 ],
        [ 3, 4 ] ],
      [ [ 5, 6, 7 ],
        [ 8, 9 ] ],
      [ [ 10 ] ] ]

for i in range(len(a)):
    for j in range(len(a[i])):
        for k in range(len(a[i][j])):
            print("a[%d][%d][%d] = %d" % (i, j, k, a[i][j][k]))