- 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. At 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. At 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)
- 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)
- 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!
- Printing 2d Lists
- Basic Version:
# 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, cols = len(a), len(a[0])
fieldWidth = maxItemLength(a)
print('[')
for row in range(rows):
print(' [ ', end='')
for col in range(cols):
if (col > 0): print(', ', end='')
print(str(a[row][col]).rjust(fieldWidth), end='')
print(' ]')
print(']')
# Let's give the new function a try!
a = [ [ 1, 2, 3 ] , [ 4, 5, 67 ] ]
print2dList(a)
- Fancy Version (with outline and row and col labels):
# 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
for row in range(len(a)):
for col in range(len(a[row])):
maxLen = max(maxLen, len(repr(a[row][col])))
return maxLen
def print2dList(a):
if a == []:
print([])
return
print()
rows, cols = len(a), len(a[0])
maxCols = max([len(row) for row in a])
fieldWidth = max(maxItemLength(a), len(f'col={maxCols-1}'))
rowLabelSize = 5 + len(str(rows-1))
rowPrefix = ' '*rowLabelSize+' '
rowSeparator = rowPrefix + '|' + ('-'*(fieldWidth+3) + '|')*maxCols
print(rowPrefix, end=' ')
# Prints the column labels centered
for col in range(maxCols):
print(f'col={col}'.center(fieldWidth+2), end=' ')
print('\n' + rowSeparator)
for row in range(rows):
# Prints the row labels
print(f'row={row}'.center(rowLabelSize), end=' | ')
# Prints each item of the row flushed-right but the same width
for col in range(len(a[row])):
print(repr(a[row][col]).center(fieldWidth+1), end=' | ')
# Prints out missing cells in each column in case the list is ragged
missingCellChar = chr(10006)
for col in range(len(a[row]), maxCols):
print(missingCellChar*(fieldWidth+1), end=' | ')
print('\n' + rowSeparator)
print()
# Let's give the new function a try!
a = [ [ 1, -1023, 3 ] , [ 4, 5, 678 ] ]
b = [ [123, 4567, 891011], [567890, 'ABC'], ['Amazing!', True, '', -3.14, None]]
print2dList(a)
print2dList(b)
- 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)
- 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)
- 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()
- 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(f'a[{i}][{j}][{k}] = {a[i][j][k]}')