-
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 xrange(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
-
Even Better: make2dList()
def make2dList(rows, cols):
a=[]
for row in
xrange(rows): a += [[0]*cols]
return a
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
- Another
option: use a list comprehension
rows = 3
cols = 2
a = [ ([0] * cols) for row in xrange(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
-
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
-
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 xrange(rows):
for col in xrange(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
-
Copying
2d Lists
-
copy.deepcopy example
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
c =
copy.deepcopy(a) # Ok
# At first, things seem ok
print "At first..."
print " a =", a
print " b =", b
print " c =", c
# Now modify a[0][0]
a[0][0] = 9
print "But after a[0][0] = 9"
print " a =", a
print " b =", b
print " c =", c
- limitations of
copy.deepcopy of aliases
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]]
# now one more time with a simple deepcopy alternative that does
# what we thought deepcopy did...
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]]
# what's going on with deepcopy? Answer: if the original list has aliases,
# the deepcopied list will have aliases (of a single copy, not the
original).
# So copy.deepcopy preserves alias structure!
- 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
xrange(rows):
for col in
xrange(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 "[ ",
for row in
xrange(rows):
if (row > 0): print "\n ",
print "[ ",
for col in
xrange(cols):
if (col > 0): print ",",
# The next 2 lines print a[row][col] with the given fieldWidth
format = "%" + str(fieldWidth) + "s"
print format % str(a[row][col]),
print "]",
print "]"
# Let's give the new function a try!
a = [ [ 1, 2, 3 ] , [ 4, 5, 6 ] ]
print2dList(a)
- Accessing 2d Lists by Row
or Column
- Accessing a whole row
# alias (not a copy!); cheap (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!); expensive (new list created)
a = [ [ 1, 2, 3 ] , [ 4, 5, 6 ] ]
col = 1
colList = [ ]
for i in xrange(len(a)):
colList += [ a[i][col] ]
print colList
- Accessing a whole column with a list comprehension
# still a copy, still expensive, but cheaper and cleaner with a list
comprehension!
a = [ [ 1, 2, 3 ] , [ 4, 5, 6 ] ]
col = 1
colList = [ a[i][col] for i in
xrange(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 xrange(rows):
cols = len(a[row]) # now cols depends on each row
print "Row", row, "has", cols, "columns: ",
for col in xrange(cols):
print a[row][col],
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 xrange(len(a)):
for j in xrange(len(a[i])):
for k in xrange(len(a[i][j])):
print "a[%d][%d][%d] = %d" % (i, j, k, a[i][j][k])
- Examples Using 2d Lists
- WordSearch
- Connect4
-
Optional: Matrices