1. Program Design Invasion Percolation: Aliasing
Copyright © Software Carpentry 2010
This work is licensed under the Creative Commons Attribution License
See for more information.

2. Use lists of lists to implement a 2D grid 3 5 7 7 5 8 6 3 2 4 2 6 5 4

3. Use lists of lists to implement a 2D grid 3 5 7 7 5 8 6 3 2 4 2 6 5 4

4. Use lists of lists to implement a 2D grid 3 5 7 7 5 8 6 3 2 4 2 6 5 4

5. assert N > 0, "Grid size must be positive"
# Correct code
assert N > 0, "Grid size must be positive"
assert N%2 == 1, "Grid size must be odd"
grid = []
for x in range(N):
grid.append([])
for y in range(N):
grid[-1].append(1) 5

6. assert N > 0, "Grid size must be positive"
# Incorrect code
assert N > 0, "Grid size must be positive"
assert N%2 == 1, "Grid size must be odd"
grid = []
EMPTY = []
for x in range(N):
grid.append(EMPTY)
for y in range(N):
grid[-1].append(1) 6

7. assert N > 0, "Grid size must be positive"
# Incorrect code
assert N > 0, "Grid size must be positive"
assert N%2 == 1, "Grid size must be odd"
grid = []
EMPTY = []
for x in range(N):
grid.append(EMPTY)
for y in range(N):
grid[-1].append(1) 7

8. "Aren't meaningful variable names supposed to be a good thing?"
# Incorrect code
assert N > 0, "Grid size must be positive"
assert N%2 == 1, "Grid size must be odd"
grid = []
EMPTY = []
for x in range(N):
grid.append(EMPTY)
for y in range(N):
grid[-1].append(1)
"Aren't meaningful
variable names
supposed to be
a good thing?" 8

9. grid = []
grid 9

10. grid = []
EMPTY = []
grid
EMPTY 10

11. grid = [] EMPTY = [] for x in range(N): # x == 0 grid.append(EMPTY)11

12. grid = [] EMPTY = [] for x in range(N): # x == 0 grid.append(EMPTY)
for y in range(N): # y == 0
grid[-1].append(1)
grid
EMPTY 1 12

13. grid = [] EMPTY = [] for x in range(N): # x == 0 grid.append(EMPTY)
for y in range(N): # y == 1
grid[-1].append(1)
grid
EMPTY 1 1 13

14. grid = [] EMPTY = [] for x in range(N): # x == 0 grid.append(EMPTY)
for y in range(N): # y == 2
grid[-1].append(1)
grid
EMPTY 1 1 1 14

15. grid = [] EMPTY = [] for x in range(N): # x == 1 grid.append(EMPTY)15

16. grid = [] EMPTY = [] for x in range(N): # x == 1 grid.append(EMPTY)
for y in range(N): # y == 0
grid[-1].append(1)
grid
EMPTY 1 1 1 1 16

17. You see the problem... grid = [] EMPTY = []
for x in range(N): # x == 1
grid.append(EMPTY)
for y in range(N): # y == 0
grid[-1].append(1)
grid
EMPTY 1 1 1 1
You see the problem... 17

18. Indirection allows aliasing

19. Indirection allows aliasing
Aliasing can be useful

20. Indirection allows aliasing
Aliasing can be useful
(In fact, sometimes it's indispensible)

21. Indirection allows aliasing
Aliasing can be useful
(In fact, sometimes it's indispensible)
But it's also a rich source of bugs

22. Indirection allows aliasing
Aliasing can be useful
(In fact, sometimes it's indispensible)
But it's also a rich source of bugs
When in doubt, draw a picture!

23. Indirection allows aliasing
Aliasing can be useful
(In fact, sometimes it's indispensible)
But it's also a rich source of bugs
When in doubt, draw a picture!
Tools that do this automatically exist...

24. Indirection allows aliasing
Aliasing can be useful
(In fact, sometimes it's indispensible)
But it's also a rich source of bugs
When in doubt, draw a picture!
Tools that do this automatically exist...
...but none has really taken off (yet)

25. Greg Wilson created by May 2010 Copyright © Software Carpentry 2010
This work is licensed under the Creative Commons Attribution License
See for more information.