1. Computer Programming Fundamentals
Loops

2. Last Time The ‘if’ statement Lists ‘else’ ‘elif’ Conditions
individual items
L[2]
slices
L[1:5] 2

3. ‘if’and‘else’ We can use‘if’ and ‘else’to express alternatives paths:
if x > y :
bigger = x
print("chose x")
else:
bigger = y
print("chose y")
print("Continue from here...") 3

4. The ‘elif’ statement
When there are more than two cases we use the ‘elif’ statement
if score < 10:
print("Pathetic")
elif score < 20:
print("My gran could do better")
elif score < 50:
print("OK")
else:
print("Legendary!!!") 4

5. Today’s Agenda The ‘while’ loop More about lists The ‘for’ loop
Formatting strings 5

6. The ‘while’ loop We can make the computer repeat statements
One way to do this is to use a ‘while’ loop
For example
The body of the loop is executed while the condition x > 0 is true
a = 1
x = 10
while x > 0 :
a = a*x
x = x - 1 6

7. Flow chart for ‘while’ loop
Start
Loop action
while ... :
Finish 7

8. More about lists Suppose our list is:
L = ["Abc", 1, 3.8, True, "XYZ", 21]
We can use negative index numbers
Then
L[-2]
L[-3]
L[-3:-1]
is "XYZ"
is True
is [True, "XYZ"] 8

9. More about lists L = ["Abc", 1, 3.8, True, "XYZ", 21]
Slices have sensible default values:
L[-2:] is ["XYZ", 21]
L[:3] is ["Abc", 1, 3.8]
L[:] is the whole list
L[:i]+L[i:] is always equal to L
Slices are more forgiving than indices:
L[99] would raise an error
L[-1:99] is [21] 9

10. More about lists We can construct lists in various ways: From a string
Q = list("Abc")
['A','b','c']
Using the built-in function ‘range’
N = list(range(4))
[0,1,2,3] 10

11. The ‘for’ loop Another way to write a loop is to use a ‘for’ loop
This kind of loop iterates over the elements of a list
L = ["Abc", 1, 3.8, True, "XYZ", 21]
for e in L:
print(e) 11

12. More about ‘range’
We can use the ‘range’ built-in function to make a ‘for’ loop iterate over integers:
for x in range(4):
print(x)
This will print the numbers 0,1,2,3
We can use other parameters:
range(4,0,-1)
Would give 4,3,2,1 12

13. More about loops
You can get out of your loop early using the keyword: ‘break’
for x in range(len(L)):
if L[x] == "value":
print(x)
break
The keyword ‘continue’ finishes the current iteration 13

14. String formatting
Often we will want to make sure the output sent to the console formats nicely
Use these string functions
myString.ljust(12)
myString.center(12)
myString.rjust(12)
Another useful one is:
print("".join(L)) 14

15. String formatting example
myString = "<string>"
print("< >"*3)
print(myString.ljust(12),end="")
print(myString.center(12),end="")
print(myString.rjust(12))
< >< >< >
<string> <string> <string> 15

16. Exercises Some things to try on the console Writing a for loop
A multiplication table
Writing a ‘Hangman’ game 16