Python Week 5 loops and iterations
Introduces loops and iterations, example of infinite loop and using break&continue.
Week 5 Loops and iterations
1. While loops (indefinite)
Loops (repeated steps) have iteration variables that change each time through a loop. Often these iteration
While loops are called “indefinite loops” because they keep going until a logical condition becomes False
n = 5
while n > 0 :
print n
n = n – 1
print 'Blastoff!'
print n
2. Infinite loops
n = 5
while n > 0 :
print 'Lather’
print 'Rinse'
print 'Dry off!'
3. Using break
- The break statement ends the current loop and jumps to the statement immediately following the loop
- It is like a loop test that can happen anywhere in the body of the loop
while True:
line = raw_input('> ')
if line == 'done' :
break
print line
print 'Done!'
> hello there
hello there
> finished
finished
> done
Done!
4. Using continue
The continue statement ends the current iteration and jumps to the top of the loop and starts the next iteration
while True:
line = raw_input('> ')
if line[0] == '#' :
continue
if line == 'done'
: break
print line
print 'Done!'
> hello there
hello there
> # don't print this
> print this!
print this!
> done
Done!
5. For loops (definite)
- Quite often we have a list of items of the lines in a file effectively a finite set of things
- We can write a loop to run the loop once for each of the items in a set using the Python for construct
- These loops are called “definite loops” because they execute an exact number of times
- We say that “definite loops iterate through the members of a set”
friends = ['Joseph', 'Glenn', 'Sally']
for friend in friends :
print 'Happy New Year:', friend
print 'Done!'
Happy New Year: Joseph
Happy New Year: Glenn
Happy New Year: Sally
Done!
6. Iteration variables
- The iteration variable “iterates” though the sequence (ordered set)
- The block (body) of code is executed once for each value in the sequence
- The iteration variable moves through all of the values in the sequence
6.1 Counting in a Loop
zork = 0
print 'Before', zork
for thing in [9, 41, 12, 3, 74, 15] :
zork = zork + 1
print zork, thing
print 'After', zork
#To count how many times we execute a loop we introduce a counter variable that starts at 0 and we add one to it each time through the loop.
$ python countloop.py
Before 0
1 9
2 41
3 12
4 3
5 74
6 15
After 6
6.2 Summing in a Loop
zork = 0
print 'Before', zork
for thing in [9, 41, 12, 3, 74, 15] :
zork = zork + thing
print zork, thing
print 'After', zork
#To add up a value we encounter in a loop, we introduce a sum variable that starts at 0 and we add the value to the sum each time through the loop.
$ python countloop.py
Before 0
9 9
50 41
62 12
65 3
139 74
154 15
After 154
6.3 Finding the Average in a Loop
count = 0
sum = 0
print 'Before', count, sum
for value in [9, 41, 12, 3, 74, 15] :
count = count + 1
sum = sum + value
print count, sum, value
print 'After', count, sum, sum / count
#An average just combines the counting and sum patterns and divides when the loop is done.
$ python averageloop.py
Before 0 0
1 9 9
2 50 41
3 62 12
4 65 3
5 139 74
6 154 15
After 6 154 25
6.4 Filtering in a Loop
print 'Before'
for value in [9, 41, 12, 3, 74, 15] :
if value > 20:
print 'Large number',value
print 'After'
# We use an if statement in the loop to catch / filter the values we are looking for.
$ python search1.py
Before
Large number 41
Large number 74
After
6.5 Search Using a Boolean Variable
found = False
print 'Before', found
for value in [9, 41, 12, 3, 74, 15] :
if value == 3 :
found = True
print found, value
print 'After', found
#If we just want to search and know if a value was found - we use a variable that starts at False and is set to True as soon as we find what we are looking for.
$ python search1.py
Before False
False 9
False 41
False 12
True 3
True 74
True 15
After True
7. Largest or smallest
smallest = None #特殊类型变量
print 'Before'
for value in [9, 41, 12, 3, 74, 15] :
if smallest is None : #IS是逻辑运算符，少用 主要是为了检查是否是 none 或 false
smallest = value
elif value < smallest :
smallest = value
print smallest, value
print 'After', smallest
# We still have a variable that is the smallest so far. The first time through the loop smallest is None so we take the first value to be the smallest.
$ python smallest.py
Before
9 9
9 41
9 12
3 3
3 74
3 15
After 3