Basically, a for loop just does something for every value in a list. (Lists will be discussed in the next class.) For instance:
new_list = [1950, 'There Will Come Soft Rains', 'Ray Bradbury', 2026]
for element in new_list:
print(element)Try that out and you will see that the above for loop prints every item in new_list. There are two important keywords here: for and in. The keyword for signifies the kind of loop we're going to be executing and in show us that we're going to be putting each item of new_list into the variable element one at a time.
Let's print the numbers 1 through 10:
for number in [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]:
print(number)That was easy enough, but there is a better way to write that in Python:
for number in range(1, 11):
print(number)The range function is really helpful in Python. Much like slice that we saw with strings, range can take three inputs: start, end, and step. It turns out that start and step are optional:
>>> range(5)
range(0, 5)
>>> type(range(5))
<class 'range'>
>>> list(range(5))
[0, 1, 2, 3, 4]
>>> list(range(1, 5))
[1, 2, 3, 4]
>>> list(range(1, 5, 2))
[1, 3]
>>> list(range(0, 30, 5))
[0, 5, 10, 15, 20, 25](NOTE: As you can see above, the start value is inclusive and the end value is exclusive. The default start is zero and the default step is 1.)
A while loop does something until its main condition is no longer met:
a = 0
while a < 10:
a = a + 1
print(a)The above loop will print the numbers 1 through 10 and then comparison between a and 10 will be no longer true, and the loop will terminate. You can also create more complicated predicates for the while loop:
n = 15
while n > 0 and n < 30:
n -= 3 # this is the same as `n = n - 3`
print(n)The above loop will print the numbers 12, 9, 6, 3, 0 and then the complicated predicate will no longer be true, and the loop will terminate.
Unlike your typical for loop, while loops can fail in an interesting way. What if the predicate is never false?
>>> n = 7
>>> while n > 0:
... n += 1
... This is called an infinte loop, because it would never terminate on its own. This is, obviously, not a good thing.
(Also, notice how we have to put an extra line at the end of this for the interpretter to know the loop is done.)
In order to write useful programs, we almost always need the ability to check conditions and change the behavior of the program accordingly. Conditional statements give us that ability. The simplest form is the if statement, which has the genaral form:
if PREDICATE:
BODYIn this case, only if the predicate is true is the body of the statement executed. For example:
food = 'spam'
if food == 'spam':
print('Ummmm, my favorite!')
print('I feel like saying it 100 times...')
print(100 * (food + '! '))Above is the basic if statement. You will use these to control the logical flow of the program. But what if you want to execute a different block of code if the predicate isn't satisfied? Well, you could do this:
food = 'spam'
if food == 'spam':
print('Joy!')
if food != 'spam':
print('Sadness...')The logic desired above is so common Python provides a shortcut:
if food == 'spam':
print('Ummmm, my favorite!')
else:
print("No, I won't have it. I want spam!")There is another option. What if you want to have several different conditionals chained together? That is the purpose of the elif symbol:
if food == 'spam':
print('Mmmmm, my favorite!')
elif food == 'spam and eggs':
print('Acceptable. I can pick out the eggs.')
elif food == 'spam and bacon':
print('Acceptable. I can pick out the bacon.')
else:
print("No, I won't have it. I want spam!")The else symbol is optional, but it is usually good practice to include it:
if food == 'spam':
print('Mmmmm, my favorite!')
elif food == 'spam and eggs':
print('Acceptable. I can pick out the eggs.')So far, we have seen Python's three major control statements:
for, while, if...elif...elseBut Python also has three minor control statements:
continue, break, passThese are used in conjunction with the major control statements to allow for great flexibility in the logical flow of our programs.
The continue statement is used inside a for or a while loop, skipping the rest of this iteration and fast forwarding to the next item in the list. Let us look at a couple examples:
for letter in 'Python':
if letter == 'h':
continue
print('Current Letter: ' + letter)This will print:
Current Letter : P
Current Letter : y
Current Letter : t
Current Letter : o
Current Letter : nHere the print statement was hit for every letter in Python except the letter h, because of the continue statement. Let's try another example:
var = 10
while var > 0:
var = var - 1
if var % 2 == 0:
continue
print('Current variable value: ' + str(var))This will produce following result, because the continue statement will skip all of the even values:
Current variable value: 9
Current variable value: 7
Current variable value: 5
Current variable value: 3
Current variable value: 1Another important way to control the flow of a program is to use break. When a break statement is reached, you immediately terminate the for or while loop:
x = 0
while True:
x += 1
print(x)
if x > 99:
breakThis will print the numbers 1 to 100 and then stop. You could, for instance, use this to figure how many integers (starting with 1) you need to add to get 10,000:
sum = 0
i = 1
while True:
sum += i
i += 1
if sum > 10000:
break
print('The sum of the first ' + str(i) + ' integers is over 10,000.')(Notice the extra line before the print statement. The interpreter needs that extra line to know the while loop is complete.)
There is one final logical control we want to discuss. What if, when a certain set of predicates are true, we want to do nothing? That's possible. But we can't just leave the block empty, so use pass statement:
if x == "not spam for dinner":
print("I will destroy the universe")
elif x == "spam for dinner":
# I'm fine with that. I'll do nothing
pass
else:
print("I demand spam!")Without that pass statement the elif block would be empty and Python would throw an error. Try that and prove that to yourself.
Did you notice in the for and while loops above how the content inside the loop was indented? That indentation is how Python decides what logic is contained within a loop, function, or class (more on those later). Unlike in other languages you might have seen, the indentation really matters in Python.
White Space is very important in Python.
Here is an example of a correctly indented piece of Python:
if n == 0:
print(0)
elif n == 1:
print(1)
else:
print((n - 1) + (n - 2))Notice how the indents line up. If we didn't align the if, elif, or else together, Python would throw an error:
if n == 0:
print(0)
elif n == 1:
print (1)
else: # ERROR: inconsistent dedent
print((n - 1) + (n - 2))Or if we didn't indent the code inside the blocks Python would throw an error:
if n == 0:
print (0 )
elif n == 1:
print(1 ) # ERROR: no indent
else:
print((n - 1) + (n - 2))You are what you comment.
If you write code and there are no comments and no documentation, the code doesn't exist. No one will want to touch it, and they won't be able to understand it if they do.
# this is the first comment
spam = 1 # and this is the second comment
# ... and now a third!
text = "# This is not a comment because it's inside quotes."
"""This is an especially long comment,
that takes up two lines."""
'''This is another especially long comment,
that takes up multiple
lines.
'''