Our previous lessons have shown us how to manipulate data, define our own functions, and repeat things. However, the programs we have written so far always do the same things, regardless of what data they're given. We want programs to make choices based on the values they are manipulating. To help us see what decisions they're making, we'll start by looking at how computers manipulate images.
The tool Python gives us for doing this is called a conditional statement, and looks like this:
In [1]:
num = 37
if num > 100:
print 'greater'
else:
print 'not greater'
print 'done'
The second line of this code uses the keyword if to tell Python that we want to make a choice.
If the test that follows it is true,
the body of the if
(i.e., the lines indented underneath it) are executed.
If the test is false,
the body of the else is executed instead.
Only one or the other is ever executed:
Conditional statements don't have to include an else.
If there isn't one,
Python simply does nothing if the test is false:
In [2]:
num = 53
print 'before conditional...'
if num > 100:
print '53 is greater than 100'
print '...after conditional'
We can also chain several tests together using elif,
which is short for "else if".
This makes it simple to write a function that returns the sign of a number:
In [3]:
def sign(num):
if num > 0:
return 1
elif num == 0:
return 0
else:
return -1
print 'sign of -3:', sign(-3)
One important thing to notice in the code above is that we use a double equals sign == to test for equality
rather than a single equals sign
because the latter is used to mean assignment.
This convention was inherited from C,
and while many other programming languages work the same way,
it does take a bit of getting used to...
We can also combine tests using and and or.
and is only true if both parts are true:
In [4]:
if (1 > 0) and (-1 > 0):
print 'both parts are true'
else:
print 'one part is not true'
while or is true if either part is true:
In [5]:
if (1 < 0) or ('left' < 'right'):
print 'at least one test is true'
In this case, "either" means "either or both", not "either one or the other but not both".
True and False aren't the only values in Python that are true and false.
In fact, any value can be used in an if or elif.
After reading and running the code below,
explain what the rule is for which values are considered true and which are considered false.
(Note that if the body of a conditional is a single statement, we can write it on the same line as the if.)
if '': print 'empty string is true'
if 'word': print 'word is true'
if []: print 'empty list is true'
if [1, 2, 3]: print 'non-empty list is true'
if 0: print 'zero is true'
if 1: print 'one is true'
Write a function called near that returns True if its first parameter is within 10% of its second
and False otherwise.
Compare your implementation with your partner's:
do you return the same answer for all possible pairs of numbers?
Another thing to realize is that if statements can be combined with loops
just as easily as they can be combined with functions.
For example,
if we want to sum the positive numbers in a list,
we can write this:
In [6]:
numbers = [-5, 3, 2, -1, 9, 6]
total = 0
for n in numbers:
if n >= 0:
total = total + n
print 'sum of positive values:', total
We could equally well calculate the positive and negative sums in a single loop:
In [7]:
pos_total = 0
neg_total = 0
for n in numbers:
if n >= 0:
pos_total = pos_total + n
else:
neg_total = neg_total + n
print 'negative and positive sums are:', neg_total, pos_total
We can even put one loop inside another:
In [8]:
for consonant in'bcd':
for vowel in 'ae':
print consonant + vowel
As the diagram below shows, the inner loop runs from start to finish each time the outer loop runs once:
We can combine nesting and conditionals to implement the logic of a desired execution pattern, e.g. to access certain data, make certain calculations (e.g. on a 2-dimensional or 3-dimensional grid), etc.
Python (and most other languages in the C family) provides in-place operators that work like this:
x = 1 # original value
x += 1 # add one to x, assigning result back to x
x *= 3 # multiply x by 3
print x
6
Rewrite the code that sums the positive and negative numbers in a list using in-place operators. Do you think the result is more or less readable than the original?
if condition to start a conditional statement, elif condition to provide additional tests, and else to provide a default.== to test for equality.X and Y is only true if both X and Y are true.X or Y is true if either X or Y, or both, are true.