If you need Python to choose between two actions, then you can use an if/else statement.
Try running this example code:
a = 42
if a < 10:
print 'the number is less than 10'
else:
print 'the number is greater or equal to 10'
Note the indentation and punctuation (especially the colons), because they are important.
If we leave out an else, then the program continues on. Try running this program with different
initial values of a and b:
if a + b == 4:
print 'printed when a + b equals four'
print 'always printed'
If you want to repeat an action several times, you can use a while loop. The following
program prints Hello once, then adds 1 to the greetings counter. It then prints Hello twice
because greetings is equal to 2, then adds 1 to greetings. After printing Hello three times,
greetings becomes 4, and the while condition of greetings <= 3 is no longer satisfied,
so the program would continue past the while loop.
greetings = 1
while greetings <= 3:
print 'Hello! ' * greetings
greetings = greetings + 1
Be careful! If you accidentally create an infinite loop, your program will freeze and you will have to abort it.
Here's an example of an infinite loop. Make sure you see why it will never exit the while loop:
greetings = 1
while greetings <= 3:
print 'Hello! ' * greetings
greetings = greetings + 0 # Bug here
If you want to carry out some action on every element of a list, the for loop will be handy
names = ['Alice', 'Bob', 'Charley']
for name in names:
print 'Hello, ' + name
And if you want to repeat an action exactly n times, you can use the following template:
n = 10
for i in range(n):
print i
In the above code, range is a function that creates a list of integers between 0 and n,
where n is not included.
Finally, try seeing what the following code prints when you run it:
More information about loops and conditions can be found in the Python documentation.
