Solving problems with Python

Now with many useful tools in hand, let us see how to make them work together to solve problems.

Flow control

The if statement is fundamental to making decisions within a program. It works simply

x=0.1
y=10.
z=0.
if x > 0.:
  y = 1./x
elif x < -1.:
  pass
elif x == 0:
  print 'Cannot divide by zero.'
  exit()
else:
  y = 1./x
z = y

Notice that indentation (by any fixed number of spaces) is used to separate the functions within the statement, and that each branch is defined by a :. The end of a branch occurs when the indentation goes back to the previous level. Each decision is based on a logical boolean value such as (x > 0.), which is True when x is greater than 0. and False otherwise. Within the if processing, a break is a way to get out of that level without doing anything, and an exit() leaves the entire program.

A while statement tests whether its argument is true, and sets up a loop that continues as long as it is. Program

flag = True
x = 0.
while flag:
  x = x + 1.
  if x > 10.:
    flag = False
print x

increases x until it is 11. and then prints the value.

Loops such as this may include a try block. This enables handling an exception, such as in this program to calculate x² with input from keyboard.

while True:
  try:
    x = int(raw_input("Please enter a number: "))
    break
  except ValueError:
  print "Oops!  That was no valid number.  Try again..."
y=x**2
print y

Here a break exits the loop from the try block unless an exception is thrown. A while statement can also test for something that is changed in the loop.

Examples

For examples of Python illustrating flow control, functions, and iteration, see the examples section.

Assignments

For the assigned homework to use these ideas, see the assignments section.