Now with many useful tools in hand, let us see how to make them work together to solve problems.
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.
Within a Python program you can define your own functions. Here's one to take an angle in degrees and reduce it an angle between 0 and 360.
if (angle < 0.0): n = int(angle / 360.0) - 1 return (angle - float(n) * 360.0) elif (angle >= 360.0): n = int(angle / 360.0) return (angle - float(n) * 360.0) else: return (angle)
Functions may have any number of objects as arguments, of any data type. Once defined, you may use a function anywhere in a program.
For examples of Python illustrating flow control, functions, and iteration, see the examples section.
For the assigned homework to use these ideas, see the assignments section.