First Steps Towards Programming

Of course, we can use Python for more complicated tasks than adding two and two together. For instance, we can write an initial subsequence of the Fibonacci series as follows:

>>> # Fibonacci series:
... # the sum of two elements defines the next
... a, b = 0, 1
>>> while b < 10:
...       print b
...       a, b = b, a+b
... 
1
1
2
3
5
8
>>>

• The first line contains a multiple assignment: the variables a and b simultaneously get the new values 0 and 1. On the last line this is used again, demonstrating that the expressions on the right-hand side are all evaluated first before any of the assignments take place.

• The while loop executes as long as the condition (here: b < 100) remains true. In Python, like in C, any non-zero integer value is true; zero is false. The condition may also be a string or list value, in fact any sequence; anything with a non-zero length is true, empty sequences are false. The test used in the example is a simple comparison. The standard comparison operators are written the same as in C: <, >, ==, <=, >= and !=.

• The body of the loop is indented: indentation is Python's way of grouping statements. Python does not (yet!) provide an intelligent input line editing facility, so you have to type a tab or space(s) for each indented line. In practice you will prepare more complicated input for Python with a text editor; most text editors have an auto-indent facility. When a compound statement is entered interactively, it must be followed by a blank line to indicate completion (since the parser cannot guess when you have typed the last line).

• The print statement writes the value of the expression(s) it is given. It differs from just writing the expression you want to write (as we did earlier in the calculator examples) in the way it handles multiple expressions and strings. Strings are written without quotes, and a space is inserted between items, so you can format things nicely, like this:

  >>> i = 256*256
  >>> print 'The value of i is', i
  The value of i is 65536
  >>>
  

  A trailing comma avoids the newline after the output:

  >>> a, b = 0, 1
  >>> while b < 1000:
  ...     print b,
  ...     a, b = b, a+b
  ... 
  1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987
  >>>
  

  Note that the interpreter inserts a newline before it prints the next prompt if the last line was not completed.