In a program, data is held in variables. variables can be manipulated by functions appropriate for their data type (i.e. math on numbers, string functions on strings). Fire up your editor in your class_files directory and try this (you do not need the shebang if you're in windows). Save the file as variables.py (you can swipe the code with the mouse if you like. In X-window, the tabs are replaced by spaces, which may cause problems with your python interpreter. You could edit the mouse swiped code to replace all occurences of 8 spaces with a tab.)
#! /usr/bin/python
""" variable.py
Kirby. 2008
class exercise in variables
"""
#
#put in a name (your own if you like).
name="Kirby" #name is a string, it needs quotes.
#Put in an age.
age=12 #age is an integer, however it will be output as a string.
age_next_year=age + 1 #another integer
print "Hi, my name is " + name + ", a word with " + repr(len(name)) + " letters."
print "I'm " + repr(age) + " years old."
print "On my next birthday I will be " + repr(age_next_year) + " years old"
print "and will have been alive for " + repr(int(round(age_next_year * 365.25))) + " days."
# variable.py -------------------------
|
If it didn't run
- fix any syntax errors (mistakes in code that python doesn't understand)
- if there are no syntax errors and you're in a unix environment, did you make the file executable.
What's the code doing?
Start with documentation (needed for all code) so in 6 months you (and other people) will know why you wrote it.
Comments
- multiline blocks start and end with """. These blocks are picked up by python utilities which scan code and assemble documentation for a project of many files. These comments describe what the code does (not how).
- single line comments start with '#'.
Put the filename somewhere and a description of what the code does (not how it does it - that belongs in the code). If you're ever going to give this code to anyone else, you need to put the author and date in the code.
- in the next 3 lines, some variables are declared (you tell python that the variables exist), and in the same line these variables are given values.
In the print lines, the variables were output, along with some text for the user.
- repr() turns a number into a string
- Due to years not being an integer number of days long, the function round() drops the 0.25,0.5 or 0.75 at the end of the number of days you've been alive. This will give you a number ending in ".0". To drop the ".0", you turn the number into an integer with the function int().
- The comment at the end (variable.py) lets the reader know they're at the end of the program. In the old days, when code was transmitted by unreliable phone lines, you would sometimes get code that just finished, and you couldn't tell if this was supposed to be the end of the code, or the phone line had dropped during transmission. Put something at the end of the code (even "end of code") to let the reader know that they really did get all the code.
Rerun the program without round() or int() to see what happens. When you modify working code, you comment (#) out the current code (keeping it incase you decided to return to it - always keep working code, till you're sure that you have better code), make a copy and then modify the copy. Here's how you'd start modifying the last line of the code above.
#print "and will have been alive for " + repr(int(round(age_next_year * 365.25))) + " days." print "and will have been alive for " + repr(round(age_next_year * 365.25)) + " days." |
When you have the code you want, you delete all the commented attempts.
Change the line
print "On my next birthday I will be " + repr(age_next_year) + " years old" |
to output your age on your next birthday, by using the variable "age" and some arithmetic, rather than the variable "age_next_year" [53] .
change this line again (including the text) to give your age in 2050 [54] .
 | Note |
|---|---|
| Did you get a reasonable answer? Starting with your age now, count your age to the same day at the end of the decade (say 2010), then add 40. The 2008 in the above line must be the year of your last birthday. If it's 2008 and your last birthday was in 2007, then you will need to put 2007 in the above line. | |