All computer languages allow comparison of the contents of variables with the contents of other variables (or with numbers, strings...). Tests are

Following the test, code execution can branch in various ways e.g. (here shown in pseudo code)

Note: python doesn't have this problem:

Here is a common coding bug. The comparison operators are unique to comparisons, but the "==" and "=" operators are similar, at least visually.

x=0 means "assign x the value 0"

x==0 means "test if x has the value 0"

In a conditional you want

if (x==0) do something else do something else

If instead you do

if (x=0) do something else do something else

then x will be assigned the value 0. The language must know whether a command succeeded (allowing execution to continue, or to bail out with an error) and since assigning x=0 will always succeed, then execution will branch to the "do something" branch, independant of the original value of x.

Since you're unlikely to ever want to execute the instruction

if (x=0)

the language should trap this construct as a syntax error. None of them do, at least till python came along. This is why rockets continue to blow up, cars have different bumper heights and we had lead in paint for so long.

Here's the official Python Tutorial, section 4.1 If Statements (http://docs.python.org/tut/node6.html#SECTION006100000000000000000)

Note

I live in one of the few (the only?) country that uses Fahrenheit, miles, lbs... (and the country's currency is loosing value, while the economy is only surviving by exporting jobs to our competitors.) If you live in the rest of the world, please substitute other values in these examples. I also live in a part of the world where you need an airconditioner in the summer and heater in winter (I lived quite happily in another part of the world for the first 30yrs of my life without these things and had some trouble adjusting to living indoors). I still wonder why people settled in such places when they didn't have to.

Here's the python syntax for a single branching conditional statement.

if x < 0:
	print 'Negative number found'

fire up your editor to code up the file temperature_controller.py

Here's my code ^[58] .

Here's python code for a two branch conditional.

if x < 0:
       print "Negative number found"
    else:
       print "non negative number found"

starting with your current temperature_controller.py, add code to output the string "no action taken" if the temperature is 80 or below. Check your code with values of temp above and below 80°.

Here's my code ^[59] .

Here's another python conditional construct

if x < 0:
      print 'Negative number found'
elif x == 0:
      print 'Zero'
else:
      print 'Positive number found'

Use this construct to add a branch to temperature_controller.py which turns on the heater at 60° or below and outputs a message describing its action.

Here's my code ^[60] .

Change the preceding code to open the windows if the temperature is 61-80°. Test your code to confirm that all branches execute at the expected temperatures. Here's my code. ^[61] .

In our current version of temperature_controller.py, the string "the temperature is " is in all branches, so will be output no matter what. In this case the string could be output once, below the declaration of temp. Here's a print statement using a trailing ',' that doesn't put a carriage return at the end of the output line.

name = "Homer"
print "my name is", name,

Use this code fragment to only have one copy of the string "the temperature is" in the code. Here's my version ^[62] .

Note that you can't get the period correctly spaced after the temperature. Python thinks it knows what you want better than you do and outputs a gratuituous blank that you didn't ask it to output.

Formatted output allows you more control over your print statements. Use this code fragment to produce a better output for temperature_controller.py (the %d says to put the decimal value of the argument in that place in the string).

>>> temp = 65
>>> print "the temperature is %d." % temp
the temperature is 65.

Here's the improved code ^[63] . Note that python still outputs a gratuituous blank when it executes the comma.

	Note
	End Lesson 8