Our first consideration of user interface and user interaction.
As you've seen when you ran the program, the disappointing answer is that
not much appears on-screen. When we run the program it will watch the input
channel and wait for a value to appear there. Specifically, it will wait for
the Enter key on the keyboard to be pressed, and then look in the keyboard
buffer to see what keys were pressed before the Enter key. From these
keypresses it will attempt to construct an integer value which it will store in
temp_in_f. It will not instruct us to type anything; we must just
remember to do so. When we do type, we will see the characters we type appear
on-screen, and have until we press the Enter key to Backspace over the line
making changes. Pressing the Enter key forwards the line we have typed to the
processor.
If we have typed 40, then after it has calculated the equivalent temperature on the Celsius scale it will display 4 beneath our 40 on the screen. So the screen might look something like this:
40
4Hardly inspiring, and no casual user would realize they were supposed to type anything, nor, on the off chance they did type a number, would they realize what the output was.
(This is a good chance to experience another error message. Press F5 again
to run your program and instead of typing a number type the word
help and then press the Enter key.)
To make our program more useful we need to provide instructions to the user, and explain the output. We do this by inserting extra print statements into the program. For example:
print "This program converts temperatures from Fahrenheit to Celsius."
print "Enter a temperature in Fahrenheit (e.g. 10) and press Enter."
temp_in_f = input("Temperature in Fahrenheit: ")
temp_in_c = (temp_in_f - 32) * 5 / 9
print temp_in_f, "degrees Fahrenheit =", temp_in_c, "degrees Celsius."Now the screen will look like this:
This program converts temperatures from Fahrenheit to Celsius.
Enter a temperature in Fahrenheit (e.g. 10) and press Enter.
Temperature in Fahrenheit: 40
40 degrees Fahrenheit = 4 degrees Celsius.Still not earth-shattering, but a marked improvement.
There are quite a few things to note in this small example:
print statements aren't limited to printing values that
are in memory, they can also print messages (formally: string literals). To
do so just enclose the string of characters you would like to display
on-screen in quotes. This means the statements print "rate"
and print rate do very different things. The first one prints
the letters r, a, t, and
e on-screen. The second one looks for a value named
rate and displays the value (and if it can't find one displays
an error message!). Try adding and removing some of the quotes in the
sample program above and see what happens.
You can combine values and string literals in one print statement, as the last line of the program does, to produce more meaningful output. To do so just separate successive items with commas. The commas are required.
An input statement can include a string literal that acts as a prompt to the user, in which case the string will be displayed and then the computer will watch for input from the user. This gives you a way to instruct the user. Note that the string must be enclosed in quotes. If your prompt is too long to fit inside the parentheses just put some of it into print statements preceding the input statement.
To get a sense of how precision matters in programming try removing the space after the colon (:) in the input prompt and re-running your program. Do you notice how the on-screen appearance is different? You should, and you should prefer it with the space.
I've added some blank lines into the program to visually separate the I, P and O blocks of the program to make it easier to see the program's structure. I do this for me and for you; Python could care less.
Using string literals judiciously it is possible to create clear input instructions, and clearly (if modestly) formatted output.