Example: One program to convert F to C or C to F

Consider this program to convert temperatures in Fahrenheit to Celsius.

print "This program converts temperatures from Fahrenheit to Celsius."
temp_in_f = input( "Enter a temperature in Fahrenheit (e.g. 10) and press Enter: " )
temp_in_c = (temp_in_f - 32) * 5 / 9
print temp_in_f, " degrees Fahrenheit = ", temp_in_c, " degrees Celsius."

(For simplicity I have removed the opening comments).

If instead we wanted to convert temperatures in Celsius to Fahrenheit we could write a second very similar program to do so:

print "This program converts temperatures from Celsius to Fahrenheit ."
temp_in_c= input( "Enter a temperature in Celsius (e.g. 10) and press Enter: " )
temp_in_f = temp_in_c * 9 / 5 + 32;
print temp_in_c, " degrees Celsius = ", temp_in_f, " degrees Fahrenheit."

But what if we want a single program that can do either conversion? We will need to place both sets of conversion code in one program and then choose which one to execute. How will we choose? Based on user input, i.e. we will present the choices and let the user choose. The resulting program might look like this.

###########################################################
## tmpcnvrt.py -- allows the user to convert a temperature
##        in Fahrenheit to Celsius or vice versa.
##
## CPSC 128 Demonstration Program
##
## Tim Topper, Winter 2013
###########################################################
print "This program converts temperatures from Fahrenheit to Celsius,"
print "or from Celsius to Fahrenheit."
print "Choose"
print "1 to convert Fahrenheit to Celsius"
print "2 to convert Celsius to Fahrenheit"

choice = input( "Your choice? " )

if choice == 1:
    print "This program converts temperatures from Fahrenheit to Celsius."
    temp_in_f = input( "Enter a temperature in Fahrenheit (e.g. 10) and press Enter: " )
    temp_in_c = (temp_in_f - 32) * 5 / 9
    print temp_in_f, " degrees Fahrenheit = ", temp_in_c, " degrees Celsius."

elif choice == 2:
    print "This program converts temperatures from Celsius to Fahrenheit ."
    temp_in_c= input( "Enter a temperature in Celsius (e.g. 10) and press Enter: " )
    temp_in_f = temp_in_c * 9 / 5 + 32
    print temp_in_c, " degrees Celsius = ", temp_in_f, " degrees Fahrenheit."

else:
    print "Error: Your choice not recognized!"

The program begins by displaying a text menu offering the user numbered choices. The user types a number and the program tests the number. If it is a 1, the block of indented code after the if statement is executed, and the blocks of code after the elif and else below it are skipped.

If it is not a 1, the block of indented code after the if is skipped, and the elif is executed. If this test is True (i.e. choice contains 2) the block of code after the elif is executed, and the else is skipped.

If the value of choice is not 2, then the code after the else is executed and the message Error: Your choice not recognized is displayed.

Note that the block of code after the if and after the elif contain several statements, so they must be consistently indented to turn them into single compound statements.

Also, take careful note of the program formatting. The statements after an if, elif or else are all indented 4 spaces.

In summary, this program selects one of three actions to carry out based on user input.

• If the initial value entered is 1, it performs a conversion from Fahrenheit to Celsius.
• If the initial value entered is 2, it performs a conversion from Celsius to Fahrenheit.
• If any other value is entered it displays an error message.

When more than one test is used to choose between multiple possibilities, as above, the if, elifs and else are arranged in a cascade. Our next program will illustrate how to create compound tests, i.e. multiple comparisons combined with logical operators (and, or, and not), and how to structure multiple ifs efficiently in a cascade.