Let's go through the steps of the software development process with a simple real-world example. Suzie Programmer has a problem. Suzie is an American computer science student spending a year studying in Europe. She has no problems with language, as she is fluent in many languages (including Python). Her problem is that she has a hard time figuring out the temperature in the morning so that she knows how to dress for the day. Suzie listens to the weather report each morning, but the temperatures are given in degrees Celsius, and she is used to Fahrenheit.
Fortunately, Suzie has an idea to solve the problem. Being a computer science major, she never goes anywhere without her laptop computer. She thinks it might be possible that a computer program could help her out.
Suzie begins with the requirements of her problem. In this case, the problem is pretty clear: the radio announcer gives temperatures in degrees Celsius, but Suzie only comprehends temperatures that are in degrees Fahrenheit. That's the crux of the problem.
Next, Suzie considers the specifications of a program that might help her out. What should the input be? She decides that her program will allow her to type in the temperature in degrees Celsius. And the output? The program will display the temperature converted into degrees Fahrenheit. Now she needs to specify the exact relationship of the output to the input. Suzie does some quick figuring to derive the formula F — (9/5)C+ 32 (Can you see how?). That seems an adequate specification.
Notice that this describes one of many possible programs that could solve this problem. If Suzie had background in the field of Artificial Intelligence (AI), she might consider writing a program that would actually listen to the radio announcer to get the current temperature using speech recognition algorithms. For output, she might have the computer control a robot that goes to her closet and picks an appropriate outfit based on the converted temperature. This would be a much more ambitious project, to say the least!
Certainly the robot program would also solve the problem identified in the requirements. The purpose of specification is to decide exactly what this particular program will do to solve a problem. Suzie knows better than to just dive in and start writing a program without first having a clear idea of what she is trying to build.
Suzie is now ready to design an algorithm for her problem. She immediately realizes that this is a simple algorithm that follows a standard pattern: Input, Process, Output (IPO). Her program will prompt the user for some input information (the Celsius temperature), process it to convert to a Fahrenheit temperature, and then output the result by displaying it on the computer screen.
Suzie could write her algorithm down in a computer language. However, the precision of writing it out formally tends to stifle the creative process of developing the algorithm. Instead, she writes her algorithm using pseudocode. Pseudocode is just precise English that describes what a program does. It is meant to communicate algorithms without all the extra mental overhead of getting the details right in any particular programming language.
Here is Suzie's completed algorithm:
Input the temperature in degrees Celsius (call it celsius) Calculate fahrenheit as 9/5 celsius + 32 Output fahrenheit
The next step is to translate this design into a Python program. This is straightforward, as each line of the algorithm turns into a corresponding line of Python code.
# A program to convert Celsius temps to Fahrenheit
# by: Suzie Programmer def main():
celsius = input("What is the Celsius temperature? ") fahrenheit = 9.0 / 5.0 * celsius + 32
print "The temperature is", fahrenheit, "degrees Fahrenheit." main()
See if you can figure out what each line of this program does. Don't worry if some parts are a bit confusing. They will be discussed in detail in the next section.
After completing her program, Suzie tests it to see how well it works. She uses some inputs for which she knows the correct answers. Here is the output from two of her tests.
What is the Celsius temperature? 0
The temperature is 32.0 degrees fahrenheit.
What is the Celsius temperature? 100
You can see that Suzie used the values of 0 and 100 to test her program. It looks pretty good, and she is satisfied with her solution. Apparently, no debugging is necessary.
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