Understanding Functions in Computer Programming

1. CSC 160Computer Programmingfor Non-MajorsLecture #2: What are Functions? Prof. Adam M. Wittenstein Wittenstein@adelphi.edu http://www.adelphi.edu/~wittensa/csc160/

2. Programs Are Functions • Any Scheme program is made up of one or more functions. • There are two types of functions: user defined and pre-defined. • Certain “pre-defined” functions, like +, and, string-append, image-width, and so on are already written into DrScheme. We look at some of these today. • If you wish to write a program that requires a function that is not pre-defined, then you need to write it yourself. (We will see how to do this in Chapter 4 of Simply Scheme.)

3. Exploring Functions • Before we learn how to write functions, we need to understand what they are. • In high school, your math teacher wrote f(x) = x + 3 on the board. But what does this mean? And what about functions that involve no numbers or math at all? • We will answer these questions today!

4. Downloads for the semester • Download DrScheme Version 301: http://download.plt-scheme.org/drscheme/v301.html • Download and install the additional file with some predefined functions to practice with: http://hkn.eecs.berkeley.edu/~dyoo/cs3/simply-scheme/ • Close and reopen DrScheme. • Go to Language>Choose Language>Simply Scheme. Then click Run.

5. Using Someone’s Program • Today, we will actually use a computer program that someone else wrote to learn what functions are. (We could just as easily learn topics not even remotely related to Computers with a computer program.) • This program is called “functions.scm”. • It was written by the author of our textbook for our use. • Notice that we can use programs without knowing how they were written.

6. Downloading “functions.scm” Only repeat when changing computers • Go to the links page on the class website. • Download “functions.scm” and save it to the folder “C:\ProgramFiles\PLT”. Repeat every time you use a computer • In DrScheme’s Interactions Window, type: --(load “C:\\Program Files\\PLT\\functions.scm”) & Enter. Be careful of spaces and capital letters! --(functions) & Enter twice.

7. Example 1: The + function • Type in the function name, +. Then Enter. • Type in the first argument, 3. Then Enter. • Type in the second argument, 5. Then Enter. • You should get an answer of 8. • Try your own example with multiplication, use the * symbol.

8. Example 2: first • Type in the function name, first. Enter. • Type in the argument, hello. Enter. • You should get an answer of h (the first letter). • Try your own example with last. • Try your own example with butfirst.

9. Data Types • We have seen two types of data: numbers and words. • There are many other types of data that Scheme officially recognizes. • Functions can take arguments of any recognized data type. • Try the + function again, with arguments 6 and seven. What happens?

10. Domain and Range • The domain is all acceptable inputs (also known as arguments). • The range is all acceptable outputs (also known as answers). • The domain of + is “all pairs of numbers”. • What is the range of +? • What is the range of first?

11. Example 3: sentence • Type in the function name, sentence. Enter. • Type in the first argument, (when I get). Enter. • Type in the second argument, home. Enter. • You should get the answer: (when I get home). • Try the same example, but put parentheses around home. • Try butfirst again but use the arguments: (when I get) and (home).

12. Example 4: The < function • Type in the function name, <. Enter. • Type in the argument, 3. Enter. • Type in the argument, 5. Enter. • You should get an answer of #t (true). • Try your own examples with =, >, >=, and <=.

13. Example 5: number-of-arguments • Function name: number-of-arguments. • Argument: -. • You should get an answer of 2, since - takes in 2 arguments. • Try the other functions you have seen today: first, last, sentence, butfirst, and >.

14. So what are functions? • The idea of function is at the heart of Computer Science (just as with Math). • However, in Computer Science, functions work on all types of data, not just numbers. • Functions give the Programmer a way to think about process. That is, a function takes something and does something to it. • For example, + takes something (the numbers 3 and 5) and does something to it (adds them).

15. More on Data Types • Now we have seen 5 data types: --numbers --booleans --words --functions --sentences • While functions are themselves a data type (“function as object”), we will rarely use them as a data type this semester. • The first goal of a programmer is to understand “function as process”. That means, takes something and does something to it.

16. Preparing for Next Class

17. In summary… • Today, we learned what a function is. Next time… • We will work with functions directly in DrScheme. • We will see what happens when a function has more than one operation.

18. Reminders… • Homework #1 is due SUNDAY at 11:59pm. Send by e-mail to: Wittenstein@adelphi.edu • Before next class, please read: --Simply Scheme, Chapter 3 (all) --How to Design Programs, Chapter 2 (introduction and Section 1 only) • Have a happy Labor Day!