Class 05: An Introduction to C

Held: Monday, 27 January 2003

Summary: Today we conclude our discussion of the simple model of computation and start exploring the C programming language.

Assignments:

• Read Gries 2 (due Tue).

Notes:

• Do you prefer physical or electronic whiteboards?
• Any thoughts on the course Web? I'm always looking for ways to improve it.
• I've extended my office hours. They now include MTuWF 9:00-9:50.
• Are there questions on homework 2?

Overview:

• Three sample programs.
• A notation for instructions.
• Three sample programs, revisited.
• An introduction to C.
• C vs. Java.

A Simple Model of Computing, Revisited

Three Sample Programs

• Read two integers and print out their sum.
  • Read the first integer into one register (say A).
  • Read the second integer into another register (say B).
  • Add the two together.
  • Print out the contents of the result register.
• Read integers until you read 0. Print out their sum.
  • Put 0 into one register (say A).
  • Label: MAIN_LOOP
  • Read a value into another register (say B).
  • If B contains 0, jump to the end of the program
  • Add B to A, storing the result in A
  • Go back to MAIN_LOOP
  • Label: END_OF_PROGRAM
  • print contents of A
• Read integers until you read 0. Print them out in reverse.
  • Trick: store in sequential spaces in memory. Register A keeps
  • track of the most recent one.
  • Put 0 into A
  • Label: MAIN_LOOP
  • ...

A Notation for Instructions

• Labels are followed by a colon.
• Everything else is indented.
• Register names are preceded by a percent sign.
• Integer constants are preceded by a dollar sign.
• To refer to memory, surround a value by parentheses. E.g., ($5) is the contents of memory location 5. Similarly, (%A) is the contents of the memory location whose address is stored in register A.

Three Sample Programs, Revisited

• We'll generate these on the fly during class.

An Introduction to C

• C is a widely-used imperative programming language.
• Its syntax formed the basis of many syntaxes, including C++, Java, C#, Javascript, Perl, the C shell, ...
• It is relatively concise.
• It assumes that you pay attention to the machine.

A Short History of C

• Designed at Bell labs as a systems programming language.
• (Unix implemented in C.)
• Based on B and BCPL.
  • Added types. (Everything in BCPL was bits that you said how to interpret.)
• Lots of support in the research community, primarily because Unix was widely distributed.
• Early 1980's: Standards organizations attack C.

C vs. Java

• C and Java look very similar and act very similar, but also have some key differences.
• Key Similarities
  • Both require a main
  • Most operations (assignment, loops, etc.) behave similarly.
• Key Differences
  • C requires you to declare all variables at the start of a block.
  • C gives you direct access to memory through a special type called the pointer.
  • C does not provide exceptions.
  • C does not include classes.
  • Pure C includes only one kind of comment.
    • Our compiler provides others.
  • Many others.