Week 1 Notes — C

Saturday Jan. 2 to Friday, Jan. 8.

Links

• Return to CS50x Cohort home page
• Harvard CS50x week 1 page

Week 1 Video (duration 2:16:48)

Stop the video after each example and type in what Malan has typed in. Don’t even think of skirting this by finding somewhere to copy and paste the code from. Part of the learning process is typing in the simple examples yourself — correctly — you will be surprised to discover myriad ways to make mistakes even in simple examples. Stylistically, you should type in what Malan has written character for character. When he puts a curly brace on a line by itself (a convention that is going overboard on whitespace, IMHO, but you should nonetheless follow for now), or indents a line of code by four spaces (by hitting the tab key), or puts in one space around each side of an equals sign, you should too. The C compiler does not care about any of the aforementioned whitespace. It is for readability.

If you would like a more structured text to complement Malan’s introduction to C, the standard pedagogical and authoritative reference for C is Kernighan and Ritchie’s The C Programming Language, 2nd Edition. Dennis Ritchie authored C. It is a successor to a language called B. ANSI C became standard in 1989. Many things you could get away with in the original C are discouraged in ANSI C, at the cost of some verbosity. The 2nd Edition of Kernighan and Ritchie is for ANSI C. A language called C++ was grafted onto C and is pretty popular, despite its many failings. When you are learning C, you should ignore the existence of C++.

• 0:03:00 Malan opens with good design and makes an analogy with good paper writing. Code should be correct, well-designed, and have good style. Then he introduces the CS50 IDE which is a cloud version of something that is usually run on your own machine. To continue in the video and do the same steps that Malan is doing, you will need to get your own copy of the cloud IDE. When you click Login on the CS50 IDE web page, you will be prompted to connect to your GitHub account (that you created when submitting the previous week’s homework), and you will get your own copy (for free, thank you Amazon, Cloud9, and GitHub). Your GitHub account is already proving its value.
• 0:11:30 Using the compiler to convert C source code to a program that the computer more readily understands. (One could imagine doing the compilation step when the program was run, but that would mean you compile it every time you run it. In C, the typical pattern is to only compile the program when you change it. You might run it many times between changes, and if you haven’t changed it, there is no need to re-compile it. You then use what the compiler created, which is called an executable.) There is actually a wrapper program that runs the compiler called make. The command Malan types is make hello and make presumes that if you want to make hello that there is a file called hello.c. Make produces hello, and then Malan executes that by doing ./hello at the prompt.
• 0:15:00 Functions and arguments (aka parameters). C syntax for a printf statement. Side effects of functions (as opposed to outputs or return values). Introducing strings and a CS50 library function, get_string. Declaring a variable and assigning a return value to it, e.g., string answer = get_string("What's your name? ");. Formatting of string output using printf and %s. At this point, you should stop the video and get everything in this screenshot to work in the IDE.
• 0:36:00 Introducing the int main(void) boilerplate. Introducing header files, starting with stdio.h. Introducing compiler error output, and help50, a command provided by the cs50 staff that premasticates compiler error output for newbies. Introducing code comments and style50 to help you write stylistically good quality code. Introducing check50 which will do much of the grading of your problem solutions automatically. Showing what the binary file hello looks like in the IDE.
• 0:55:00 Introducing the shell, including ls, rm, mv, mkdir, cd, and .. for the parent directory.
• 1:03:30 Introducing types other than string: char, float, double, int, and long. Introducing more CS50 conveniences analogous to get_string: get_char, get_float, get_double, get_int, and get_long. Introducing printf specifiers analogous to %s: %c, %f, %lf, %i, %li. Introducing operators for division, remainder, etc. Example: addition.c. Illustrating truncation in integer division: truncation.c. Using a cast to fix the truncation bug.
• 1:25:30 Declaring, initializing, and incrementing a counter. Conditions, if, else if, else. Logical or operator. The while and for statements. Declaring and defining a function. Function arguments. Implementing get_positive_int using the do-while statement and returning a value from a function. Variable scope.
• 2:00:45 An example mario.c which printed out various number of rows and columns of bricks. Another example imprecision.c that illustrates the limitations of floating point arithmetic, and finally an example overflow.c that illustrates the limitation of integer arithmetic. The 2038 problem (analogous to the Y2K problem).
• 2:15:45 There are 14 people listed in the credits, not counting the American Repertory Theater (which operates the stage, space, and lighting), the Harvard-Radcliffe Orchestra (doing the soundtrack), or Malan himself. A big reason this course is acclaimed is that budget has been allocated to make it well-produced.

Around the 55-minute point in this lecture, some shell commands were introduced. If you are hungering for a more systematic introduction to shell commands, try reading the first several sections of the Bash Reference Manual.

By the way, there are many shells. zsh is arguably the best and most popular shell nowadays, but there are many people that would vociferously choose bash instead. To understand why, you’d have to read The GNU Manifesto. The CS50 IDE is based on Ubuntu Linux, and its standard shell is bash. The common ancestor of both zsh and bash is the original sh, and it is really lacking in features. Having briefly looked at quite a few introductions to bash, I can recommend Introduction to the Bash Command Line as a pedestrian place to start. The Bash Reference Manual would be your next stop, but only if you want to become a power user of bash.

Problem Set 1

Problem Set 1 is to write two programs. One is an elaboration of mario.c. The other is to write a program credit.c to test the type and validity of credit card numbers. More specific directions are at the link above.

My mario.c output for a height of 15:

My credit.c output for a few inputs: