20010110: What's the difference between using a computer and programming a computer? User: A B C D E Y A B C D E Z A B C D E. Programmer: program Q is A B C D E; Q Y Q Z Q. Desirable features of programs (* for features irrelevant to most programs): correct; easy to write; easy to read; easy to test; easy to debug; easy to modify; modular; *portable (works on many computers); *reliable (handles unusual input); *secure (handles malicious input); *fast. Magic incantations to be explained later:
int main()
{
...
return 0;
}
UNIX tip: uppercase and lowercase are different in filenames.
Definitions of variables in easter.c.
Memory picture.
A variable is a collection of bytes in the computer's memory.
A byte is, at each time, 0 or 1 or 2 or ... or 255.
int year; defines a 4-byte variable.
It also declares the variable:
the program can use year later.
20010112: Assignment: read sections 1.12 and 14.7. How to start Exceed. Picture of memory changing over time. Instructions operate on memory. Memory picture after several int variable declarations. What
scanf("%d",&year);
does to memory.
More instructions.
= to put a value into a variable.
/ to divide.
Division of int throws away remainder.
When in doubt, put in parentheses.
* to multiply.
Can't leave out the *.
% for the remainder from division.
An if with == and &&.
-- to subtract 1 from a variable.
-= to subtract anything.
A variable name refers to the current value of the variable.
A printf example.
What debuggers do: step through your program, printing memory pictures.
20010115: No class.
20010117: UNIX tip: at the shell prompt, type ^W to erase a word. Notes on programming from home. UNIX tip: man gcc for the gcc section of the online manual. Assignment: read chapter 2. Basic types of symbols in C programs: preprocessor symbols; names (``identifiers''); numbers; strings; comments; miscellany. Examples of preprocessor symbols: #include lines; #define lines; etc. Don't add semicolons. Examples of names. Name is a letter or underscore, followed by any number of letters or underscores or digits. Warning about length limits. Uppercase letters are different from lowercase letters. Examples of numbers. Warning about 0 for base 8. Warning about 0x for base 16. Some floating-point numbers. Examples of quoted characters as numbers. Examples of quoted backslashed characters.
20010119: UNIX tip: creating shell scripts. Examples of strings. Backslash combinations in strings. String starts with double quote, continues until closing double quote. Backslashed quote does not stop a string. Splitting a string into two. Using backslashes to split a line into two. Warning about space after backslash. Warning about spaces on next line after backslash. Examples of comments. Comment starts with /*, continues until */.
20010122: UNIX tip: mv -i and mv. /* /* /* */ versus /*/*/**/. Use comments, indentation, and blank lines to help the reader. Example: consistent indentation inside main(){}; a ``Julian correction'' comment. Example: consistent indentation inside an if(){}; a ``should not happen'' comment. All the miscellaneous symbols; how to read them. A byte (``char'') variable. Characters are letters, symbols, etc. The characters 7, C, space, 3, D, newline are represented inside a computer as bytes 55, 67, 32, 51, 58, 10. What scanf("%c",...) does. What printf("%c",...) does. Example: program that reads and writes one character. What program does if user types 7, C, newline.
20010124: Assignment: read chapter 3. Warning about lab schedule. UNIX tip: rm -i and rm. An int variable. Possibilities for an int: -2147483648, -2147483647, ..., -2, -1, 0, 1, 2, ..., 2147483647. Portability warning: different compilers may have different int sizes. What printf("%d",...) does. What scanf("%d",...) does. Example: program that reads and writes one number in decimal. What program does if user types space, 3, 2, 1, space, 7, 3, newline. printf("Hello"), printf("%%"), printf("\n"). Combining multiple printf statements.
20010126: UNIX tip: cp -i and cp. Output buffering: printf saves results in a buffer. Situations where the buffer is flushed: end of program, print newline, scanf() on some systems, fflush(stdout). #include <stdio.h> before using stdout. Input buffering: user's line is not fed to scanf() until user presses return. Example of user editing a line. Using float variables. What can fit into a float variable. Using double variables. What can fit into a double variable. scanf("%f",...). printf("%f",...). scanf("%lf",...). printf("%lf",...). A few safe mixtures of printf() types. Example: program that reads an integer, writes the corresponding byte. Example: program that reads a real number and prints its cosine. Warning about -lm.
20010129: UNIX tip: ls, ls *.c, ls /bin. Control flow: if, while, do, etc. Example of a while(){} loop. The while() condition is not tested continuously; it is tested at the top of the loop. Example: i = 0; while (i < 10) { ++i; printf("%d\n",i); }. Memory pictures. After i reaches 10, it is printed, and then the condition is tested. Example of a do{}while() loop. Equivalence of while() with if()do{}while(). Ways to leave a loop: break; goto; setjmp/longjmp; return. Try to avoid goto, setjmp, longjmp.
20010131: UNIX tip: lpr. goto. Rephrashing while(){} in terms of goto. Rephrashing do(){} in terms of goto. The break and continue statements. for(){} loops. Rephrashing for(){} in terms of goto. An almost equivalent while(){} loop; what happens with continue. An equivalent while(){} loop. break looks for nearest while, do, for, switch; continue looks for nearest while, do, for.
20010202: UNIX tip: < infile > outfile. Examples of switch(){}. Using break inside switch(){}. Examples of control flow.
20010205: UNIX tip: pipelines. More examples of control flow; using scanf() as a number. Functions with no parameters returning void.
20010207: UNIX tip: gzip foo; gunzip foo; quota -v. Functions with parameters returning void. Arguments are copied to parameters. The return statement.
20010209: UNIX tip: echo. Functions with parameters returning int.
20010212: UNIX tip: tail -5. printf("%d\n",scanf(...)). Equivalent: r = scanf(...); printf("%d\n",r). Picture of the scanf data flow. Side effects: anything produced by a function other than its return value. Differences between mathematical functions and C functions. Sample midterm problem: write a program that reads a sequence of integers and prints the last 5 integers. Building the solution.
20010214: UNIX tip: head -5. Midterm review.
20010216: Midterm review.
20010219: Midterm #1.
20010221: Comments on midterm.
20010223: Comments on midterm.
20010226: Comments on midterm.
20010228: Comments on midterm.
20010302: UNIX tip: grep. Final comments on midterm. When arrays are useful. Example: program to read 3 integers, print them in opposite order. Same program with an array of 3 integers. Same program with variable array indexing. What variable array indexing means.
20010305: UNIX tip: review of shell scripts; $1 and $2 in shell scripts. Recap of arrays. Terminology: elements, array of floats. Security warning: running off the end of an array might cause program to change or remove files, make network connections, etc.; attacker who can provide input may be able to take control of the program; simplified memory picture; range checking. Program to compute and print the first 10 powers of 2. Memory pictures.
20010307: UNIX tip: while ... do ... done in shell scripts. Partially filled arrays.
20010309: UNIX tip: if ... then ... else ... fi in shell scripts. Partially filled arrays of char.
20010312: No class.
20010314: No class.
20010316: No class.
20010319: UNIX tip: for i in ... do ... done in shell scripts. Two-dimensional arrays. A program to read and print 10 lines.
20010321: Midterm review.
20010323: Midterm #2.
20010326: Comments on midterm.
20010328: Comments on midterm.
20010330: Comments on midterm.
20010402: UNIX tip: the directory hierarchy; the current working directory; cd. Example of function to print a fixed-length array. Example of function to add the elements of a fixed-length array. Example of function to read numbers into a fixed-length array. Arrays are effectively copied out of function upon return; actually, array parameter overlaps array argument. Memory pictures.
20010404: UNIX tip: mkdir; rmdir. int sum5(int x[5]) same as int sum5(int x[]). Can pass array of any length as argument, if array elements have the proper type. void printarray(int x[],int xlen). int sumarray(int x[],int xlen). void readarray(int x[],int xlen). Reminder that array parameter overlaps array argument. Memory pictures for readarray(u,3); readarray(v,5).
20010406: An extended example of a program passing a partially filled array to functions: main() calling pfa_read(), pfa_print(), pfa_sum().
20010409: Grading scale for course. Picture of main() above pfa_read() etc.; high level, low level. Picture of communication between main() and pfa_read(). An extended example of a program with several functions manipulating a partially filled array defined outside the functions: main() calling x_read(), x_print(), x_sum(). Basic advantage of variables outside functions: easy to use; no need to define parameters. Basic advantage of arguments and parameters: can use the function for any array of the right type.
20010411: UNIX tip: while read line in shell scripts. ``Global variables'' are variables defined outside functions: visible to all subsequent functions. ``Local variables'' are parameters and other variables defined inside a function: visible only to that function. Variable name refers to a global variable or a local variable in the same function. Local variable q in function f is unrelated to local variable q in function g. Advantage of global variables: can collect all important variables at the top of the program. Advantage of local variables: can easily see all variables used by a function. Advantage of global variables: can recognize any use of a variable. Advantage of local variables: other functions cannot change variables without permission. Example of an unexpected change to a global variable. Examples of expressions, non-expressions. Each expression has a type: e.g., void, int, float. Each non-void expression has a value. Each expression has a sequence of side effects, maybe none. Variable names as expressions: type is type of variable; value is current contents of variable; no side effects (except for volatile variables).
20010413: Expressions of the form p+q: possible types, value, side effects. Warning that side effects of p may occur before, after, or interleaved with side effects of q. Example of incorrect code: inc() + dbl() affecting the same variable. Recap: how to evaluate p+q; similarly p-q, p*q, etc. Expressions of the form v=q: type, value, side effects. Example: m=n=0. Warning that side effects of q may occur before, after, or interleaved with change of v. Example: m=n++. Example of incorrect code: n=n++. Similarly v+=q, v-=q, v*=q, etc.; ++v same as v+=1; --v same as v-=1. Example: m=++n.
20010416: Expressions of the form v++: type, value, side effects. Example: j++ + ++k. Similarly v--. Mnemonic to remember v++ and ++v. Expressions of the form x[p]=q, x[p], x[p]+=q. Example: x[i++]=j; common for partially filled arrays. Example: x[++i]=j. Expressions of the form p?q:r. Examples.
20010418: Expressions of the form p&&q. Expressions of the form p||q. Expressions of the form p,q.
20010420: Expressions of the form f(p,q). Casts. Expressions of the form sizeof p. Examples of statements.
20010423: Every statement has a sequence of effects. Statements of the form p;. Examples. Block statements: {s t u} etc. Examples. Defining variables in a statement. Examples of initializers. Examples of array initializers. Statements of the form while(p)s. Example: while(n--) { printf("%d\n",x[n]); } Example: while(n) { --n; printf("%d\n",x[n]); } Similarly for if(p)s etc.
20010425: putchar(). What getchar() does: sample code. Typical use: int c; while ((c = getchar()) != EOF) { ... } What goes wrong with char c. What tolower() does: sample code. What toupper() does. What islower() does: sample code. What isupper() does. Review.
20010427: Review.