程序设计实践笔记

=================

I. Style

Names


-  use descriptive names for globals, short names for locals.

.. code:: c

    int i = 0;
    for (; i < 10; i++)
        ;

-  be consistent. give related things related names that show their relationship
   and highlight their diffrence.

-  use active names for functions. function names should be based on active
   verbs, perhaps followed by nouns.

-  be accurate(保证准确).

Expressions and Statements
  • indent to show structure (python have a native support for it \(^o^)/~).
  • use the nature form for expressions.

.. code:: c

// do not use expressions like
// if (!(block_id < actblks) |!(block_id >= unblocks))
// but in this way:
if ((block_id >= actblks) |(block_id < unblocks))
  • parenthesize to resolve ambiguity. parentheses specify grouping and can be
    used to make the indent clear even when they are not required.

.. code:: c

// instead of writing expressions like
// leap_year = y % 4 == 0 && y % 100 != 0 |y % 400 == 0;
// write:
leap_year = ((y % 4 == 0) && (y % 100 != 0) |(y % 400 == 0));
  • break up complex expressions.
  • be careful with side effects. like:

.. code:: c

array[i++] = array[i++] = ' ';

Consistency and Idioms


-  use a consistent indentation and brace style. the programs consistency is
   more import than your own, because it makes life easire for those who follow.

-  use idioms for consistency(采用惯用写法).

.. code:: c

    // instead of wrting :
    // int i = 0;
    // for (i = 0; i < n; )
    //     array[i++] = 1;
    int i = 0;
    for (i = 0; i < n; i++)
        array[i] = 1;

.. code:: c

    // write
    while (1):
        //
    // instead of writing for (;;)

-  sprawling layouts also force code onto multiple screens or pages, and thus
   detract from readability. so, set cc to 79 in your vim! :)

.. code:: vim

    set cc=79

-  the return value from ``malloc``, ``realloc``, ``strdup`` or any other allocation
   routine should always be checked!

-  use else-ifs for multi-way decisions. put the most possible choice in the
   first statement can improve performance.

-  cases should always end with a ``break``, though longer.

.. code:: c

    switch (c) {
        case 'a': blablabla; break;
        case 'b': blablabla; break;
        ...
    }

but, an acceptable use of fall-through occurs when serveral cases have
identical(相同的) code, the conventional layout is like this:

.. code:: c

    switch (c) {
        case '0':
        case '1':
        case '2':
            blablabla
            break;
    }

Function Macros
  • avoid function macros.

    in c++, inline functions render function macros unnecessary;
    in java, there are no macros;
    in c, they cause more problems than they solve.

  • parenthesize the macro body and arguments.

.. code:: c

1/square(x) // works well if square is a function, but not macro:
// #define square(x) (x)*(x), it will be evaluated to:
1/(x) * (x)
// this version works well:
// #define square(x) ((x) * (x))

Magic Numbers


- ``magic numbers`` are the constants, array sizes, character posiitions,
  conversion factors, and other literal numeric values that appear in programs.

- give name toi magic numbers. by given names to the principal numbers in the
  calculation, we can make the code easier to follow.

- define numbers as constants, not macros. macros are dangerous ways to program
  because they change the lexical structure of the program underfoot.

- use character constants, not integers.

.. code:: c

    // instead of using:
    if (c >= 65 && c <= 90)
    // using:
    if (c >= 'A' && c <= 'Z')
    // this way is the best(use the standard library):
    if (isupper(c))

- use the language to calculate the size of an object.

.. code:: c

    #define NELEMS(array) (sizeof(array) / sizeof(array[0]))

Comments
  • the best comments aid the understanding of a program by briefly pointing out
    salient details or by providing a larger-scale view of the proceedings.
  • don't belabor the obvious. comments should't report self0evident information,
    such as the usage of i++.
  • comment functions and global data. we comment functions, global variables,
    constant definitions, fields in structures and classes, and anything else

where a brief summary can aid understanding.

  • don't comment bad code, rewrite it.
  • don't contradict the code(代码与注释要保持同步修改,以免冲突). comments
    should not only agree with code, they should support it.
  • clarify, don't confuse. comments are supposed to help readers over the hard
    parts, not create more obstacles. when it takes more than a few words to

explain what's happening, it's often an indication that the code should be
rewritten.

II. Algorithms and Data Structures

Chapter2


-  if you are developing programs in a field that's new to you, you must find out
   what is already known, lest you waste your time doing poorly what others have
   already done well.

-  if repeated searches are going to be made in some data set, it will be
   profitable to sort once and then use binary search.

-  `big-o notation cheat sheet <http://bigocheatsheet.com/>`__

-  这一章主要介绍了常用的数据结构和主要操作,例如List, Tree, Hash
   Table.---

Chapter3
  • whoever opens an input file should do the corresponding close:
    matching tasks should be done at the same level or place.
  • as a principle, library routines should not just die when an error
    occursl error status should be returned to the caller for appropriate action.
  • do the same thing the same way everywhere. keep consistency.

III. Debugging

Easy bugs


-  look for familiar patterns. ask yourself, "have I seen this before"
   when you get a bug.

-  examine the most recent change. source code control systems and
   other history mechanisms are helpful here. e.g. git.

-  don't make the same mistake twice. easy code can have bugs if its
   familiarity causes us to let down out guard. even when code is so
   simple you could write it in your sleep, don't fall asleep while writing it.

-  debug it now, not later. don't ignore a crash when it happens; track it
  down right away, since it may not happen again until it's too late.o

-  get a stack trace. the source line number of the failure, often
   part a stack trace, is the most useful single piece of debugging infomation;
   improbable(难以置信的,不会的) values of arguments are also a big
   clue(zero pointers, integers that are huge when they should be small,
   or negative when they should be positive, character strings
   that aren't alphabetic).

-  read before typing. one effective but under-appreciated debugging
   technique is to read the code very carefully and think about it
   for a while without making changes. resist the urge to
   start typing, thinking is a worthwhile alternative.

-  explain your code to someone else.
   `小黄鸭调试法?哈哈哈哈 <https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CB4QFjAAahUKEwjK8PS09LTIAhWM5oAKHWwpACU&url=https%3A%2F%2Fzh.wikipedia.org%2Fzh%2F%25E5%25B0%258F%25E9%25BB%2584%25E9%25B8%25AD%25E8%25B0%2583%25E8%25AF%2595%25E6%25B3%2595&usg=AFQjCNHJAF8oTPEFyICQ_QJ9tz_gwKlcvw&sig2=REOYXrZfbO6yu1AsA7QNLQ>`__

Hard bugs
  • make the bug reproducible. if the bug can't be made to happen every
    time, try to understand

why not. does some set of conditions make it happen more often than
others? using a log system
to log some unreproducible values(such as a random number).

  • divide and conquer. narrow down the possibilities by creating the
    smallest input where the bug still shows up.
  • study the numerology of failures(研究错误出现的规律).
  • display output to localize your search. e.g. use grep.
  • write self-checking code. personally, I think assert is useful,
    and write it with a DEBUG macro, just like:

.. code:: c

#ifdef DEBUG
......
#endif

and here is a trick for assert:

.. code:: c

assert(a > b), "a should bigger than b";

so the string after assert(a > b) will be displayed if assert works.

  • write a log file. be sure to flush I/O buffers so the final log
    records appear in the log

file.

  • draw a picture. sometimes pictures are more effective than text for
    testing and debugging.
  • use tools. like diff, grep , etc. write tricial programs to
    test hypotheses or confirm
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your understanding of how something works(善用工具, 弄清楚哪些坑).

  • keep records. if the search for a bug goes on for any length of
    time, you will begin to lose

track of what you tried and what you learned.

Last Resorts


what do you do if none of this advice helps? this may be the time to use a good debubger to
step through the program. it's tough to find this kind of bug, because your brain takes you
right around the mistake, to follow what the program is doing, not what you think it is doing.

if you can't find a bug after considerable work, take a break, clear your mind,
do something else, talk to a friend and ask for help.

Other People's Bugs

if you think that you have found a bug in someone else's program, the first step is to make
absolutely sure it is a genuine bug, so you don't waste the author's time and lose your own
credibility.