CS156: How to write a program

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CS156 How to write a program

• Lecture

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Hello World 1

• A simple hello world example

/
hello_world.c Purpose Say hello to the
world. /
include ltstdio.hgt int main() // Say
hello printf(Hello, world.\n) return 0
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Hello World 2

• An obfuscated hello world example
• Whaaaa?
• Many programs will do what we want, but few will
  be good programs.

includeltstdio.hgt main() int
x0,y14,zy(z)0x48(z)yx0x1D
(z)yx0x07(z)yx0x00(z)y
x0x03 (z)yx-0x43(z)yx-0x
0C(z)yx0x57 (z)yx-0x08(z
)yx0x03(z)yx-0x06
(z)yx-0x08(z)yx-0x43(z)yx-
0x21 x(--z)while(yx!NULL)putchar(yx)

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Signs of a Good Program

• Effective - The first goal of a program is to be
  correct.
• Simple The program should be only as complex as
  necessary to solve the problem.
• Documented A programmer unfamiliar with the
  program should be able to easily understand what
  it does and how.
• Robust The program should handle unexpected
  events gracefully.

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Signs of a Bad Program

• Lack of useful comments and documentation
• Overly complex, e.g. sorting a list to find the
  maximum element
• Clever code is hard to interpret and modify.
• Many things can be done multiple ways in C,
  choosing the shortest way is not always the best
  idea.

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Why does good programming matter?

• Programs often outlive their expected lifetime.
• Programs often outlive the programmers short term
  memory.
• Most useful code will end up being modified by
  someone who did not write it.

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A Few Style Tips

• Break your program into blocks
• Document the purpose of each block and how it
  works.
• Use meaningful variable names.
• Use the simplest effective code.

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Practical Program Writing What to do.

• Understand what the program should do.
• Describe the program in pseudocode.
• Write the code from the top down.
• Work in steps. Write/Compile/Test/Repeat

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Practical Program Writing What not to do.

• Dont try to write the program without any
  planning of forethought.
• Dont try to write the program all at once.
• Dont send me your code and ask me to tell you
  what is wrong with it.
• Dont wait until the last minute.

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Bugs and Errors

• Bug is a programmers euphemism for error
• Syntax errors occur when a program does not
  follow the rules defined by the C language.
  Syntax highlighting helps.
• Logic errors occur when the program does not
  perform as expected. Planning and pseudocode
  helps.

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Correcting Syntax Errors

• The compiler detects and describes syntax errors
  during compilation.
• Often, a single error will cause the compiler to
  detect many errors later in the code.
• It is best to try to correct the errors one at a
  time. This will keep you from spending time
  trying to correct phantom errors.

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Example
/
example3-1.c a bad example
/ in
clude ltstdio.hgt int main() float
friction foat number // assign some
values friction .04 number friction 3.2
2 // print number printf(The final number
was f\n, number) printf(The value of
\friction\ was f.\n,friction) return 0
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Example
preisnergt gcc example3-1.c example3-1.c In
function main example3-1.c10 error foat
undeclared (first use in this function) example3-1
.c10 error (Each undeclared identifier is
reported only once example3-1.c10 error for
each function it appears in.) example3-1.c10
error syntax error before number example3-1.c1
4 error number undeclared (first use in this
function) example3-1.c14 error syntax error
before numeric constant example3-1.c17 error
missing terminating " character example3-1.c18
error syntax error before token

• The error messages give the file, function, and
  line number of the error. A description of the
  error is also given.
• Sometimes, an error is not detected until several
  lines after it actually occurred. The line
  number can be misleading.

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Finding and Correcting Logical Errors

• Writing a program in stages help in avoiding
  logical errors.
• To correct logical errors, try to localize the
  error to a small portion of code.
• Test just this piece by commenting out other
  parts of the code or making a separate test
  program.
• Use printf to give you the values of variables at
  various points in the program.
• Use your brain, dont work at random.