Homework

1
Homework

• Done the reading?
• KR 1.1-1.9
• Glass Chapters 1 and 2
• Applied for cs240? (If not, keep at it!)
• Gotten a UNIX account? (If not, keep at it!)
• Making progress on HW1? Due next class!
• HW2 will be on line by next class.

2
gcc

• At UNIX prompt, type gcc hello.c o hello
• If you get any compiler error messages
• Figure out what they mean
• Study and correct your source code
• Repeat gcc hello.c o hello until you get no
  messages
• If you get no output except a new prompt
• The compiler has accepted your source code
• You should now have a file named hello

3
Running your program hello

• At UNIX prompt, type hello
• If you get the printout Hello World! and a new
  prompt, your program ran successfully
• If not,
• Study any UNIX error messages for clues
• Study your source code for logical errors
• Probably logical errors - compiler didnt catch
• Fix your source code and recompile / rerun

4
Handing in Assignments

• Turn in assignments as hard copy of typescript
  file
• script (Start recording typescript file)
• Script started, file is typescript
• ls l (list directory entries)
• cat hello.c (display source file)
• gcc hello.c o hello (compile source file)
• hello (run executable file)
• exit
• script done on Thu Aug 22 143002 2002
• lpr typescript (printout typescript file)

5
Debugging a C program error

• There is a big difference between
• The program compiling correctly
• The program doing what you want it to do
• You hope the compiler will catch your errors
• These errors will be easier to find
• If the compiler does not catch your errors
• These errors will be harder to find

6
Compiler Error Messages

• A compiler error message may direct you to a
  specific error in your program
• A compiler error message may be vague about what
  the error is and why it is an error
• Some compilers are better than others at
  providing useful error messages!

7
Compiler Error Messages

• Introduced error is a missing closing brace
• include ltstdio.hgt
• int main ( )
• printf("Hello, World!")
• return 0
• / missing /
• gcc hello.c o hello
• hello.c In function main'
• hello.c6 parse error at end of input
• Not a very helpful message Gotta figure it out!

8
Variables

• Defined Data Type, Name, and ( value)
• int lower 0 / Note and /
• lower case by convention for readability
• An executable statement
• Memory location assigned to hold the value
• Value can be changed as program executes
• lower 20 / Legal /

9
Symbolic Constants

• Defined Name and Value
• define LOWER 0 / Note No or /
• UPPER CASE by convention for readability
• Not an executable statement
• No memory location assigned to hold value
• Value cant be changed as program executes
• LOWER 20 / NOT Legal /

10
Example Program (KR, P 15)

• include ltstdio.hgt
• define LOWER 0 / Symbolic Constants /
• define UPPER 300
• define STEP 20
• / Print out Fahrenheit Celsius Conversion
  Table /
• int main ( )
• int fahr / Variable /
• for (fahr LOWER fahr lt UPPER fahr fahr
  STEP)
• printf(3d,6.1f\n, fahr, (5.0/9.0)(fahr
  32))
• return 0

11
For Statement

• for (fahr LOWER fahr ltUPPER fahr fahr
  STEP)
• statements within the loop
• next statement after the loop terminates
• for (A B C) - controls executing statement(s)
  within the loop
• A is initialization (executed once when loop is
  started)
• B is the loop test statement (when to stop
  looping)
• C is a statement to execute at end of each loop

12
Printf statement

• printf (3d, 6.1f\n, fahr, (5.0/9.0) (fahr -
  32))
• First Argument to printf 3d, 6.1f\n
• 3d integer format with 3 digits
• 6.1f floating point format with 6 digits and
  1 decimal
• \n end of line character just as in Hello
  World!
• Second Argument to printf fahr
• variable to print in 3d format
• Third Argument to printf (5.0/9.0)(fahr
  32.0)
• expression to calculate and print in 6.1f
  format
• Note For open book tests, remember KR, Page 154!

13
printf formats

• printf ("Values?3d,?6.1f\n", fahr,
  (5.0/9.0)(fahr-32))
• Where the characters specified by "?" are what we
  write to show a space explicitly. Quoted string
  "Value . . ." is just like "Hello, world", but
  3d and 6.1f are special placeholders, called
  conversion specifications.
• This means that the two expressions following the
  quoted string, fahr, and (5.0/9.0)(fahr-32), are
  to be printed according to the prescription
  given.
• The table here would look like
• Values???0,??-17.8
• Values??20,???-6.7
• Values??40,????4.4

14
printf formats

• Other characters in "Values . . .", such as ","
  and "?" are printed literally.
• The 3d means that an integer is printed so as to
  take up 3 spaces, right adjusted -- "?40", but
  no initial space for "100" -- still have space
  before 100 because came after "Values?".
• The 6.1f means to print a float number (floating
  point or double by default, represented with a
  fractional part), with a total of 6 spaces used
  up and 1 digit after the decimal point thus
  "?-17.8" uses 6 spaces.

15
Functions

• A function is a separate block of code that you
  can call as part of your program
• A function executes and returns to next line
  after you call it in your program
• Arguments within parentheses may be passed in
• Arguments are passed by value!
• function_name (arguments)
• A return value may be passed back
• return_value function_name (arguments)

16
Character I/O getchar( )

• A standard function/macro defined in ltstdio.hgt
• No arguments are passed to getchar ( )
• getchar ( ) returns an int value from stdin
• int c
• c getchar ( )
• Next character input is returned as value of c

17
Character I/O putchar( )

• A standard function/macro defined in ltstdio.hgt
• Character to print to stdout is passed as
  argument
• There is no return value
• int c
• putchar (c)
• Next character output is based on value of c

18
File Copying

• include ltstdio.hgt
• int main ( )
• int c
• c getchar( )
• while (c ! EOF)
• putchar (c)
• c getchar ( )
• return 0

19
File Copying - Alternative

• include ltstdio.hgt
• int main ( )
• int c
• while ( (c getchar( ) ) ! EOF)
• putchar (c)
• return 0

20
Redirecting stdin and stdout

• copy lthello.c gtjunk.c
• Copies
• FROM
• the contents of existing file hello.c as stdin
• TO
• the contents of a new file junk.c as stdout
• (Note will overwrite any existing file junk.c)