CS 151

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CS 151

• Midterm exam Friday (Nov 7)
• A practice midterm is located on web pags
• Answers will be posted later
• No quiz next week.
• No programming assignment next week.

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More functions

• time functions
• "random numbers"

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The time() Function

• Returns the number of seconds since 1/1/1970.
• At 545 p.m., 4/27/2003, the value was
  1,051,465,590
• Usage (requires include lttime.hgt)
• int t
• t time(NULL)

null pointer
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Tick Tock

• include ltstdio.hgt
• include lttime.hgt
• int main(void)
• int i, t
• for(i0 i lt 10 i) / loop for ticks and
  tocks /
• t time(NULL) 1 / t is 1 second from now
  /
• while (time(NULL) lt t) / wait /
• printf("TICK\n") / then print TICK /
• t time(NULL) 1 / again 1 sec in future
  /
• while(time(NULL) lt t) / wait again /
• printf("TOCK\n") / print TOCK and repeat /
• return 0

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The clock() Function

• Also in the lttime.hgt library
• Returns number of msec since computer was turned
  on.
• Useful for timing things, e.g. reaction time.
• wait random time up to 10 sec
• get clock value and print a ''
• wait for key press
• subtract current clock val from previous and print

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Reaction Time

• include ltstdio.hgt
• include lttime.hgt
• include ltstdlib.hgt
• int main(void)
• int ranTime, reactTime
• ranTime clock() 1000 rand() 10000
• while(clock() lt ranTime) / wait 1 to 11 sec /
• reactTime clock() / start measuring time /
• printf("")
• getchar() / wait for ENTER to be pressed /
• reactTime clock() - reactTime / calculate
  the reaction time /
• printf("Your reaction time was d msec",
  reactTime)
• return 0

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The rand() Function

• Use
• i rand() /requires include ltstdlib.hgt /
• Returns different (random) number each time
  called (between 0 and RAND_MAX)
• RAND_MAX 32,767 in CodeWarrior.
• Always produces same sequence.
• 16838 5758 10113 17515 31051 5627 23010 7419
  16212 4086 etc.

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The srand() function

• Sets starting point for random sequence, e.g.
• srand(seed)
• 'seed' will determine the first number rand()
  returns. (It won't be the value of 'seed').

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A Random Number ExampleSuppose that I want a
list of 20 random 3-digit numbers.

• What do I want to do?
• Initialize the random number generator
• Generate and print 20 3-digit numbers
• Design decisions
• Loop in main or loop in function?
• Constants?

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/ Program to generate 20 random 3-digit
numbers./ include ltstdio.hgt include
ltstdlib.hgt define LOVAL 100 define HIVAL
999 / prototypes / void initRandom() int
myRandom(int, int) int main(void) int
k initRandom() for (k 1 k lt 20
k) printf("d\n", myRandom(LOVAL,
HIVAL)) return 0 / stubs / void
initRandom() int myRandom(int, int) return 0
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/ Program to generate 20 random 3-digit
numbers./ include ltstdio.hgt include
ltstdlib.hgt include lttime.hgt / / define LOVAL
100 define HIVAL 999 int myRandom(int,
int) void initRandom() int main(void) int
k initRandom() for (k 1 k lt 20
k) printf("d\n", myRandom(LOVAL,
HIVAL)) return 0 void initRandom()
srand(time(NULL)) int myRandom(int low,
int high) int range high low 1 return
rand() range low
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Program Style

• Use comments.
• Line up code in loops and functions (use tabs to
  indent).
• Use blank lines to separate programming
  ideas.

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Indentation shows Nesting

• int i,t
• for(i0 i lt 10 i) / loop for ticks and
  tocks /
• t time(NULL) 1 / t is 1 second from now
  /
• while (time(NULL) lt t) / wait /
• printf("TICK\n") / then print TICK /
• t time(NULL) 1 / again 1 sec in future /
• while(time(NULL) lt t) / wait again /
• printf("TOCK\n") / print TOCK and repeat /

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Style 2

• int i,t
• for(i0 i lt 10 i)
• / loop for ticks and tocks /
• t time(NULL) 1 / t is 1 second from now
  /
• while (time(NULL) lt t) / wait /
• printf("TICK\n") / then print TICK /
• t time(NULL) 1 / again 1 sec in future /
• while(time(NULL) lt t) / wait again /
• printf("TOCK\n") / print TOCK and repeat /

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Style 3

• int i,t
• for(i0 i lt 10 i)
• / loop for ticks and tocks /
• t time(NULL) 1 / t is 1 second from now
  /
• while (time(NULL) lt t) / wait /
• printf("TICK\n") / then print TICK /
• t time(NULL) 1 / again 1 sec in future /
• while(time(NULL) lt t) / wait again /
• printf("TOCK\n") / print TOCK and repeat /

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Program Design

• Top-down design (or step-wise refinement)
• Start with total problem and break into
  sub-problems, then into sub-sub-problems, etc.
• Make functions for each sub-problem.
• main() solves whole by calling f1(), f2(), f3().
• f1() solves sub-problem by calling f4(), f5(),
  etc.

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Parameter passing by reference

• Address of actual parameter is passed to formal
  parameter.
• Pass exactly one address to a formal parameter
• Constants, literals, and expressions are not
  allowed.
• Types must match exactly.
• Actual parameter has (remember scanf ?)
• Formal parameter has .
• Formal parameter is an alias for the actual
  parameter.
• Inside the function, the value of a variable is
  accessed by dereferencing with .
• Changes in formal reference parameters are
  changes in the associated actual parameters.

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What gets printed?

• / prototypes, etc. defined here /
• int main(void)
• int a 7, b 4, c 5
• int z testRef(a, b, c)
• printf("d d d d\n", a, b, c, z)
• int testRef(int p, int q, int r)
• int a p q
• int b a r
• p 0
• q 2
• r q
• return b

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Questions?