prior In Class Exercise ICE: Saving Pennies

1
prior In Class Exercise (ICE) Saving Pennies

• Write a program that computes the amount of money
  you would have in a year if you saved 1 penny the
  first day, 2 more pennies the second day (for a
  total of 3 cents), 3 more pennies the third day
  (for a total of 6 cents), and so forth.
• Compile and run your program using g
• Update We will do this at the end of the period,
  if there is time.

2
C Pointer Review

• Pointers
• De-referencing operator
• Address of operator
• Indirection
• 
  

3
PointerSpeak Reading C pointer syntax

• declarations
• double a a is a memory location that holds a
  floating point (holds garbage until told
  otherwise).
•  double p p is a pointer to a double, can
  hold the memory address of a double (but hasnt
  yet been told to point to any particular address)
  
• executable statements
• p a p gets assigned the value of the address
  of a, therefore, p now points to the object a
• p a2 object p points to (here, a) gets
  assigned 2 times the previous contents of a

4
Pointer Declarations

• Pointers can point to any type (primitive
  datatypes or even user-defined classes)
• int, double, char, Ship,
• Pointers must be declared before being used
• char letterPtr
• letterPtr is potentially capable of pointing to a
  character.
• double dblPtr1, dblPtr2, myQpr
• two double pointers and 1 actual double

5
Pointer Initialization

• Should be initialized (made to point somewhere)
  when declared
• int myInt 5
• int intPtr myInt // intPtr points to myInt
• int intPtr 0
• int intPtr NULL // equivalent statements
• Accessing a pointer that does not point to a
  valid address results in a segmentation violation
  
• Result a run time error that crashes your program

6
Indirection/Dereferencing Operators

• The operator (dereference)
• Dereferences the object the operand points to
• Dereferences means that expression evaluates as
  the contents of the memory cell
• Cant dereference non-pointers
• The operator (address of)
• Returns the address of the memory cell

• double vBigNum 34.56
• double myPtr vBigNum
• //What prints out?
• cout ltlt myPtr ltlt endl
• //What happens here?
• myPtr 78.90
• //What is happening here?
• cin gtgt myPtr

7
Some nitty gritty pointer operations

• double vBigNum 34.56 // pointer
  demo1
• double myPtr vBigNum
  
• cout ltlt "\nThe Pointer " ltlt myPtr ltlt endl
• cout ltlt "The variable " ltlt vBigNum ltlt endl
• cout ltlt "\nThe address of the pointer " ltlt myPtr
  ltlt endl
• cout ltlt "The address of the variable " ltlt
  vBigNum ltlt endl
• cout ltlt "\nDerefing the address (myPtr) " ltlt
  myPtr ltlt endl
• cout ltlt "and the addr of the deref (myPtr) " ltlt
  myPtr ltlt endl
• myPtr 78.90
• cout ltlt "\nPointer " ltlt myPtr ltlt " Variable " ltlt
  vBigNum ltlt endl

8
Pointers in function calls ptrdemo2.cpp

• int callByVal (int) //call by value function
  prototype declaration
• void callByRef (int ) //call by reference
  function prototype
• int main ()
• // g -c ptrdemo2.cpp
• int myVar 1024 // g -o ptrdemo2
  ptrdemo2.o
• int myPtr myVar
• callByVal (myVar)
• cout ltlt "After call by value function " ltlt
  myVar ltlt endl
• callByRef (myPtr)
• cout ltlt "After call by reference passing a ptr
  "ltltmyVar ltltendl
• return 0 // what if you leave the function
  definitions out?

int callByVal (int number) number
number return number //end callByValue

• void callByRef (int numPtr)
• numPtr numPtr
• //end callByRef

9
ICE Working With Pointers

• Determine what the following C code produces
  at run time.
• int a, p
• a 8
• p a
• p 3 p - a
• cout ltlt p