Homework due

1
Homework due

• Test the random number generator
• Create a 1D array of n ints
• Fill the array with random numbers between 0 and
  100
• Compute and report the average of the array
  contents
• Do this for n 10, 100, 1000, 10000, 100000

2
Homework due

• Create a 2D array of m x n ints
• Fill the array with random numbers between 0 and
  100
• Compute and report the average of the array
  contents
• Do this for all combinations of m 1, 10, 100
  and n 1, 10, 100, 1000

3
Homework special instructions

• Do not use any hard coded values or constants
  when calculating loop indices
• Use the techniques discussed for computing the
  number of elements in rows/columns of an array
• Due Thursday, next class meeting Ill look at
  your results in class

4
C

• Arrays Part II

5
Run time allocation

• Allocate an array of 20 ints
• int array20
• Allocates an array of 20 elements in main memory
  at compile time
• But what if we dont know how big the array
  should be when we are compiling the program?
• Java has no issues with this since everything is
  run time allocate

6
Run time allocation

• The dreaded P wordPointers!
• In C a pointer is nothing more than a variable
  that holds an address in memory
• Whereas most variables are themselves addresses
  in memory and hold data
• Its often referred to as an indirect address

7
Regular (non-pointer) variables
int x 10 int y 20 int z 30 int a3
0
8
Pointer variables
int px int py int pz int pa
?

• Notes
• int x is the same as int x
• int x, y declares 1 pointer (x) and 1
  non-pointer (y) variable

9
Pointers what do you do with them?
int px int py int pz int pa int x
10 int y 20 int z 30 int a3 0 px
x py y pz z pa a
px
x
py
y
pz
z
pa
a

• You can use them to refer to (point to) other
  variables
• The is called the address-of operator and
  returns the address of the variable it is
  operating on
• Note that the array name is already and address
  and needs no

10
Pointing to other variables

• These assignments will change the contents of the
  variables to whom they point
• x is now 27
• y is now 28
• x is now 29
• a now holds 30, 31, 32
• The is called the indirection operator and
  refers to the contents of the variable it is
  operating on
• Note that the array index operators and
  perform a similar task on array type variables

px 27 py 28 pz 29 pa0 30 pa1
31 pa2 32
11
Why?

• Typically, you wont use pointers to access
  individual (primitive) variables
• You might use pointers to access arrays
• At one point in time it used to be faster to do
  so rather than use indexing not clear if this
  is still true
• You could use an array of pointers to create a
  ragged edged array (like you can do in Java)
• The real value of pointers is in dynamic, run
  time memory allocation

12
Array access though pointers

• int a10 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
• for (int i 0 i lt 10 i)
• stdcout ltlt ai ltlt stdendl
• int pa a
• for (int i 0 i lt 10 i)
• stdcout ltlt pa ltlt stdendl
• pa
• The tricks
• Pointer arithmetic (like pa) is smart enough to
  know how much to add to the pointer (address) to
  get to the next variable of the appropriate type
• e.g. if pointing to an int it adds 4, if pointing
  to a char it adds 1, if pointing to a double it
  adds 8
• Similar for things like pa 2 (adds 8 for an
  int, 2 for a char, 16 for a double)
• Be sure to initialize the pointer properly
• Know how far you can go before overrunning the
  end of the array

13
Dynamic memory allocation

• Its really quite simple
• int x 5
• int pa new intx
• for (int i 0 i lt x i)
• (pa i) i
• delete pa
• Note that you have to keep track of the size
  sizeof(pa) / sizeof(pa0) will not work

14
What not to do

• int x 5
• int pa new intx
• for (int i 0 i lt x i)
• pa i
• pa
• delete pa
• Why is this a very, very, very bad thing to do?

15
To fix it

• int x 5
• int pa new intx
• for (int i 0 i lt x i)
• (pa i) i
• delete pa

16
Dynamic multi-dimensional arrays

• Basically, you need to dynamically allocate an
  array of pointers
• The method shown in the book is (in my opinion)
  hooky at best
• Heres how I do it

17
Dynamic multi-dimensional arrays
int rows 4 int cols 5 // -- requires a
pointer to a pointer int ma // -- requires
two separate allocations // and some pointer
assignments ma new introws ma0 new
introws cols for (int i 1 i lt rows i)
mai mai - 1 cols
18
Dynamic multi-dimensional arrays
// -- now you can access it as a normal 2D
array for (int i 0 i lt rows i) for (int
j 0 j lt cols j) maij i cols
j for (int i 0 i lt rows i) for
(int j 0 j lt cols j) stdcout ltlt
maij ltlt "\t" stdcout ltlt stdendl
19
Dynamic multi-dimensional arrays
// -- you have to delete both allocations in //
reverse order delete ma0 delete ma
20
Homework

• Same as the last assignment but this time use
  dynamically allocated arrays and ask the user how
  big they should be
• i.e. you only need to write and compile 2
  programs, 1 for the 1D case and 1 for the 2D case
• All the different run sizes will be handled at
  run time based on user inputs
• Due next class period