1. CS1010 Discussion Group 11
Week 9 – Pointers

2. Slides are at http://www.comp.nus.edu.sg/~yanhwa/
HELLO!
Slides are at

3. Lab 5
“Right. Since you have learned pointers already you may use pointer parameters.”

4. Pointers might be confusing but…
Need to get used to them! :) you can do itttt

5. For example, declared as int *a_ptr;
Lecture Summary
Pointers Declaration
For example, declared as int *a_ptr;
a pointer variable a_ptr which may point to any int variable
Good practice to name a pointer with suffix _ptr or _p so that you won’t forget it’s not a normal variable! (assuming “snake case” and not camel case)

6. “pointer is set to the address of a”
Lecture Summary
Initialization of pointer
What is the correct way?
*a_ptr = &a;
a_ptr = a;
*a_ptr = a;
a_ptr = &a;
“pointer is set to the address of a”

7. “declare a pointer b and initialise it with address of a”
Lecture Summary
Declaration and initialisation
double a;
double b = &a;
Is this allowed / legal?
Is b a pointer?
double a;
double *b = &a;
“declare a pointer b and initialise it with address of a”

8. “go to what a_ptr is pointing at”
Lecture Summary
Indirection / deferencing operator
a_ptr a
Are they the same since a_ptr points to a?
Why?
Need to use indirection operator. *a_ptr
“go to what a_ptr is pointing at”

9. “go to where a_ptr is pointing to and increment it”
Lecture Summary
Changing the value at memory location
a_ptr++
(*a_ptr)++
Note that compiler sees *p++ as *(p++) as ++ takes precedence over *. Always use brackets for clarity.
“go to where a_ptr is pointing to and increment it”

10. What is hexadecimal? Base-16 positional numeral system.
Extra info
What is hexadecimal?
Base-16 positional numeral system.
Normal numbers is base-10 (decimal numbers), has digits 0 to 9.
Thus base-16 has digits from 0 to 15.
What are the digits after 9?
a b c d e f
Can assume Byte addressing. An address represents a unique byte in memory or the memory space, for example ffbff627
The next byte will be ffbff628

11. incrementing a pointer variable
Lecture Summary
incrementing a pointer variable
a_ptr++
When incrementing a int pointer variable, it will point to the next int
When incrementing a char pointer variable, it will point to the next char.
ffbff62c to ffbff630
ffbff627 to ffbff628
Unit 4 Exercise #1:
int takes up 4 bytes
float takes up 4 bytes
char takes up 1 byte
double takes up 8 bytes

12. Function can modify variables out of its scope thus….
Lecture Summary
Why do we need pointers?
Function can modify variables out of its scope thus….
Function can “return” more than one value
Pointers can lead to efficient code, but affects modularity, cohesiveness and at this point we shall value cohesion more.
Use pointers only when necessary
Rmr function prototype parameters!
void findMaxAndAverage(int [], int, int *, double *);

13. Lab 5 Worksheet
Game Of Life

14. Answer is 310 0 5 . Try tracing it for yourself!
Tutorial 2
Answer is Try tracing it for yourself!

15. Tutorial 3 – compute_surface_area_and _diagonal()

16. Tutorial 4 - Triangle incenter
The red lines are the internal angle bisectors.
The Cartesian coordinates of the incenter are a weighted average of the coordinates of the three vertices, using the side lengths of the triangle opp these vertices as weights
What other function do we need besides the function to get the incenter of a triangle?
Why do we need to use pointers? How is this related?

17. Tutorial 4
Nice comment too

18. Another disadvantage:
Tutorial 5
If a function only returns one value as an answer, is it good to still use pointers?
Usually, no.
Another disadvantage:
The caller would have to declare a variable, and pass the address of that variable into the new function. What if there is really no intention/need to have such a variable in the caller in the first place, as shown below?
printf("The GCD is %d\n", brusco_gcd(num1, num2));

19. Tutorial 6
You should know by now….
Now, a function should perform one specific task, and not a mixture of tasks. Magic word is cohesive
We should refrain from mixing a computation task and an input/output task in a function. The GCD is a computation task. Printing the answer is an output task.

20. Tutorial 6
Another reason: reusability of code
DON”T REPEAT YOURSELF = DRY principle
What if an application needs to compute the GCD of two numbers as part of a bigger algorithm? Compute it multiple times? Having the printf() statement in the GCD function would upset such a plan..

21. Tutorial 7 – Why should we write separate functions?
Separate the functions
Combine the functions
Cohesive = do one task
2 (or more) values to be
returned (via the pointer parameters) are so intimately related to one another that
they are considered as one group of information. For example, the triangle center coordinates
Might save on unnecessary variables
Efficiency (if specified that it is valued over cohesion)
Trade-off! No hard-and-fast rule…