Pointer (III)

1
Pointer (III)

• CGS 3460, Lecture 30
• Mar 27, 2006
• Hen-I Yang

2
Previously

• Address and Indirection Operators.
• Use of Pointers.

3
Quiz 4

• On May 31, Friday, at the beginning of the class
• 20 Minutes, 3 Questions
• Covers
• Homework 4
• Textbook Chapter 7 10,
• Slides up to Lecture 26 (May 10)
• All rules from previous quizzes apply (Gator 1
  ID, no electronic devices, etc)

4
Agenda

• Use of Pointers.
• Array and Pointers

5
Why do we use pointers?

• Pass as argument or Return the value
• Dynamic variables
• More efficient
• Refer to part of the a larger variable

6
Use of Pointers as argument

• Pointer as argument
• So the function can modify the value of the
  argument passed
• C functions are Pass by Value
• scanf(d, i)
• Const modifier

7
Use of Pointers as return value

• Use pointer as return value
• int max (int a, int b)
• if (a gt b)
• return a
• else
• return b
• int p, x, y
• p max (x, y)
• Do not return local variable or local pointer
  variable when return type is a pointer

8
Arrays and Pointers

• Closely related in C (not so in other language)
• Pointer is more efficient to use (array has catch
  up with new compilers)
• Insight into C
• Critical in mastering C

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Arrays and Pointers
a
int a int b4 int p int q p b0 p
5 p p 10 p b3 q p -1 q 3 p
- 3 if (p lt q) printf(d, p)
int a int b4 int p int q p b0 p
5 p p 10 p b3 q p -1 q 3 p
- 3 if (p lt q) printf(d, p)
5
10
b 5
b
(b1) 5
b b 3
3
q b-1
b -3
(bltq)
b
b)
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How do these pointers work?

• p, (p)
• (p)
• p, (p)
• p, (p)

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Array and Pointer (III)

• int a10 0,1,0,1,0,1,0,1
• int p
• int sum
• for (p a0 p lt a9 p)
• sum p

Equivalent
for (p a p lt a 9 p) sum p
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Array and Pointer (IV)

• Declaration int a10 is not equal to int a
• if p a can we use p2 to point to the third
  element in array a?

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Multidimensional Array and Pointers (I)
A00
A01
A02
A03

• Pointers can point to multidimensional arrays too
• Row major
• E.g. char A33

A10
A11
A12
A13
A20
A21
A22
A23
A00 A01 A02 A03
A10 A11 A12 A13
A20 A21 A22 A23
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Multidimensional Array and Pointers (II)
A00
A01
A02
A03

• char p
• p A00
• How do we get to A01?
• p 1
• How do we get to A10?
• p 3
• How do we get to
• A23
• p 7 or
• p A23

A10
A11
A12
A13
A20
A21
A22
A23
A00 A01 A02 A03
A10 A11 A12 A13
A20 A21 A22 A23
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Multidimensional Array and Pointers (III)

• int a1010 0,1,0,1,0,1,0,1
• int p
• for (row 0 row lt 10 row)
• for (column 0 column lt 10 column)
• arowcolumn 0

Equivalent
for (p a00 p lt a99 p) p
0
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Multidimensional Array and Pointers (IV)

• Remember if
• int a10
• Then automatically pointer a points to a0
• (a is equivalent to a0)

• What about
• int b1010
• Is there an automatic pointer b?
• Yes, but b b0, not b00
• b is of type (int ) !! Pointer of pointers

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Summary

• Use of Pointers.
• Arrays and Pointers

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Before you go

• Read Chapter 12.
• Exercise 12.1, 12.7
• Homework 4 due tomorrow night, and late
  submission on Thursday night.