Pointers vs. Arrays - Binghamton Universitytbarten1/CS211_Fall_2015/lectures/L20 -...

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Binghamton University CS-211 Fall 2015 Pointers vs. Arrays 1

Transcript of Pointers vs. Arrays - Binghamton Universitytbarten1/CS211_Fall_2015/lectures/L20 -...

Page 1: Pointers vs. Arrays - Binghamton Universitytbarten1/CS211_Fall_2015/lectures/L20 - Pointers...Pointers vs. Arrays 1. Binghamton University CS-211 Fall 2015 Pointers in C •Pointers

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Pointers vs. Arrays

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Page 2: Pointers vs. Arrays - Binghamton Universitytbarten1/CS211_Fall_2015/lectures/L20 - Pointers...Pointers vs. Arrays 1. Binghamton University CS-211 Fall 2015 Pointers in C •Pointers

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Pointers in C

• Pointers are a special data type

• The VALUE of a pointer is an address

• The TYPE of a pointer is “pointer to <target_type>• pointer to character

• pointer to integer

• pointer to float

• pointer to array of integers

• …

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Memory

• Array of bytes

• Each element has a value

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x00 x04 x1C x02 … … … … x96 x00 x30 x04

0 1 2 3 xffff ffffxffff fffexffff fffd

Page 4: Pointers vs. Arrays - Binghamton Universitytbarten1/CS211_Fall_2015/lectures/L20 - Pointers...Pointers vs. Arrays 1. Binghamton University CS-211 Fall 2015 Pointers in C •Pointers

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age

Variables In Memory

• Every variable starts at a specific location in memory

• Type of variable tells how many bytes (spaces) in memory

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x00 x04 x1C x02 … … x54 x00 x00 x00 x39 … … x96 x00 x30 x04

0 1 2 3 xffff ffffxffff fffexffff fffd

First_Initial

x0034 ffe0 x004d 18c4

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Array Values are “Contiguous”

• Right next to each other in memory

• int vec[6]

• int m [4][3];

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vec[0] vec[1] vec[2] vec[3] vec[4] vec[5]

m[0][0] m[0][1] m[0][2] m[1][0] m[1][1] m[1][2] m[2][0] m[2][1] m[2][2] m[3][0] m[3][1] m[3][2]

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char name[8]=“Thomas”;

printf(“name is at %p\n”,&name[0]);

name is at 0x34ffe0

Example Array in Memory

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x00 x04 x1C x02 … … x54 x68 x6f x6d x61 x73 x00 x00 … … x96 x00 x30 x04

0 1 2 3 xffff ffffxffff fffexffff fffd

name

x0034 ffe0 x0034 ffe1

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char name[8]=“Thomas”;

char *np=&name[0];

printf(“np is %p\n”,np);

np is at 0x34ffe0

Pointer to Array in Memory

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x00 x04 x1C x02 … … x54 x68 x6f x6d x61 x73 x00 x00 … … x96 x00 x30 x04

0 1 2 3 xffff ffffxffff fffexffff fffd

name

x0034 ffe0 x0034 ffe1

np

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char name[8]=“Thomas”;

char *np=&name[0];

printf(“np -> %c\n”,(*np));

np -> T

What are we Pointing At?

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x00 x04 x1C x02 … … x54 x68 x6f x6d x61 x73 x00 x00 … … x96 x00 x30 x04

0 1 2 3 xffff ffffxffff fffexffff fffd

name

x0034 ffe0 x0034 ffe1

np

Question:Does np point to a single character?

Page 9: Pointers vs. Arrays - Binghamton Universitytbarten1/CS211_Fall_2015/lectures/L20 - Pointers...Pointers vs. Arrays 1. Binghamton University CS-211 Fall 2015 Pointers in C •Pointers

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char name[8]=“Thomas”;

char *np=&name[0];

printf(“np ->%c-%c-%c…\n”,*np,*(np+1),*(np+2));

np -> T-h-o…

What are we Pointing At?

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x00 x04 x1C x02 … … x54 x68 x6f x6d x61 x73 x00 x00 … … x96 x00 x30 x04

0 1 2 3 xffff ffffxffff fffexffff fffd

name

x0034 ffe0 x0034 ffe1

np

Question:Does np point to a single character?

Or an array of characters?

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What is a “string”?

• A “string” is just a vector of ASCII characters• Followed by a “null terminator” – a byte with the value 0x00

char str[14]=“This a string”;

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Index 0 1 2 3 4 5 6 7 8 9 10 11 12 13

ASCII T h i s a s t r i n g

Hex x54 x68 x69 x73 x20 x61 x20 x73 x74 x72 x69 x6e x67 x00

{‘T’, ’h’, ’i’, ’s’, ’ ‘, ’a’, ’ ‘, ’s’, ’t’ ,’r’ ,’i’ ,’n’ ,’g’, x00}

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char name[8]=“Thomas”;

char *np=&name[0];

printf(“np ->%s\n”,np);

np -> Thomas

What are we Pointing At?

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x00 x04 x1C x02 … … x54 x68 x6f x6d x61 x73 x00 x00 … … x96 x00 x30 x04

0 1 2 3 xffff ffffxffff fffexffff fffd

name

x0034 ffe0 x0034 ffe1

np

Question:Does np point to a single character?

Or an array of characters?Or a string?

Page 12: Pointers vs. Arrays - Binghamton Universitytbarten1/CS211_Fall_2015/lectures/L20 - Pointers...Pointers vs. Arrays 1. Binghamton University CS-211 Fall 2015 Pointers in C •Pointers

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A Pointer points to one or more elements of a specific

type

(Actually, zero or more, but who’s counting)

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Integer Vector in Memory

int v[4]={11,12,13,14};

int *vp=&v[0]; // vp=x00c0 0014

printf(“vp-> %d %d %d … \n”,*vp,*(vp+1),*(vp+2));

vp-> 11 12 13

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v[0] v[1] v[2] v[3]

x00 00 00 0B x00 00 00 0C x00 00 00 0D x00 00 00 0E

x00c0 0014 x00c0 0018 x00c0 001c x00c0 0020

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Pointer Arithmetic

• “Unit” (1) is the size of a single element in bytes• For char, unit=1

• For int, unit=4

• For float, unit=4

• For pointers, unit=4

• When we add “1” to an integer pointer, it increases by 4!• points to the next integer address!

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Page 15: Pointers vs. Arrays - Binghamton Universitytbarten1/CS211_Fall_2015/lectures/L20 - Pointers...Pointers vs. Arrays 1. Binghamton University CS-211 Fall 2015 Pointers in C •Pointers

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2D Arrays in Memory

int m[4][3]={11,12,13,21,22,23,31,32,33,41,42,43};

int *mp=&m[0][0]; // mp=x00c0 0014printf(“mp-> %d %d %d … \n”,*mp,*(mp+1),*(mp+2));printf(“row 3: %d %d %d\n”,*(mp+6),*(mp+7),*(mp+8));

mp-> 11 12 13row 3: 31 32 33

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m[0][0] m[0][1] m[0][2] m[1][0] m[1][1] m[1][2] m[2][0] m[2][1] m[2][2] m[3][0] m[3][1] m[3][2]

11 12 13 21 22 23 31 32 33 41 42 43

c0 0014 c0 0018 c0 001c c0 0020 c0 0024 c0 0028 c0 002c c0 0030 c0 0034 c0 0038 c0 003c c0 0040

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What is an un-sub-scripted Array?

char name[8]=“Thomas”;

printf(“name[0] is at %p\n”,&name[0]);

printf(“name value is %x\n”,name);

name[0] is at 0x23cb10

name value is 23cb10

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Page 17: Pointers vs. Arrays - Binghamton Universitytbarten1/CS211_Fall_2015/lectures/L20 - Pointers...Pointers vs. Arrays 1. Binghamton University CS-211 Fall 2015 Pointers in C •Pointers

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array == &array[0]

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What is a String?

char name[8]=“Thomas”;

printf(“name[0] is at %p\n”,&name[0]);

printf(“name value is %x\n”,name);

printf(“name string is %s\n”,name);

name[0] is at 0x23cb10

name value is 23cb10

name string is Thomas

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Page 19: Pointers vs. Arrays - Binghamton Universitytbarten1/CS211_Fall_2015/lectures/L20 - Pointers...Pointers vs. Arrays 1. Binghamton University CS-211 Fall 2015 Pointers in C •Pointers

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A string is a pointer to one or more characters

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Page 20: Pointers vs. Arrays - Binghamton Universitytbarten1/CS211_Fall_2015/lectures/L20 - Pointers...Pointers vs. Arrays 1. Binghamton University CS-211 Fall 2015 Pointers in C •Pointers

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Fall 2015

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What is a String?

char *name=“Thomas”;

printf(“name[0] is at %p\n”,&name[0]);

printf(“name value is %x\n”,name);

printf(“name string is %s\n”,name);

name[0] is at 0x23cb10

name value is 23cb10

name string is Thomas

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In C, Pointers and Arrays are Virtually Interchangeable!

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Array vs. Pointer Notation

Array

&array[0]

array[i]

Pointer

array

*(array+i)

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Page 23: Pointers vs. Arrays - Binghamton Universitytbarten1/CS211_Fall_2015/lectures/L20 - Pointers...Pointers vs. Arrays 1. Binghamton University CS-211 Fall 2015 Pointers in C •Pointers

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strlen implementations

Using array notation

int strlen(char str[]) {

int i=0;

while(str[i]!=x00) i++;

return i;

}

Using pointer notation

int strlen(char *str) {

int i=0;

while((*str)!=0) {

i++; str++;

}

return i;

}

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Page 24: Pointers vs. Arrays - Binghamton Universitytbarten1/CS211_Fall_2015/lectures/L20 - Pointers...Pointers vs. Arrays 1. Binghamton University CS-211 Fall 2015 Pointers in C •Pointers

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Resources

• Programming in C, Chapter 10

• Wikepedia Pointers : https://en.wikipedia.org/wiki/Pointer_(computer_programming)

• C Pointer Tutorial : http://www.tutorialspoint.com/cprogramming/c_pointers.htm

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https://en.wikipedia.org/wiki/Pointer_(computer_programming)
http://www.tutorialspoint.com/cprogramming/c_pointers.htm

Lecture 2 Pointers Pointers with Arrays Dynamic Memory Allocation.

Lecture 2 Pointers Pointers with Arrays Dynamic Memory Allocation.

Arrays and Pointers

Arrays and Pointers

Lecture 2: arrays and pointers

Lecture 2: arrays and pointers

Variables, Pointers, and Arrays

Variables, Pointers, and Arrays

Arrays and Pointers (part 2)

Arrays and Pointers (part 2)

Arrays, Pointers and References

Arrays, Pointers and References

Pointers. Topics Pointers Pointer Arithmetic Pointers and Arrays.

Pointers. Topics Pointers Pointer Arithmetic Pointers and Arrays.

Chapter 17 Pointers and Arrays

Chapter 17 Pointers and Arrays

Pointers, Arrays, Multidimensional

Pointers, Arrays, Multidimensional

Pointers, Arrays, Multidimensional - Michigan State …cse251/lecture11.pdf · Pointers, Arrays, Multidimensional Arrays ... 1 Programming in C. Review: Pointers ... pointer to the

Pointers, Arrays, Multidimensional - Michigan State …cse251/lecture11.pdf · Pointers, Arrays, Multidimensional Arrays ... 1 Programming in C. Review: Pointers ... pointer to the

Chapter 9 - Arrays and Pointers

Chapter 9 - Arrays and Pointers

Pointers & Dynamic Arrays

Pointers & Dynamic Arrays

Pointers in C++. Topics Covered Introduction to Pointers Pointers and arrays Character Pointers, Arrays and Strings Examples.

Pointers in C++. Topics Covered Introduction to Pointers Pointers and arrays Character Pointers, Arrays and Strings Examples.

Pointers to pointers & multi- dimensional arrays · Pointers to pointers & multi-dimensional arrays. Pointers to pointers 2 i: 3 j: 4 k: 5 ip1: ip2: ipp: int i=3; int j=4; ... •

Pointers to pointers & multi- dimensional arrays · Pointers to pointers & multi-dimensional arrays. Pointers to pointers 2 i: 3 j: 4 k: 5 ip1: ip2: ipp: int i=3; int j=4; ... •

Pointers, Arrays, Multidimensionalcse251/lecture11.pdf · Pointers, Arrays, Multidimensional Arrays • Pointers versus arrays – Lots of similarities • How to deal with 2D, 3D,

Pointers, Arrays, Multidimensionalcse251/lecture11.pdf · Pointers, Arrays, Multidimensional Arrays • Pointers versus arrays – Lots of similarities • How to deal with 2D, 3D,

Chapter 16 Pointers and Arrays

Chapter 16 Pointers and Arrays

Pointers to pointers & multi- dimensional arrays - ariel.ac.il · Pointers to pointers & multi-dimensional arrays . Pointers to pointers 2 i: 3 j: 4 k: 5 ip1: ip2: ipp: int i=3; int

Pointers to pointers & multi- dimensional arrays - ariel.ac.il · Pointers to pointers & multi-dimensional arrays . Pointers to pointers 2 i: 3 j: 4 k: 5 ip1: ip2: ipp: int i=3; int

Lecture 23: Pointers. 2 Lecture Contents: t Pointers and addresses t Pointers and function arguments t Pointers and arrays t Pointer arrays t Demo programs.

Lecture 23: Pointers. 2 Lecture Contents: t Pointers and addresses t Pointers and function arguments t Pointers and arrays t Pointer arrays t Demo programs.

0 Chap. 5 Pointers and Arrays 5.3Pointers and Arrays 5.4Address Arithmetic 5.5Character Pointers and Functions 5.6Pointer Arrays; Pointers to Pointers.

0 Chap. 5 Pointers and Arrays 5.3Pointers and Arrays 5.4Address Arithmetic 5.5Character Pointers and Functions 5.6Pointer Arrays; Pointers to Pointers.

C: Pointers, Arrays, and strings - Boise State CScs.boisestate.edu/~cs253/handouts/c-pointers-arrays-strings-handout.pdf · C: Pointers, Arrays, and strings Department of Computer

C: Pointers, Arrays, and strings - Boise State CScs.boisestate.edu/~cs253/handouts/c-pointers-arrays-strings-handout.pdf · C: Pointers, Arrays, and strings Department of Computer