Array Pointers

8/8/2019 Array Pointers

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Copyright : Nahrstedt, Angrave, Abdelzaher 1

C Survival Guide

Lawrence Angrave


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Copyright : Angrave, Abdelzaher,Nahrstedt

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C in 3 slides: Pointers& 'address-of ' (reference operator)

* 'contents-of ' (dereference operator) Automatic variables are temporary and storedin the stackchar* p; p is a point er to a charac t er.

*p =0; c ont ents-of p s e t to 0. (Kab oo m!) After declaring a pointer, initialize it tosomething before using it. (Doh!)


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Instant C #2: StringsC strings are terminated with a null byte

strcpy( "hello", "world" ) will crashstrcmp(s1,s2) returns 0 if sameargv[0] is the program name

argv[argc] is a null pointer


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Instant C #3: Heap Allocationm alloc( bytes ) to reserve heap memorylater f ree(ptr)

static char * ptr;static variables are not stored in stack


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Common Causes of 'Death '*Uninitialized pointers

strcpy(dest,"hello");

C Strings need null byte at the endBuffer overflowUn-initialized memoryToo confident: not checking returnvaluesMiss-use of static vs. stack variables.

*Talk like a Professional : "Segfault" / "Signal 11"


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R eview1. in t p 1 ;

What does &p1 mean?

2. char**argv ;What type is argv ?What type is *argv ?What type is **argv ?

3 . ma in ( in t argc,char**argv) {what is *argv ?what is argv[argc ] ?

4. What are the differences between x

and y ?char* f () {char *x;stat i c char*y;retur n y;

}

Fix the following code if necessary

1. if (strc m p("a","a"))

pr in t f ("sa m e!");

2. in t i =4;in t * i ptr;i ptr = & i ;* i ptr = 5;// n ow i =5

3 . char *p;p=(char*) m alloc(99);strcpy("Hello",p);pr in t f ("%s World",p);f ree(p);

4 . char m sg[5];

strcpy ( m sg,"Hello");


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Writing C code is easy!void* myfunction() {

char *p;*p = 0;return (void *) &p;

Good news: You've used C++ in CS225 so


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Writing C code is easy !void* myfunction() {

char *p;*p = 0;return (void *) &p;

that crashes / is unpredictable


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How do I write good C

programs?Fluency in C syntax

Stack vs. HeapKey skill: read code for bugs(compiler warnings,if any, and testing areinsufficient)


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100 minute challengePointersC character arrays & string functionsStack & heap allocationTools:make, gcc, (gdb)

Exchange rate: 10 minutes learning = 100 minutes of debugging an MP


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C is powerful - it 's the System

Programmer 's choice language

Sun Operating System

Linux Operating SystemBSD & Apple OS X

R uby, Python, JVM, gccbash shell, dir

Apache web serverhttp://svn.apache.org/repos/asf/httpd/httpd/trunk/server/util.c

http://www.google.com/codesearch?q=lang:c


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The C Language Spirit Made by professional programmers for professionalprogrammers

Very flexible, very efficient and portableDoes not protect the programmers from themselves.R ationale: programmers know what they are doing.

UNIX and most serious system software (servers,compilers, etc) are written in C.Can do everything Java and C++ can. It ll just lookuglier in C


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Compilergcc

PreprocessorCompilerLinkerSee manual man for options: man gcc

"Ansi-C" standards C89 versus C99C99: Mix variable declarations and code (for int i= )C++ inline comments //a comment

make a compilation utilityGoogle 'makefile '


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Programming in CC = Variables + Instructions


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intchar

float

string

pointer array


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intchar

float

string

pointer array

printf/scanf assignment

if

switch

for while


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10,000

10,002

10,008

10,010

10,012

Variables

Value1

Value2

Value3

Value4

Value5

x

y

zp

d

Memory Address

Name

Value


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10,000

10,002

10,008

10,010

10,012

The & Operator:

R eads Address of

Value1

Value2

Value3

Value4

Value5

x

y

zp

d

Name

Value

&y


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10,000

10,002

10,008

10,010

10,012

Pointers

Value1

Value2

Value3

10,002

Value5

x

y

zp

d

Name

Value

A pointer is a variablewhose value is the

address of another


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10,000

10,002

10,008

10,010

10,012

The * Operator

R eads Variable pointed to by

Value1

Value2

Value3

10,002

Value5

x

y

zp

d

Name

Value

A pointer is a variablewhose value is the

address of another*P


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What is the Output?main() {

int *p, q, x;x=10;p=&x;*p=x+1;

q=x;printf ( Q = %d\n , q);

}


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int *p, q, x;x=10;p=&x;*p=x+1;


}

#@ *%!

#@%$!

@*%^

p

q

x


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int *p, q, x;x=10;p=&x;*p=x+1;


}

#@ *%!

#@%$!

10

p

q

x


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int *p, q, x;x=10;p=&x;*p=x+1;


}

#@%$!

10

p

q

x


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int *p, q, x;x=10;p=&x;*p=x+1;


}

#@%$!

11

p

q

x


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int *p, q, x;x=10;p=&x;*p=x+1;


}

11

11

p

q

x


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Cardinal R ule: Must Initialize

Pointers before Using themint *p;*p = 10;

BAD


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#@ *%!p??

Pointing somewhererandom


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#@ *%!p

#@ *%!10


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How to Initialize Pointers


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How to Initialize PointersSet pointer equal to location of knownvariable

int *p;int x;

p=&x;


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How to Initialize PointersUse malloc()

int *p;

p=(int *) malloc (sizeof (int));


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How to Initialize PointersCreate an Array

int p[10];

Same as:int *p;p=(int *) malloc (10 *sizeof (int));


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Arraysint p[5];

p[0]p[1]p[2]p[3]p[4]

Name of array (is a pointer)

p

Shorthand:*(p+1) is called p[1]*(p+2) is called p[2]etc..


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Exampleint y[4];y[1]=6;y[2]=2;

6

2

y[0]y[1]y[2]y[3]

y


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Array Name as PointerWhat s the difference between the examples below:

Example 1:int z[8];int *q;q=z;

Example 2:int z[8];int *q;q=&z[0];


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Array Name as PointerWhat s the difference between the examples below:

Example 1:int z[8];int *q;q=z;

Example 2:int z[8];int *q;q=&z[0];

NOTHING!!

x (the array name) is a po intert o the beginning of the array , which is & x[ 0]


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Example:How much is y at the end:int y, x, *p;

x = 20;

*p = 10;y = x + *p;


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Question:What s the difference between

int* q;int q[5];

What s wrong with:

int ptr[2];ptr[1] = 1;ptr[2] = 2;


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Question:What is the value of b[2] at the end?

int b[3];int* q;

b[0]=48; b[1]=113; b[2]=1;q=b;*(q+1)=2;b[2]=*bb[2]=b[2]+b[1];


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C i h A Abd l h N h d


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ConventionsStrings

string c

Character c

C i ht A Abd l h N h t dt


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String Operationsstrcpystrlenstrcat strcmp

Cop right : Angra e Abdel aher Nahrstedt


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strcpy, strlenSyntax:strcpy(ptr1, ptr2);

where ptr1 and ptr2 are pointers to charvalue = strlen(ptr);

where value is an integer andptr is a pointer to char

Example:int len;char str[15];strcpy (str, "Hello, world!");

len = strlen(str);

Copyright : Angrave Abdelzaher Nahrstedt


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strcpy, strlenWhat s wrong with

char str[5];

strcpy (str, "Hello");



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strncpySyntax:strncpy(ptr1, ptr2, num);

where ptr1 and ptr2 are pointers to charnum is the number of characters to be copied

Example:int len;char str1[15], str2[15];strcpy (str1, "Hello, world!");

strncpy (str2, str1, 5);



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strncpySyntax:strncpy(ptr1, ptr2, num);

where ptr1 and ptr2 are pointers to charnum is the number of characters to be copied

Example:int len;char str1[15], str2[15];strcpy (str1, "Hello, world!");

strncpy (str2, str1, 5);

Caution: strncpy blindly copies thecharacters. It does not voluntarily appendthe string-terminating null character.



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strcat Syntax: strcat(ptr1, ptr2);

where ptr1 and ptr2 are pointers to char

Concatenates the two null terminates stringsyielding one string (pointed to by ptr1).

char S[25] = "world!";char D[25] = "Hello, ";strcat(D, S);



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Copy g t : g ave, bde a e , a stedt

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strcat Example

What s wrong with:

char S[25] = "world!";strcat(Hello, , S);


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py g g

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Math: Increment and

Decrement OperatorsExample 1:int x, y, z, w;

y=10; w=2;x=++y;z=--w;

Example 2:int x, y;y=10; w=2;x=y++;

z=w--;



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Math: Increment and

Decrement OperatorsExample 1:int x, y, z, w;

y=10; w=2;x=++y;z=--w;

Example 2:int x, y;y=10; w=2;x=y++;

z=w--;

First increment/decrement then assign result x is11, z is 1

First assign then increment/decrement x is 10, z is 2



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Math: Increment and Decrement

Operators on PointersExample 1:int a[2];int number1, number2, *p;a[0]=1; a[1]=10; a[2]=100;p=a;number1 = *p++;number2 = *p;

What will number1 and number2 be at the end?



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Math: Increment and Decrement

Operators on PointersExample 1:int a[2];int number1, number2, *p;a[0]=1; a[1]=10; a[2]=100;p=a;number1 = *p++;number2 = *p;

What will number1 and number2 be at the end?

Hint: ++ increments pointer p not variable *p



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Logic: R elational (Condition)

Operators== equal to!= not equal to> greater than< less than

>= greater than or equal to


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Logic Exampleif (a == b) printf (Equal.);else printf (Not Equal.);

Question: what will happen if I replaced the above

with:if (a = b) printf (Equal.);else printf (Not Equal.);

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Array Pointers