Pointersandarrays
CS1121IntroductiontoCComputing
ContinueOVERVIEWofC
Lecture7
FunctionsinC(Contd.)
Outline
Functions
callbyvalue
callbyreference
witharrayarguments
Functionswithrecursivecalls
Factorial,Sum
Recursivesearchinastring
POINTERS
Pointersareconfusing
Theyareveryusefulinmanydifferentways
(e.g.withpointerswecaneffectivelyreturnmore
thanonethingfromafunction)
Arraysareaformofpointer
Wewillseealotmoreofpointerslaterin
theclass
SimplePointers
EveryVariablehasanassociatedaddress
(andamemorylocation)
Thevariablereferstothevalueinsidethat
memorylocation
Pointersrefertotheaddressofamemory
location
Eachvariablehasapointerareference
tothememoryaddressItcanbe
accessedwith&beforethenameofthe
variable.
4
SimplePointers
Pointersareusuallyrepresentedas
unsignedintegers(becausetheyare
addresses,andaddressesarenumbers,
usuallyverybignumbers)
Therecanbepointersofalltypes.The
typeofapointerdependsofthedatatype
ofthememorylocationitispointingto.
Therearegenericpointersvoidpointers
thatcanbeassignedtoanypointervalue.
5
SimplePointers
Genericpointers
Youcan'tcomparepointersofdifferent
types.eg.Youcantcompareanintand
charpointer.
Inthiscase,youcandeclareonepointer
asvoidanddoacomparison.
6
TypesofPointers
int*pointertoaninteger
char*pointertoacharacter
float*pointertoasingleprecisionfloating
pointnumber
double*pointertoadoubleprecision
floatingpointnumber
void*genericpointertoalltypes.
7
&meansaddressofavariable
*meansvalueinthespacepointedat
p is a pointer to an int
int *p;
q is an int
int q = 5;
p= &q;
p now points at q
printf ("p is %d\n",*p);
Therefore address p has the same value of
*p= 5;
5
We use *p to mean "the value that p is pointing at"
q=5
p
Wesawusing
pointerswithfunctions
Whenweuse&topassapointer(address)to
avariableintoafunction.Thisallowedusto
changethevalueofthevariablewithinthe
function.
Thisiscalledpassbyreference.
Thisiswhatwasgoingonwhenweuse&in
scanf.
Anotherexampleof
Changingavariable
withinafunction
void square_num (int *);
Prototype of a function
taking a POINTER
int main()
{
int p = 5;
Pass the address of
square_num (&p);
function
printf (p is now %d\n",p);
return 0;
}
Now the function square_num has changed p to
void square_num (int *num)
{
(*num)= (*num) * (*num);
}
p to our
25
Remember * gives
the value the pointer
points at
//FunctionExample,power(x,y)=xy
withoutpointers
#include<stdio.h>
intpower(intbase,intp);/*prototypedeclaration*/
main(){
#include<stdio.h>
inti;/*handyvariable*/
intpower(intbase,intp)
for(i=0;i<=10;++i)
{intn;
printf("2^%d=%d\n",i,power(2,i));
for(n=1;p>0;p)n=n*base
returnn;
return0;
}
}
main(){
intpower(intbase,intp)//functdefinition inti;/*handyvariable*/
for(i=0;i<=10;++i)
{intn;//handyvariable
printf("2^%d=%d\n",i,power
for(n=1;p>0;p)n=n*base;
return0;
returnn;
}
}
Whatarepointers?
Pointers"pointat"areasofyourcomputer's
memory.
int *p;
says p is a pointer to an int
Imagineyourcomputer'smemoryasa
seriesofboxeswhichallholdints
56 71 12 3
21 7
p points at one of the ints
Pointers
Pointers
intx=3;
int*X_addr;/*X_addrpointstothememory
locationofaninteger*/
X_addr=&x;
x(0xffbb00cc)X_addr
3
0xffbb00cc
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Pointers
Pointerspointtomemorylocations.
Tofindthevalueinsidethememory
locationpointedtobythepointer,usethe
dereferencingoperator(*)
(*x_addr)referstothevalueataddress
(memorylocation)x_addr.
Howabout*(&x)?
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POINTERS
SimpleProgramw/Pointers
OUTPUT:
#include<stdio.h>
intmain(void)
jhasthevalue1andisstoredatffbffa04
{
khasthevalue2andisstoredatffbffa00
ptrhasthevalueffbffa00andisstoredatffbff9fc
intj=1,k=2;
Valueoftheintegerpointedtobyptris2
int*ptr;
ptr=&k;
printf("\n");
printf("jhasthevalue%dandisstoredat%p\n",j,&j);
printf("khasthevalue%dandisstoredat%p\n",k,&k);
printf("ptrhasthevalue%pandisstoredat%p\n",ptr,&ptr);
printf(Valueoftheintegerpointedtobyptris%d\n",*ptr);
return0;
}
POINTERS
POINTERS
POINTERS
POINTERS&ARRAYS
POINTERS&ARRAYS
PointersArithmetic
PointersArithmetic
PointersArithmetic
ExamplesofCPointerDeclarations
1.
2.
3.
4.
int*ppisapointertoint
int*p[13]pisanarrayof[13]pointerstoint
int**ppisapointertoapointertoanint
int(*p)[13]isapointertoanarray[13]ofint
825
POINTERS&ARRAYS
#include<stdio.h>
OUTPUT:
#include<string.h>
$./a.out
intmain(void)
s1is=ffbff9e8ands2is=ff
{
s1hasthevalueHelloworld
chars1[32]="Helloworld!\0"; s3hasthevalueHello
s2hasthevalueHelloworld
char*s2;
chars3[32];
s2=s1;
strncpy(s3,s1,5);
printf("s1is=%pands2is=%p\n",s1,s2);
printf("s1hasthevalue%s\n",s1);
printf("s3hasthevalue%s\n",s3);
printf("s2hasthevalue%s\n",s2);
return0;
/*passingarraystofunctions#1*/
voidstrcopy(charb[],chara[]);
main()
{
charstr1[]="Hello,world",str2[20];
strcopy(str2,str1);
printf("%s\n",str2);
}
voidstrcopy(charb[],chara[])
{inti;
for(i=0;a[i]!='\0';i++)
b[i]=a[i];
b[i]='\0';
}
627
./a.out
Hello,world
/*passingarraystofunctions#2*/
voidstrcopy(char*b,char*a);
main()
{
charstr1[]="Hello,world",str2[20];
strcopy(str2,str1);/*No&needed*/
printf("%s\n",str2);
}
voidstrcopy(char*b,char*a)
{
while((*b++=*a++)!='\0')
;
628
}
$./a.out
Hello,world
Exampleofafunction"swap#1":
exchangesthevaluesoftheparameters:
voidswap1(intp1,intp
#include<stdio.h>
voidswap1(int,int);/*prototype*/
intmain()
{
intx=5,y=10;
printf("x=%d,y=%d\n",x,y);
swap1(x,y);
printf("x=%d,y=%d\n",x,y);
return0;
}
629
{
inttemp;
temp=p1;
p1=p2;
p2=temp;
}
$./a.out
x=5,y=10
x=5,y=10
Exampleofafunction"swap#2:
exchangesthevaluesoftheparameters:
voidswap2(int*p1,int
#include<stdio.h>
{
inttemp;
temp=*p1;
*p1=*p2;
*p2=temp;
}
voidswap2(int*,int*);/*prototype*/
intmain()
{
intx=5,y=10;
printf("x=%d,y=%d\n",x,y);
swap2(&x,&y);
printf("x=%d,y=%d\n",x,y);
return0;
}
$./a.out
x=5,y=10
x=10,y=5
630
RecursionAfunctionmay
callitselfdirectlyorindirectly
Example:factorials
5!=5*4*3*2*1
Noticethat
5!=5*4!
4!=4*3!...
Wecancomputefactorialsrecursively
Solvebasecase(1!=0!=1)thenplugin
2!=2*1!=2*1=2;
3!=3*2!=3*2=6;
Recursion
RecursionSimpleexample
/*sum*/
intsum(intn){
if(n==0)
return0;
returnn+sum(n1);
}
/*iterativesum1*/
intsum(intn){
inti,s=0;
for(i=0;i<=n;i++)
s+=i;
returns;
}
/*iterativesum2*/
intsum(intn){
ints=n;
while(n>0){
n;s+=n;
}
returns;
}
Recursionvs.Iteration
Anyrecursivefunctioncanbewritten
withiteration.
Anyiterativefunctioncanbewritten
withrecursion.
Shouldweuseiterationorrecursion?
Easeandclarityofimplementation.
Time/spaceefficiency.
Recursion
NormalCMemoryManagement
Aprogramsaddressspace
contains4regions:
stack:localvariables,grows
downward
heap:spacerequestedfor
pointersviamalloc();
resizesdynamically,grows
upward
staticdata:variablesdeclared
outsidemain,doesnotgrow
orshrink
code:loadedwhenprogram
starts,doesnotchange
stack
~FFFF
FFFFhex
heap
staticdata
code
~0hex
SampleofRecursion
#include<stdio.h>
voidexam(intn){
if(n>1)
exam(n1);
printf("%d",n);
}
intmain(){
intn=3;
printf("nisnow%d\n",n);
exam(n);
printf("\n");
n=5;
printf("nisnow%d\n",n);
exam(n);
printf("\n");
Whatistheoutput
637
SampleofRecursion
#include<stdio.h>
voidexam(intn){
if(n>1)
exam(n1);
printf("%d",n);
}
intmain(){
intn=3;
printf("nisnow%d\n",n);
exam(n);
printf("\n");
n=5;
printf("nisnow%d\n",n);
exam(n);
printf("\n");
Whatistheoutput
OUTPUT:
nisnow3
123
nisnow5
12345
638
Anotherrecursionsample
Considerthisfunctiondeclaration:
voidquiz(inti){
staticallowsthelastv
if(i>0)
localvariabletobepres
{staticintk=0;
betweensuccessiveca
quiz(i/2)
function
}
printf("kis=%d*\n",++k);
}
Howmanyasterisksareprintedbyquiz(5)call?
A.3
B.4
C.7
D.8
E.Someothernumber
639
Considerthisfunctiondeclaration:
voidquiz(inti){
if(i>0)
kis=1*
{staticintk=0;
kis=2*
kis=3*
quiz(i/2)
kis=4*
}
printf("kis=%d*\n",++k);
}
Howmanyasterisksareprintedbyquiz(5)call?
A.3
B.4
C.7
D.8
E.Someothernumber
SampleQuestion
640
#include<stdio.h>
voidquiz(inti)
{staticintk=0;
if(i>0){
quiz(i/2);
}
printf("kis=%d*\n",++k);
}
intmain(){
intn=5;
quiz(5);
return0;
}
SampleQuestion
641
Considerthisfunctiondeclaration:
voidquiz(inti){
if(i>0)
{
quiz(i/2);
quiz(i/2);
}
printf("*);
}
Howmanyasterisksareprintedbyquiz(5)call?
A.3
B.4
C.7
D.8
E.Someothernumber
AnotherSample
Question
642
#include<stdio.h>
voidquiz(inti)
{staticintk=0;
if(i>0){
quiz(i/2);
quiz(i/2);
}
printf("kis=%d*\n",++k);
}
intmain()
{
intn=5;
quiz(5);
return0;
}
SampleQuestion
643
#include<stdio.h>
voidquiz(inti)
{staticintk=0;
if(i>0){
quiz(i/2);
quiz(i/2);
}
printf("kis=%d*\n",++k);
}
intmain()
{
intn=5;
quiz(5);
return0;
}
SampleQuestion
644
$./a.out
kis=1*
kis=2*
kis=3*
kis=4*
kis=5*
kis=6*
kis=7*
kis=8*
kis=9*
kis=10*
kis=11*
kis=12*
kis=13*
kis=14*
kis=15*
SampleQuestion
quiz(0)*
quiz(1)quiz(0)*
*
main:quiz(1)quiz(0)*
quiz(5)quiz(2)*quiz(0)*
*
quiz(2)quiz(1)quiz(0)*
quiz(0)*
**
quiz(1)quiz(0)*
*quiz(0)*
*
645