5.6PointerArrays:PointertoPointerPointerarrayisanarraywhichstorespointers.Anexampletosortasetoftextlinesintoalphabeticorder.defghijklmnopqrstabcdefghijklmnopqrstabcsortAlgorithm:readallthelinesofinputsortthemprinttheminorder5.6PointerArrays:PointertoPointer#include<stdio.h>#include<string.h>#defineMAXLINES5000char*lineptr[MAXLINES];voidreadlines(char*lineptr[],intnlines);voidwritelines(char*lineptr[],intnlines);voidqsort(char*lineptr[],intleft,intright);/*sortinputlines*/main(){intnlines; /*numberofinputlinesread*/if((nlines=readlines(lineptr,MAXLINES))>=0){qsort(lineptr,nlines);writelines(lineptr,nlines);return0;}else{printf(“error:inputtoobigtosort\n”); return1; }}5.6PointerArrays:PointertoPointer#defineMAXLINE1000 /*maxlengthofanyinputline*/int

getline(char*,int);char*alloc(int);/*readlines:readinputlines*/int

readlines(char*lineptr[],int

maxlines){int

len,nlines;char*p,line[MAXLINE];

nlines=0;

while((len=getline(line,MAXLINE))>0)

if(nlines>=maxlines||(p=alloc(len))==NULL) return-1; else{line[len-1]=‘\0’;/*deletenewline*/

strcpy(p,line);

lineptr[nlines++]=p; }returnnlines;}voidwritelines(char*lineptr[],int

nlines){inti;for(i=0;i<lines;i++)

printf(“%s\n”,lineptr[i]);}while(nlines-->0)

printf(“%s\n”,*nlineptr++);5.6PointerArrays:PointertoPointer/*qsort:sortv[left]…v[right]intoincreasingorder*/voidqsort(char*v[],inteft,intright){inti,last;voidswap(char*v[],inti,intj);if(left>=right)return;swap(v,left,(left+right)/2);last=left;for(i=left+1;i<=right;i++) if(strcmp(v[i],v[left])<0) swap(v,++lasti);swap(v,left,last);qsort(v,left,last-1);qsort(v,last+1,right);}voidswap(char*v[],inti,intj){char*temp;temp=v[i];v[i]=v[j];v[j]=temp;}5.7Multi-DimensionalArrays/*daytabisa2-demensionalarray*/staticchardaytab[2][13]={{{0,31,28,31,30,31,30,31,31,30,31,30,31},{0,31,29,31,30,31,30,31,31,30,31,30,31}};int

day_of_year(intyear,intmonth,intday){

inti,leap;

leap=year%4==0&&year%100!=0||year%400==0;

for(i=1;i<month;i++) day+=daytab[leap][i];returnday;}voidmonth_day(intyear,int

yearday,int*pmonth,int*pday){inti,leap;leap=year%4==0&&year%100!=0||year%400==0；

for(i=1;yearday>daytab[leap][i];i++)

yearday-=daytab[eap][i];*pmonth=i;*pday=yearday;}5.8InitializationofPointerArrays/*month_name:returnnameofn-thmonth*/char*month_name(intn){

staticchar*name[]={ “Illegalmonth”, “january”,“februaru”,“march”, “april”,“may”,“june”,“july”, “august”,“september”,“october”, “november”,“december” }; return(n<1||n>12)?name[0]:name[n];}5.9Pointersvs.Multi-dimensionalArrayDifferencebetweenatwo-dimensionalarrayandanarrayofpointer:inta[10][20];int*b[10];Althougha[3][4]andb[3][4]arebothsyntacticallylegalreferencetoasingleint,but:1:aisatruetwo-dimensionalarray,bisonlyaone-dimensionalarray2:ahas200int-sizedlocations,bonly10pointers.3:therowsofbmaybeofdifferentlengths.Becauseof3,themostfrequentuseofarraysofpointersistostorecharacterstringsofdiverselengths.5.9Pointersvs.Multi-dimensionalArrayExample:charaname[][15]