4.10Recursionrecursion:functioncallitselfeitherdirectlyor indirectlyexample1:#include<stdio.h>/*printd:printnindecimal*/voidprintd(intn){if(n<0){putchar(‘-’);n=-n;}if(n/10)printd(n/10);

putchar(n510+’0’);}printd(123)---------putchar(3+’0’);printf(12)-----------putchar(2+’0’);printf(1)------------putchar(1+’0’);result:1234.10Recursionexample2:/*qsort:sortv[left]..v[right]intoincreaseingorder*/voidqsort(intv[],intleft,intright){intI,last;voidswap(intv[],inti,intj);if(left>=right)return;/*donothing*/swap(v,left,(left+right)/2);/*moveparititionelemtov[0]*/last=left;for(i=left+1;i<=right;i++)/*partition*/if(v[i]<v[left])swap(v,++last,i);swap(v,left,last);qsort(v,left,last-1);qsort(v,last+1,right);}/*swap:interchangev[i]andv[j]*/voidswap(intv[],inti,intj){inttemp;temp=v[i];v[i]=v[j];v[j]=temp;}4.11TheCPreprocessor4.11.1FileInclusion#include<filename>searchfilenameinthecomplier’s“include”directory#include“filename”searchfilenameincurrentdirectory,theninthecomplier’s“include”directory4.11TheCPreprocessor4.11.2MacroSubstitutionform:#definenamereplacement_textmacrowithoutarguments:

#definePI3.14159 macrowitharguments:

#definemax(A,B)((A)>(B)?(A):(B)) max(2+5,6)<==>((2+5)>(6)?(2+5):(6)

notice:#definesquare(x)x*x square(i+j)<==>i+j*i+juse“#define”toundefineamacro #undefinegetchar/*definein<stdio.h>*/intgetchar(void){…}

4.11TheCPreprocessor#:parameterstring

#definedprint(expr)printf(#expr“=%g\n”,expr) dprint(x/y):<==>printf(“x/y”“=%g\n”,x/y); ##:concatenatetheactualargument: #definepaste(front,back)front##back paste(name,1)===>name14.11TheCPreprocessor4.11.3ConditionalInclusionform:(1)#ifconstant block1 #elifconstant block2 #else blockN#endifexample:#ifSYSTEM==SYSU#defineHDR“sysv.h”#elifSYSTEM==BSD#defineHDR“bsd.h”#elifSYSTEM==MSDOS#defineHDR“msdos.h”#else#defineHDR“default.h”#endif#includeHDR#ifdefHDR…(block1)#else…(block2)#endif(2)#ifdefname block