Exercise 5.7 - Readlines using array#
Question#
Rewrite readlines to store lines in an array supplied by main, rather than calling alloc to maintain storage. How much faster is the program?
#include<stdio.h>
#include<string.h>
#define MAXLINES 5000 /* max #lines to be sorted */
char *lineptr[MAXLINES];
char linestor[MAXLINES];
int readlines(char *lineptr[],char *linestor,int nlines);
void writelines(char *lineptr[],int nlines);
void qsort(char *lineptr[],int left,int right);
/* sort input lines */
int main(void)
{
int nlines; /* number of input lines read */
if((nlines = readlines(lineptr,linestor,MAXLINES)) >= 0)
{
qsort(lineptr,0,nlines-1);
writelines(lineptr,nlines);
return 0;
}
else
{
printf("error: input too big to sort \n");
return 1;
}
}
#define MAXLEN 1000 /* max length of any input line */
#define MAXSTOR 5000
int mgetline(char *,int);
char *alloc(int);
/* readlines: read input lines */
int readlines(char *lineptr[],char *linestor,int maxlines)
{
int len,nlines;
char line[MAXLEN];
char *p = linestor;
char *linestop = linestor + MAXSTOR;
nlines=0;
while((len=mgetline(line,MAXLEN)) > 0)
if(nlines >= maxlines || p+len > linestop)
return -1;
else
{
line[len-1] = '\0';
strcpy(p,line);
lineptr[nlines++]=p;
p+=len;
}
return nlines;
}
/* writelines: write output lines */
void writelines(char *lineptr[],int nlines)
{
int i;
for(i=0;i<nlines;i++)
printf("%s\n",lineptr[i]);
}
/* qsort: sort v[left] ... v[right] into increasing order */
void qsort(char *v[],int left,int right)
{
int i,last;
void swap(char *v[],int i,int j);
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,++last,i);
swap(v,left,last);
qsort(v,left,last-1);
qsort(v,last+1,right);
}
/* swap: interchange v[i] and v[j] */
void swap(char *v[],int i,int j)
{
char *temp;
temp=v[i];
v[i]=v[j];
v[j]=temp;
}
#define ALLOCSIZE 10000 /* size of available space */
static char allocbuf[ALLOCSIZE]; /* storage for alloc */
static char *allocp = allocbuf; /* next free position */
char *alloc(int n) /* return pointer to n characters */
{
if(allocbuf + ALLOCSIZE - allocp >= n)
{
allocp += n;
return allocp -n;
}
else
return 0;
}
int mgetline(char *s,int lim)
{
int c;
char *t=s;
while(--lim >0 && (c=getchar())!=EOF && c!='\n')
*s++ = c;
if( c == '\n')
*s++ = c;
*s= '\0';
return s-t;
}
Explanation#
This uses the same qsort program. But instead of calculating the memory required using the alloc operator. It sends a predefined amount of memory from the main program.