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 maxlines);
void writelines(char *lineptr[], int nlines);
void qsort(char *v[], 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.