solutions to excercises

k&r/04-funcs-and-prog-struct/04-07.c

@@ -0,0 +1,48 @@
+#include <stdio.h>
+#include <string.h>
+
+#define MIN(A, B) ((A < B) ? (A) : (B))
+
+/* -------- INPUT WITH PUSHBACK */
+int _getchar()
+{
+    extern int buffi;
+    extern char buffer[];
+    return buffi ? buffer[--buffi] : getchar();
+}
+#undef getchar
+#define getchar() _getchar()
+
+
+/* --------- MAIN PROGRAM */
+#define SLEN 10
+void ungets(char *s);
+
+/* this program copies its input to its output,
+ * if ungets wokrks as expected */
+int main()
+{
+    int i, c;
+    char s[SLEN+1];
+
+    for (i = 0; i < SLEN && (c = getchar()) != EOF; i++)
+        s[i] = c;
+    s[i] = '\0';
+
+    ungets(s);
+    while ((c = getchar()) != EOF)
+        putchar(c);
+}
+
+
+#define BUFF_SIZE 100 
+int buffi;
+char buffer[BUFF_SIZE];
+
+/* ungets: pushes string s back to input */
+void ungets(char *s)
+{
+    int i = strlen(s);
+    for (i = MIN(i, BUFF_SIZE-buffi)-1; i >= 0; i--, buffi++)
+        buffer[buffi] = s[i];
+}

k&r/04-funcs-and-prog-struct/04-08.c

@@ -0,0 +1,22 @@
+#include <stdio.h>
+
+int main()
+{
+    ;
+}
+
+/* this handles EOF as wall */
+int buffc = -1;
+
+char getch()
+{
+    char temp;
+    return (buffc < 0) ? getchar() : (temp = buffc, buffc = -1, temp);
+}
+
+void ungetch(int c)
+{
+    if (buffc >= 0)
+        printf("warning: overriding character buffer\n");
+    buffc = c;
+}

k&r/04-funcs-and-prog-struct/04-12.c

@@ -0,0 +1,37 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+void itoar(char *s, int x);
+
+/* converts first arg to int, puts it to output with itoar */
+int main(int argc, char *argv[])
+{
+    if (argc < 2)
+        return 1;
+    int x = atoi(argv[1]);
+    char s[strlen(argv[1])+1];
+    itoar(s, x);
+    printf("%s\n", s);
+    return 0;
+}
+
+char * utoar(char *s, unsigned x);
+
+/* itoa: writes int x to string s in decimal
+ * recursive implementation */
+void itoar(char *s, int x)
+{
+    unsigned y = (x < 0) ? *(s++) = '-', -x : x;
+    s = utoar(s, y);
+    *s = '\0';
+}
+
+/* utoar: helper for itoa */
+char * utoar(char *s, unsigned x)
+{
+    if (x / 10)
+        s = utoar(s, x / 10);
+    *s = x % 10 + '0';
+    return s+1;
+}

k&r/04-funcs-and-prog-struct/04-13.c

@@ -0,0 +1,28 @@
+#include <stdio.h>
+
+int reverse(char string[], int i);
+
+/* reverses command line argument */
+int main(int argc, char *argv[])
+{
+    if (argc < 2)
+        return 1;
+    printf("length :   %d\n", reverse(argv[1], 0));
+    printf("reversed : %s\n", argv[1]);
+    return 0;
+}
+
+/* reverse: reverses string from index i, returns length 
+ * recursive implementation */
+int reverse(char s[], int i)
+{
+    if (s[i] == '\0') { /* base case */
+        return 0; 
+    }
+    int j = reverse(s, i+1);
+    if (j > i) { /* swap elements */
+        char c;
+        c = s[i], s[i] = s[j], s[j] = c;
+    }
+    return j+1;
+}

k&r/04-funcs-and-prog-struct/04-qsort.c

@@ -0,0 +1,43 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+void qsort_book(int v[], int left, int right);
+
+/* sorts command line arguments */
+int main(int argc, char *argv[])
+{
+    int i, v[argc-1];
+    for (i = 1; i < argc; i++)
+        v[i-1] = atoi(argv[i]);
+    qsort_book(v, 0, argc-2);
+    for (i = 0; i < argc-1; i++)
+        printf("%d ", v[i]);
+    putchar('\n');
+    return 0;
+}
+
+/* swap: swap v[i] and v[j] */
+void swap(int *v, int i, int j)
+{
+    int temp = v[i];
+    v[i] = v[j], v[j] = temp;
+}
+
+/* qsort: sort v[left]...v[right] into increasing order */
+void qsort_book(int v[], int left, int right)
+{
+    int i, last;
+
+    if (left >= right) /* do nothing if array contains */
+        return;        /* fewer than two elements */
+    swap(v, left, (left + right)/2); /* move partition elem */
+    last = left; /* to v[0] */
+
+    for (i = left+1; i <= right; i++) /* partition */
+        if (v[i] < v[left])
+            swap(v, ++last, i);
+    swap(v, left, last); /* restore partition elem */
+
+    qsort_book(v, left, last-1);
+    qsort_book(v, last+1, right);
+}