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authorzlg <zlg@zlg.space>2013-04-03 07:15:17 -0500
committerzlg <zlg@zlg.space>2013-04-03 07:15:17 -0500
commit3529fb7a854a969f7fadf0d5310c659b933ea276 (patch)
treef3c9f743c66ac0b3a2a4271af08f1e9e98a74fe6 /ch2
parentSolve Exercise 2-6: setbits() (diff)
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Solve Exercise 2-07: invert()
Diffstat (limited to 'ch2')
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diff --git a/ch2/2-07_invert.c b/ch2/2-07_invert.c
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+#include <stdio.h>
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 2-07: Write a function invert(x,p,n) that returns x with the n bits
+ * that begin at position p inverted (i.e. 1 changed into 0 and vice-versa),
+ * leaving the others unchanged.
+ *
+ * Answer: The trick is to start with ~0 to get all 1s, then left-shift n
+ * times. After that, flip again to get n number of 1s. Next, left shift the
+ * appropriate number of times needed to line up the mask of 1s to the proper
+ * location. Apply that mask to x with XOR and bam, job done!
+ *
+ * Here's a step by step illustration of invert(205, 5, 4), assuming 16-bit
+ * unsigned integers:
+ *
+ * 1111111111111111 ~0
+ * 1111111111110000 << 4
+ * 0000000000001111 ~
+ * 0000000000011110 << (5 - 4 == 1)
+ *
+ * Mask created. Now let's take our original number...
+ * 0000000011001101 205
+ * #### ^ ...and apply the mask!
+ * 0000000011010011 End Result (211)
+ */
+
+unsigned invert(unsigned x, unsigned p, unsigned n) {
+ return x ^ ((~(~0 << n)) << (p - n));
+}
+
+int main() {
+ printf("invert(205, 5, 4) returns %d\n", invert(205, 5, 4));
+ printf("11001101 flips 110[0110]1 to produce...\n11010011\n");
+ printf("invert(1876, 7, 3) returns %d\n", invert(1876, 7, 3));
+ printf("11101010100 flips 1110[101]0100 to produce...\n11100100100\n");
+ return 0;
+}