diff --git a/k&r/02-types-operators-and-exp/02-09.c b/k&r/02-types-operators-and-exp/02-09.c
new file mode 100644
index 0000000..d91732c
--- /dev/null
+++ b/k&r/02-types-operators-and-exp/02-09.c
@@ -0,0 +1,35 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include "02-bitwise.h"
+
+int bitcount(unsigned x);
+
+/* parse command line args for bitcount, print result */
+int main(int argc, char *argv[])
+{
+    if (argc < 2) {
+        printf("error : expected argument x\n");
+        return 1;
+    }
+    unsigned x;
+    if (!int_of_string(&x, argv[1]))
+        return 2;
+    
+    printf("%d\n", bitcount(x));
+    return 0;
+}
+
+/* x &= x-1 deletes the right most 1 bit
+ *    suppose x    = y10..0
+ *    then  x-1    = y01..1
+ *    then x & x-1 = y00..0
+ * QED */ 
+
+/* bitcount: count number of set bits in x, using the abova property */
+int bitcount(unsigned x)
+{
+    int b;
+    for (b = 0; x; b++)
+        x &= x-1;
+    return b;
+}
diff --git a/k&r/02-types-operators-and-exp/02-10.c b/k&r/02-types-operators-and-exp/02-10.c
new file mode 100644
index 0000000..acf6bfe
--- /dev/null
+++ b/k&r/02-types-operators-and-exp/02-10.c
@@ -0,0 +1,16 @@
+#include <stdio.h>
+
+char lower(char c);
+
+int main()
+{
+    int c;
+    while ((c = getchar()) != EOF)
+        putchar(lower(c));
+    return 0;
+}
+
+char lower(char c)
+{
+    return (c >= 'A' && c <= 'Z') ? c + 'a' - 'A' : c;
+}