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authorKai Mallea <kmallea@gmail.com>2013-06-28 21:47:31 -0400
committerKai Mallea <kmallea@gmail.com>2013-06-28 21:47:31 -0400
commit86885dab5ca005ba72c21b80f7f55a9b92e17a6d (patch)
tree210853ebd775c4d36945ecc8d17b0fbbc98bd5f7 /c.html.markdown
parent86273dec2fafff150f3d07865237e9e499889fff (diff)
Update pointer examples and comments
Diffstat (limited to 'c.html.markdown')
-rw-r--r--c.html.markdown27
1 files changed, 21 insertions, 6 deletions
diff --git a/c.html.markdown b/c.html.markdown
index 15bfa05e..3f9b6c61 100644
--- a/c.html.markdown
+++ b/c.html.markdown
@@ -194,26 +194,41 @@ printf("%d\n", (short) 65537); // => 1 (Max short = 65535)
// Pointers
///////////////////////////////////////
-// You can retrieve the memory address of your variables,
-// then mess with them.
+// A pointer is a variable declared to store a memory address. Its declaration will
+// also tell you the type of data it points to. You can retrieve the memory address
+// of your variables, then mess with them.
int x = 0;
printf("%p\n", &x); // Use & to retrieve the address of a variable
// (%p formats a pointer)
// => Prints some address in memory;
+// Pointer types end with * in their declaration
+int* px; // px is a pointer to an int
+px = &x; // Stores the address of x in px
+printf("%p\n", px); // => Prints some address in memory
+
+// To retreive the value at the address a pointer is pointing to,
+// put * in front to de-reference it.
+printf("%d\n", *px); // => Prints 0, the value of x, which is what px is pointing to the address of
+
+// You can also change the value the pointer is pointing to.
+// We'll have to wrap the de-reference in parenthesis because
+// ++ has a higher precedence than *.
+(*px)++; // Increment the value px is pointing to by 1
+printf("%d\n", *px); // => Prints 1
+printf("%d\n", x); // => Prints 1
+
int x_array[20]; // Arrays are a good way to allocate a contiguous block of memory
int xx;
for(xx=0; xx<20; xx++){
x_array[xx] = 20 - xx;
} // Initialize x_array to 20, 19, 18,... 2, 1
-// Pointer types end with *
int* x_ptr = x_array;
-// This works because arrays are pointers to their first element.
+// This works because an array name is bound to the address of its first element
-// Put a * in front to de-reference a pointer and retrieve the value,
-// of the same type as the pointer, that the pointer is pointing at.
+// Arrays are pointers to their first element
printf("%d\n", *(x_ptr)); // => Prints 20
printf("%d\n", x_array[0]); // => Prints 20