Question and Answer
Q What are the left and right values?
A The left value refers to the address of a variable, and the right value refers to the content stored in the memory location of a variable. There are two ways to get the right value of a variable: use the variable name directly, or use the left value of the variable to refer to where the right value resides. The second way is also called the indirect way.
Q How can you obtain the address of a variable?
A By using the address-of operator, &. For instance, given an integer variable x, the &x expression returns the address of x. To print out the address of x, you can use the %p format specifier in the printf() function.
Q What is the concept of indirection in terms of using pointers?
A Before this hour, the only way you knew for reading from or writing to a variable was to invoke the variable directly. For instance, if you wanted to write a decimal, 16, to an integer variable x, you could call the statement x = 16;.
As you learned in this hour, C allows you to access a variable in another way—using pointers. Therefore, to write 16 to x, you can first declare an integer pointer (ptr) and assign the left value (address) of x to ptr—that is, ptr = &x;. Then, instead of executing the statement x = 16;, you can use another statement:
*ptr = 16;
Here the pointer *ptr refers to the memory location reserved by x, and the content stored in the memory location is updated to 16 after the statement is executed. So, you see, making use of pointers to access the memory locations of variables is a way of indirection.
Q Can a null pointer point to valid data?
A No. A null pointer cannot point to valid data. This is so because the value contained by a null pointer is 0. You'll see examples of using null pointers in arrays, strings, or memory allocation later in the book.
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