1. Computer Science 210 Computer Organization Pointers

2. The Pointer C allows the programmer to access the address of a memory location and use it as a data value Pointers are also used to refer to dynamically allocated storage (from the system heap)

3. int number = 10; printf("The value is %d\n", number); // The contents of number printf("The address is %p\n", &number); // The address of number The Address of Operator & The & operator returns the actual memory address (a large integer, probably) of a variable The printf format flag p is used to print an address

4. int number = 10; int *alias; Declaring a Pointer Variable The * operator says that the variable alias can contain a pointer to a memory location that can hold an int 10 numberalias The memory for alias contains garbage until it is set to a value

5. int number = 10; int *alias; int aCopy = number; // Copies number’s value to aCopy alias = &number; // Copies number’s address to alias Assigning an Address The variables number and aCopy name separate storage locations The variables number and alias can access the same storage location 10 numberalias The memory for alias now contains the address of number 10 aCopy

6. int number = 10; int *alias = NULL; // Set the pointer variable to empty int aCopy = number; // Copies number’s value to aCopy alias = &number; // Copies number’s address to alias printf("The value is %d\n", number); printf("The value is %d\n", *alias); // Dereference to get value Access by Indirection (Dereferencing) The * operator is also the dereference operator Its operand must be a pointer variable, and it must contain the address of another cell in memory The value NULL from stdio is used to indicate the empty pointer

7. int number = 10; int *alias = NULL; // Set the pointer variable to empty int aCopy = number; // Copies number’s value to aCopy alias = &number; // Copies number’s address to alias (*alias) = 69; // Reset the storage by indirection printf("The value is %d\n", number); // Prints 69 Aliasing and Side Effects * Has a lower precedence than = *alias and number access the same storage 69 numberalias Ouch!!! 10 aCopy

8. Input and Output Parameters Parameters are passed by value, so they’re good as input-only parameters Pointers can be used to implement pass by reference, so they’re good for output parameters (in functions that return multiple values) Example: define a function that returns two quadratic roots, given the inputs a, b, and c

9. #include int main(){ float a, b, c, root1, root2; printf("Enter a, b, and c: "); scanf("%f%f%f", &a, &b, &c); quadraticRoots(a, b, c, &root1, &root2); printf("Root1: %f\nRoot2: %f\n", root1, root2); } Use & with the Actual Parameter The function quadraticRoots has three input parameters and two output parameters The address of a variable is passed for each output parameter The function scanf also uses this mechanism

10. #include void quadraticRoots(float a, float b, float c, float *root1, float *root2); int main(){ float a, b, c, root1, root2; printf("Enter a, b, and c: "); scanf("%f%f%f", &a, &b, &c); quadraticRoots(a, b, c, &root1, &root2); printf("Root1: %f\nRoot2: %f\n", root1, root2); } Use * with the Formal Parameter Input parameters Output parameters In the function header, the root1 and root2 parameters are of type pointer ( * ) to float This allows them to receive the addresses of float variables when the function is called

11. #include void quadraticRoots(float a, float b, float c, float *root1, float *root2); int main(){ float a, b, c, root1, root2; printf("Enter a, b, and c: "); scanf("%f%f%f", &a, &b, &c); quadraticRoots(a, b, c, &root1, &root2); printf("Root1: %f\nRoot2: %f\n", root1, root2); } void quadraticRoots(float a, float b, float c, float *root1, float *root2){ (*root1) = - b + sqrt(b * b - 4 * a * c) / (2 * a); (*root2) = - b - sqrt(b * b - 4 * a * c) / (2 * a); } Use * to Access or Modify Data Input parameters Output parameters Use dereference (*) to store an integer in the implementation

12. Pointers and Arrays An array variable is just a name for the base address of the array storage in memory Normally, an array variable is not an l-value, meaning that it cannot be the target of an assignment statement But, this address can be copied to another variable, which can then be modified

13. #include int main(){ int i, max = 5, array[max]; for (i = 0; i < max; i++) array[i] = i + 1; // Print ‘em using the array and the subscript operator for (i = 0; i < max; i++) printf("%d\n", array[i]); } Process an Array Using []

14. #include int main(){ int i, max = 5, array[max]; for (i = 0; i < max; i++) array[i] = i + 1; // Print ‘em using a pointer and dereference int *arrayPtr, *lastAddress = array + max - 1; for (arrayPtr = array; arrayPtr <= lastAddress; arrayPtr++) printf("%d\n", *arrayPtr); Process the Array Using * arrayPtr is an alias for array (the address of the first cell) lastAddress is the address of the last cell in the array structure arrayPtr is incremented to move through the array structure, and * is used to access each cell