COMS 1003: Introduction to Computer Programming in CPointers 1 & 2November 1st and 3rd 2005Announcements●Good constructive feedback on the midterm comments●No class November 8th. Go vote.●Read Chapter 5 in K&R–go back and re-read the chapter on structs and unions, now paying heed to the information on pointersOutline●Recall the properties of variables–Recall the outline of main memory●Learn how to:–declare pointers–dereference pointers–obtain addresses, –do pointer arithmetic–use the sizeof operatorVariables●Represent an object that you can store data into–a cell of memory (or group of cells)●Have a name (address)●Have a type●Hold (contain) some valueMental Model: Main Memory●Array of cells with address and value●32-bit machine, 1 word = 4 bytes●Primitive types occupy some amount of memory●Use 'sizeof' operator in C to determine how many bytes a type takes up–sizeof(int) == 4, sizeof(char) == 1Intro to Pointers●A variable that holds the address of another variable–The value of a pointer variable is the address of another memory cell (or group of cells)●this is the proper, appropriate, and legal semantics of pointers–but you can store arbitrary integer values into pointers. Dangerous!Pointer Operators●Declaration operator: *–int *y_ptr = NULL;●'Addressof ' operator: &–int x = 456;y_ptr = &x;●'Dereference' operator: *–printf(“val of addr in y_ptr = %d\n”,*y_ptr);Reasons to Use Pointers●Can refer to a large data structure in a compact way●Facilitate data sharing between functions●Can dynamically allocate memory●Can be used to indicate logical relationships between data structuressizeof Operator●Can be used to determine amount of storage necessary for both types and named variablesint num_horses = 100;printf(“%d”,sizeof(int));printf(“%d”,sizeof num_horses);Pointer Arithmetic●Pointer values are treated slightly differently from regular variables●Assignment works as expected, assigning and comparing to zero●Addition, Subtraction (pointer and integer)●Subtracting or Comparing two pointers in the same arrayPointer Math Operations●All semantically legal operations listed on previous slide●No other operations are legal–cannot add two pointers–no mul, div, shift, mask, add/sub float, doubleGeneric Pointer Type●Many times, you may not know what specific pointer type you are going to point to, need flexibility●The generic pointer type is void*–void *foo = NULL; //foo can point to any type●The NULL pointer points to nothing–consistent default initialization valuePointers to Functions●Functions are just code blocks stored in memory, so it is entirely legal to have a variable that holds the location of a functionint say_hello(){ printf(“hello\n”); return 0;}Passing Arguments to Functions●C has call by value semantics for function parameters●What if you need to modify an argument's value as well as return another value?int set_zero(int var){ var = 0; return SUCCESS;}int set_zero(int *var){ *var = 0; return SUCCESS;}Passing Arguments to Functions●If a function takes a pointer argument, then you need to pass in a pointer or an addressint set_zero(int *var){ *var = 0; return SUCCESS;}int main(){ int x = 500; int result = set_zero(&x);}String Processing With