A Pointer Experiment. Take out a sheet of scrap paper. Tear it in half. Tear it in half again.

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1 Take out a sheet of scrap paper. Tear it in half. Tear it in half again. A Pointer Experiment On one of the quarter sheets, write legibly either: your name, or an integer from 1 to 100. Fold it in half. Fold it in half again. When the box comes around the first time, put the quarter sheet of paper into it. When the box comes around the second time, take a random quarter sheet of paper out of it. If you draw your own name, take out another one and put your name back in. 1

2 More on Pass By Reference in C % cat henryshouse2bad.c int henryshouse; void who(int *house); who(&henryshouse); printf("i m sure that %d people", henryshouse); printf(" live in Henry s house.\n"); void who (int *drneemanshouse) { /* who */ printf("how many people live in Dr Neeman s house? "); scanf("%d", drneemanshouse); printf("i hope you re sure that %d people\n", drneemanshouse); printf(" live in Dr. Neeman s house.\n"); } /* who */ % cc -o henryshouse2bad henryshouse2bad.c % henryshouse2bad How many people live in Dr Neeman s house? 2 I hope you re sure that people live in Dr. Neeman s house. I m sure that 2 people live in Henry s house. 2

3 Correct Pass By Reference in C % cat henryshouse2.c int henryshouse; void who(int *house); who(&henryshouse); printf("i m sure that %d people", henryshouse); printf(" live in Henry s house.\n"); void who (int *drneemanshouse) { /* who */ printf("how many people live in Dr Neeman s house? "); scanf("%d", drneemanshouse); printf("i hope you re sure that %d people\n", *drneemanshouse); printf(" live in Dr. Neeman s house.\n"); } /* who */ % cc -o henryshouse2 henryshouse2.c % henryshouse2 How many people live in Dr Neeman s house? 2 I hope you re sure that 2 people live in Dr. Neeman s house. I m sure that 2 people live in Henry s house. 3

4 What Does All This Mean? As we saw earlier, in C we have a special operator, the ampersand, named the address operator. What does the address operator do? It returns the address of the variable that it s applied to. % cat addr.c double dub = 5.0; float flo = 4.0; int in = 3; printf("dub = %f, &dub = %d\n", dub, &dub); printf("flo = %f, &flo = %d\n", flo, &flo); printf("in = %f, &in = %d\n", in, &in); % cc -o addr addr.c % addr dub = , &dub = flo = , &flo = in = , &in = How does this help us in converting from pass by value to pass by reference? Well, the value of the expression &dub is the address of dub. If we pass the value of &dub, then we re passing the address of dub, so we re passing dub by reference. Eh? 4

5 How Pass by Reference Works in C Okay, so we ve decided that, if we pass the value of &dub, then we re passing dub by reference, because we re passing the address of dub. What s that all about? Well, pass by reference (as in Fortran 90) means that the formal argument refers to the actual argument, in the sense that the formal argument has the same memory address as the actual argument. But pass by value (as in C) means that the value of the actual argument is copied into a new memory location, which is the memory location of the formal argument. So let s say we re doing pass by value. If the value that we pass is the address of the actual argument, then the formal argument knows the memory location of the actual argument. In which case, if we can figure out how to dereference the address contained in the formal argument to use it to get to the contents of that address then we d have the address of the actual argument. Which would be pass by reference. So, what we need is a way to dereference an address. Happily, C provides a dereference operator: * We use the dereference operator with pretty much the same syntax that we use for the address operator: *dub 5

6 Pass By Reference in C Redux % cat henryshouse2.c int henryshouse; void who(int *house); who(&henryshouse); printf("i m sure that %d people", henryshouse); printf(" live in Henry s house.\n"); void who (int *drneemanshouse) { /* who */ printf("how many people live in Dr Neeman s house? "); scanf("%d", drneemanshouse); printf("i hope you re sure that %d people\n", *drneemanshouse); printf(" live in Dr. Neeman s house.\n"); } /* who */ % cc -o henryshouse2 henryshouse2.c % henryshouse2 How many people live in Dr Neeman s house? 2 I hope you re sure that 2 people live in Dr. Neeman s house. I m sure that 2 people live in Henry s house. 6

7 % cat henryshouse2assn.c Pointers int henryshouse; void how_many(int *house); how_many(&henryshouse); printf("i m sure that %d people", henryshouse); printf(" live in Henry s house.\n"); void how_many (int *drneemanshouse) { /* how_many */ *drneemanshouse = 2; printf("i ve decided that %d people\n", *drneemanshouse); printf(" live in Dr. Neeman s house.\n"); } /* how_many */ % cc -o henryshouse2assn henryshouse2assn.c % henryshouse2assn I ve decided that 2 people live in Dr. Neeman s house. I m sure that 2 people live in Henry s house. The name for a variable whose value is a reference (i.e., an address of another variable) is a pointer. It points to the other variable. Notice that, to assign a value to a pointer, we apply the dereference operator * to the pointer: *drneemanshouse = 2; Likewise, to use the value of the variable pointed to by a pointer, we also apply the dereference operator to the pointer: printf("%d people\n", *drneemanshouse); 7

8 Pointer Variables We can actually declare a variable to be a pointer: % cat pointer.c int q; int *p; q = 5; p = &q; printf("q = %d, &q = %d\n", q, &q); printf(" p = %d, *p = %d\n", p, *p); % cc -o pointer pointer.c % pointer q = 5, &q = p = , *p = 5 Okay, that s kind of interesting, but not very useful. But look: % cat alloctest.c float *element; int numelts, i; printf("how many elements do you want? "); scanf("%d", &numelts); element = (float *)malloc(sizeof(float) * numelts); printf("what are the %d elements?\n", numelts); for (i = 0; i < numelts; i++) scanf("%f", &element[i]); printf("the elements are:\n"); for (i = 0; i < numelts; i++) printf("%f\t", element[i]); printf("\n"); % cc -o alloctest alloctest.c % alloctest How many elements do you want? 4 What are the 4 elements? e+7 The elements are:

9 Dynamic Memory Allocation The C standard library function malloc returns a pointer to a place in memory that is as many bytes as you asked for: malloc(sizeof(float) * numelts) returns a pointer to a place in memory big enough to hold numelts variables of type float. In other words, the return value of malloc is a pointer to an array of type float. Interestingly, if you declare an array float static element[100]; you get a pointer, but it points to a static (fixed) location in memory. Whereas, if you declare a pointer and then allocate it float *dynamic element; dynamic element = (float *)malloc(sizeof(float) * numelts); then the pointer points to a dynamic location in memory. 9

10 Dynamic Allocation Example % cat stat_dyn.c #include <math.h> #define MAX_STATIC 100 float static_element[max_static]; float *dynamic_element; int num_static_elts; int num_dynamic_elts; void get_elts(float *element, int numelts, char *name); float mean(float *element, int numelts); /* Static array. */ printf("how many elements in the 1st array"); printf(" (1 to %d)? ", MAX_STATIC); scanf("%d", &num_static_elts); if ((num_static_elts < 1) (num_static_elts > MAX_STATIC)) { printf("error: can t have %d elements\n", num_static_elts); printf(" in the 1st array.\n"); exit(-1); } /* if num_static_elts < 1... */ get_elts(static_element, num_static_elts, "1st"); printf("the mean of the 1st array is %f.\n", mean(static_element, num_static_elts)); /* Dynamic array. */ printf("how many elements in the 2nd array? "); scanf("%d", &num_dynamic_elts); if (num_dynamic_elts < 1) { printf("error: can t have %d elements\n", num_dynamic_elts); printf(" in the 2nd array.\n"); exit(-1); } /* if num_dynamic_elts < 1 */ dynamic_element = (float *)malloc(sizeof(float) * num_dynamic_elts); get_elts(dynamic_element, num_dynamic_elts, "2nd"); printf("the mean of the 2nd array is %f.\n", mean(dynamic_element, num_dynamic_elts)); 10

11 void get_elts (float *element, int numelts, char *name) { /* get_elts */ int e; printf("what are the %d elements", numelts); printf(" in the %s array?\n", name); for (e = 0; e < numelts; e++) scanf("%f", &element[e]); printf("elements of %s array:\n", name); for (e = 0; e < numelts; e++) printf("%f\t", element[e]); printf("\n"); } /* get_elts */ float mean (float *element, int numelts) { /* mean */ float sum; int e; for (e = 0, sum = 0.0; e < numelts; e++) sum += element[e]; return sum / numelts; } /* mean */ % cc -o stat_dyn stat_dyn.c % stat_dyn How many elements in the 1st array (1 to 100)? 4 What are the 4 elements in the 1st array? Elements of 1st array: The mean of the 1st array is How many elements in the 2nd array? 6 What are the 6 elements in the 2nd array? Elements of 2nd array: The mean of the 2nd array is