CSC 107 – Programming For Science

Today’s Goal  Learn how pointers really used with arrays  Exploit the similarity between pointers & arrays  Take advantage of relationship with built-in functions  Understand what NULL is & why we use NULL hardest topic of term  Pointers still hard & this is hardest topic of term

Today’s Goal  Learn how pointers really used with arrays  Exploit the similarity between pointers & arrays  Take advantage of relationship with built-in functions  Understand what NULL is & why we use NULL hardest topic of term  Pointers still hard & this is hardest topic of term  (Unless you have not learned how to write loops, yet)

Pointers & Variables  Variable gives name for address in memory  Initial values unknown if it is not set  Value at the location used when variable used  Pointer is variable like any other…  … but the pointer’s value is address of a variable  Aliases other variables so both can update location  Pointers can be used like other variables…  …but value is address and not a useful number

& and * Operators variable  & operator gets address of scalar variable  Use only with variables  Use only with variables, including array elements  Lets variable & pointer alias same memory location  Pointer used two ways: arrow & target of arrow  When code has the name alone, using the arrow  With *pointer, value at target can be used or set double x = 4, *y, *z; y = &x; z = y; cout << x << " " << y << " " << *y << endl; cout << x << " " << y << " " << *y << endl;

Arrays vs. Pointers ArraysPointers YamsSweet Potatoes

Arrays vs. Pointers Arrays = YamsPointers = Sweet Potatoes  Makes space at declaration  Variable value is address  Use [] to access entries  Can also access using *  Can be assigned a pointer  Needs target to be useful  Variable value is address  Use * to access target  Can also access using []  Can be assigned array

Arrays vs. Pointers Arrays = YamsPointers = Sweet Potatoes  Makes space at declaration  Variable value is address  Use [] to access entries  Can also access using *  Can be assigned a pointer  Needs target to be useful  Variable value is address  Use * to access target  Can also access using []  Can be assigned array Often use pointers & arrays interchangeably

Arrays + Pointers =  Pointer arithmetic enables mixing & matching  Relies on fact that * and [] are synonyms  Used processing cStrings (& with other arrays, too)  Provides another way to access array elements  Use foo[ nnn ] to access entry in foo at index nnn  But could also access entry using *(foo+ nnn )  Pointers follow same rules and work similarly  But, pointers access offset from where they point

How Pointers Get Used

Using the strstr Function  Finds where 2 nd cString is found in 1 st cString  Pointer to 1 st entry returned by this function  To be found, needs perfect match of entire cString  Looks from left to right until a match is found char myName[]="John J. Jingleheimerschmidt"; char *start = strstr(myName, "J"); char *ptr = strstr(myName, "John"); char *middle = strstr(start + 1, "J"); char *last = strstr(middle + 1, "J"); char *oops = strstr(last + 1, "J");

Slide Is NULL (But Not void)  NULL is special value used by all pointers  Not an actual address – NULL used if when no target  Not a real address, but says DO NOT USE ME  If pointer is NULL, accessing value will crash  May get vague error like "Segmentation Fault"  Most likely, however, the program simply crashes  Use NULL anywhere in code pointer is usable  Needs #include & must be in CAPS

Slide Is NULL (But Not void)  NULL is special value used by all pointers  Not an actual address – NULL used if when no target  Not a real address, but says DO NOT USE ME  If pointer is NULL, accessing value will crash  May get vague error like "Segmentation Fault"  Most likely, however, the program simply crashes  Use NULL anywhere in code pointer is usable  Needs #include & must be in CAPS (Tony apologizes,btw)

Using NULL With Pointers char * ptr = NULL; char *prof = "Matthew Hertz"; char letter = *ptr; float * toiletBulb = ptr; toiletBulb = NULL; int * pIX = null; ptr = strstr("Hi Mom!", "mom"); strstr(NULL, "Monkeys!"); strstr(prof, ptr); *ptr = NULL;

Using NULL With Pointers char * ptr = NULL; char *prof = "Matthew Hertz"; char letter = *ptr; float * toiletBulb = ptr; toiletBulb = NULL; int * pIX = null; ptr = strstr("Hi Mom!", "mom"); strstr(NULL, "Monkeys!"); strstr(prof, ptr); *ptr = NULL;

Using NULL With Pointers char * ptr = NULL; char *prof = "Matthew Hertz"; char letter = *ptr; float * toiletBulb = ptr; toiletBulb = NULL; int * pIX = null; ptr = strstr("Hi Mom!", "mom"); strstr(NULL, "Monkeys!"); strstr(prof, ptr); *ptr = NULL;

Using NULL With Pointers char * ptr = NULL; char *prof = "Matthew Hertz"; char letter = *ptr; float * toiletBulb = ptr; toiletBulb = NULL; int * pIX = null; ptr = strstr("Hi Mom!", "mom"); strstr(NULL, "Monkeys!"); strstr(prof, ptr); *ptr = NULL;

Using NULL With Pointers char * ptr = NULL; char *prof = "Matthew Hertz"; char letter = *ptr; float * toiletBulb = ptr; toiletBulb = NULL; int * pIX = null; ptr = strstr("Hi Mom!", "mom"); strstr(NULL, "Monkeys!"); strstr(prof, ptr); *ptr = NULL;

Using NULL With Pointers char * ptr = NULL; char *prof = "Matthew Hertz"; char letter = *ptr; float * toiletBulb = ptr; toiletBulb = NULL; int * pIX = null; ptr = strstr("Hi Mom!", "mom"); strstr(NULL, "Monkeys!"); strstr(prof, ptr); *ptr = NULL;

Using NULL With Pointers char * ptr = NULL; char *prof = "Matthew Hertz"; char letter = *ptr; float * toiletBulb = ptr; toiletBulb = NULL; int * pIX = null; ptr = strstr("Hi Mom!", "mom"); strstr(NULL, "Monkeys!"); strstr(prof, ptr); *ptr = NULL;

Using NULL With Pointers char * ptr = NULL; char *prof = "Matthew Hertz"; char letter = *ptr; float * toiletBulb = ptr; toiletBulb = NULL; int * pIX = null; ptr = strstr("Hi Mom!", "mom"); strstr(NULL, "Monkeys!"); strstr(prof, ptr); *ptr = NULL;

Using NULL With Pointers char * ptr = NULL; char *prof = "Matthew Hertz"; char letter = *ptr; float * toiletBulb = ptr; toiletBulb = NULL; int * pIX = null; ptr = strstr("Hi Mom!", "mom"); strstr(NULL, "Monkeys!"); strstr(prof, ptr); *ptr = NULL;

Using NULL With Pointers char * ptr = NULL; char *prof = "Matthew Hertz"; char letter = *ptr; float * toiletBulb = ptr; toiletBulb = NULL; int * pIX = null; ptr = strstr("Hi Mom!", "mom"); strstr(NULL, "Monkeys!"); strstr(prof, ptr); *ptr = NULL;

Using NULL With Pointers char * ptr = NULL; char *prof = "Matthew Hertz"; char letter = *ptr; float * toiletBulb = ptr; toiletBulb = NULL; int * pIX = null; ptr = strstr("Hi Mom!", "mom"); strstr(NULL, "Monkeys!"); strstr(prof, ptr); *ptr = NULL;

Using the strchr Function  Finds where char found in 1 st cString  Pointer to 1 st entry returned by this function  To be found, needs exact match of the char  Looks from left to right until a match is found char myName[]="John J. Jingleheimerschmidt"; char *start = strchr(myName, 'J'); char *ptr = strchr(myName, myName[4]); char *startOver = strchr(start, 'J'); char *middle = strchr(start + 1, 'J'); char *nope = strchr(myName, 'j');

Your Turn  Get into your groups and try this assignment

For Next Lecture  Dynamic memory allocation in 12.9 &  More interesting topic in all of computer science?  As program runs, can we grow & shrink arrays?  My thesis was related to which lecture this semester?  Angel also has Weekly Assignment #11 due Tues.  Last programming assignment  Last programming assignment available now