Lecture 28: CStrings & POinters

CSC 107 – Programming For Science Lecture 28:CStrings & POinters

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 • Pointers still hard & this is hardest topic of term

Arrays vs. Pointers Arrays Pointers Yams Sweet Potatoes

Arrays vs. Pointers Arrays Pointers Yams Sweet Potatoes Sweet Potatoes Yams

Arrays vs. Pointers Arrays = YamsSweet Potatoes Pointers = Sweet Potatoes Yams • 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 usepointers & 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 • Lots of built-in cString functions return pointers • Not a new array, but points within array passed in • Provides a way to use & modify cString as needed • Requires program add line to include prototypes #include <cstring> • Two main functions that are often used char* strstr(const char*, const char*);char* strchr(const char*, intchr);

Using the strstrFunction • Finds where 2ndcString is found in 1stcString • Pointer to 1st entry returned by this function • To be found, needs perfect match of entire cString • Looks from left to right until a match is foundchar 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 <cstring>& 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 <cstring>& 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 the strchrFunction • Finds where charfound in 1stcString • Pointer to 1st entry returned by this function • To be found, needs exact match of the char • Looks from left to right until a match is foundchar 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 & 12.17 • 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 #10 due Wed.

Lecture 28: CStrings & POinters