Thursday, February 28, 2013

1. Thursday, February 28, 2013 Pointers, Strings

2. NOTES • Project 2 will go out next Monday • Questions?

3. Pointers • Declaration: • <pointer type> *<pointer name> • Type: char, float, int, etc • Specifies type of variable whose addresses this pointer can store • Examples • int *intptr = &4; • Puts address of ‘4’ into intptr • char c = ‘c’; char *cptr = &c

4. Pointers • Can change variable by accessing it through address • “dereferencing” pointer • Usually, only one return value for function • By passing in pointers (addresses), variables can be modified by dereferencing • constint *q vsint * const q

5. Dereferencing Pointers int a = 4, b = 5; int *p1 = &a; int *p2 = &b; modify(p1, p2); void modify(int *ptr1, int *ptr2) { *ptr1 = 6; *ptr2 = 3; } Run modifyptr.c

6. Pointers in Memory • Pointers are variables which store the address of another variable: • Ex: int a = 3; int *p; p = &a; a p

7. Pointers and Arrays • Name of an array is a constant pointer whose value is the address of the array’s first element • a = &a[0] • Use either pointer arithmetic or indexing to refer to or modify the elements in an array • Arrays are always passed by reference to/from functions

8. Arrays • Name of array is pointer to first element in array • Ex: int a[] = {1, 2, 3, 4, 5} • Here, a = &a[0]; • What would these return? • *(a+2) => 3 • *(a+3) => 4 • Go over pointers.c

9. Strings • Strings represented by arrays of characters • Pointer to null-terminated array of characters • char string1[3] = “hi”; • Same as: char s1[3] = {‘h’, ‘i’, ‘\0’}; • Null-terminated • Note size is one more than number of letters in string

10. Input / Output • %s for strings • No ‘&’ in scanf: • When dealing with int’s: int a; scanf(“%d”, &a); • With strings: char s1[3]; scanf(“%s”, s1); • This is because s1 is already a pointer • Scanning in string longer than the length of array will override adjacent memory space!

11. #include <ctype.h> The following two functions return non-zero if c follows the condition described; return zero otherwise: • intisupper(int c); • if c is an uppercase letter • intislower(int c); • if c is a lowercase letter ************************************************************* • inttoupper(int c); • converts c to uppercase • inttolower(int c); • converts c to lowercase

12. #include <string.h> • char *strcpy(const char *dest, const char *src) • Copies the contents of the second into the array referenced by the first, and puts a null character at the end • intstrcmp(const char *s1, const char *s2) • Returns negative number if first string comes before the second alphabetically, 0 if the strings are identical through null, positive if second comes first. • char *strcat(char *s1, const char *s2) • Concatenates s2 onto the end of s1, and adds a null character at the end • intstrlen(const char *str) • Returns the length of strexcluding ‘\0’

13. Mistakes with Strings • Forgetting about the null character, or forgetting to include array space for it • Setting strings equal to each other instead of using strcpy • Single quotes vs. double quotes • Using the ampersand with scanf

14. Note the difference: char s1[] = “winter is coming soon”; char *s2 = “winter is coming soon”; • s1 is a constant pointer which will always point to a specific memory location; the first element of the character array. • s2 is a pointer which points to a string constant. You can’t modify individual elements of the array of memory that it points to! • Go over string.c

15. Coding Assignment • Create a file called data.txt with the following numbers: • 99 32 46 53 42 33 47 58 21 101 • Read in this file using file I/O and store the numbers in an array • Loop through the array (USING POINTERS ONLY) • Add 5 to those elements with an even index • Subtract 5 to those elements with an odd index • Print out the final array (again if using loops, only pointers) • So no a[0], etc!!