CS 24 – Problem Solving II (Data Structures)

1. CS 24 – Problem Solving II (Data Structures) Diana Franklin

2. Searching for a number in a phone book that has n numbers in it • Start at the beginning • while (we haven’t found it) • advance to the next one • Best case - 1 • Average case – n/2 • Worst case - n

3. Searching for a number in a sorted phone book • Look at the middle element • if it’s smaller than that one, throw out top half • if it’s larger than that one, throw out bottom half • repeat until we have only one item left • best-case - 1 • worst-case log2(n) • average-case –summation of: time*probability of that time • 1*1/n + 2*2/n + 3*4/n + 4*8/n+…+log2(n)*n/2

4. n*1/2*1/2*1/2*… = 1 • n*(1/2)^j = 1 • 1/(2^j) = 1/n • n = 2^j • log2(n) = j

5. http://www.cs.ucsb.edu/~franklin/24/24.html • Syllabus • Assignments (with dates) • Lecture resources • Definition of plagiarism • Detailed grading breakdown • Late policy • Information guides for gdb, compiling, unix • etc, etc, etc.

6. Logistics • Labs – Worth .5% each. Must attend 4 to not lose credit. Others boost assignment grade. • Mini-Exams – 2 – in between to make sure I don’t lose you • Exams – 2 midterms, 1 final • Programs – First individual, the rest pair programming

7. Late Policy • Programs – up to 24 hrs late, 10% off. Nothing accepted after 24 hrs. • If you submit late, the entire assignment is late.

8. Arrays • How is an array different from a pointer? • int *x • int x[10]; • What does this declare? • int *x[10]; • int **x;

9. Strings • How are strings stored in C? • Array of characters • The end of the string has a ‘\0’ • How do you declare an array of strings? • char **arrayOfStrings = NULL • How do you declare an array of 7 strings? • char *arrayOfStrings[7]

10. Command-line arguments void main(intargc, char *argv[]) • Explain what type char *argv[] is • How do argc and argv get set? • Draw the memory of argv for the input: • ./a.out cs24 rules

11. Pointer Review • General knowledge • Drawing the memory • Reading and Writing

12. What do you know about pointersthat was covered in CS 16? • Pointers store a memory address • You can use pointer arithmetic to access different parts of an array • & is the syntax for “address of” • * is the syntax to dereference (read the contents of the item the pointer points to) • If you have a struct, you use variablename->field rather than variable.field • Pointers can point to anything (including pointers) • You can have a large degree of indirection

13. When MUST you use pointers? • Calling a function and you want to pass a huge structure • You want a function to modify data that is being used outside the function (not a global variable) • You don’t know how big of an array to create until the program is running

14. Pointers and Types • If a pointer is not pointing anywhere useful, where should it point? • NULL • If a function is receiving a pointer, but it does not know what type the pointer is, how do you declare it? • void * • To access the data in a void *, what do you need to do? • cast it to the right type • What does void mean? • A function does not return a value • Explicitly states that there are no input arguments

15. What do you know about pointersthat was covered in CS 16? • They store an address in a variable • You can have a pointer pointing to another pointer • You can change values in arrays more easily • There are void pointers, and you need to cast them if you want to dereference them • Pointers are used for dynamic memory allocation

16. Draw the memory of each int *x, *y; x = &a; y = &b; x = y; int a, b; a = 5; b = 3; a = b; What is the difference between (a = b) and (x = y) for what the computer does? What is the difference between (a = b) and (x = y) for how we view them? The way to make working with pointers easy is to operate on them mechanically like the computer. They are no different than integers.

17. Pointer Review – what prints out? int x = 3, y = 4; int *pX = &x, *pY = &y; (pX == pY) ? Print(“Same\n”): print(“Not Same\n”); (*pX == *pY) ? Print(“Equal\n”) : print(“NEqual\n”); x = y (pX == pY) ? Print(“Same\n”): print(“Not Same\n”); (*pX == *pY) ? Print(“Equal\n”) : print(“NEqual\n”); x = 10; pX = pY; (pX == pY) ? Print(“Same\n”): print(“Not Same\n”); (*pX == *pY) ? Print(“Equal\n”) : print(“NEqual\n”);

18. Draw the memory char *c[]={ "ENTER", "NEW", "POINT", "FIRST" }; printf(“%s”,c[2]); printf(“%s “, &(c[0]3]));

19. Structs • What do you know about structs? • You can create your own data type • Make many copies of that data type • dynamically allocate structs • Variables with multiple fields – programmer defines what the fields hold

20. Structs typedefstruct _product { int weight; floatprice; } product; How do I declare three variables, named apple, banana, and melon, of type product? struct _product apple, banana, melon; product apple, banana, melon; apple.weight = 5;

21. Structs typedefstruct _product { int weight; floatprice; } product; product apple; product banana, melon;

22. Structs typedefstruct _product { int weight; floatprice; } product; product apple, banana, melon; How do I set the weight field of apple? apple.weight = 5;

23. header files (.h) • What goes into a header file? • type definitions • function definitions • global variable declarations • A guard to only be read once • What does not go into a header file? • Local variable declarations • Function implementations

24. dynamicArray.h #ifndefDYNAMICARRAY_H #define DYNAMICARRAY_H typedefstructdynamicarray_h{ int size; char **array; } dynamicArray; dynamicArray*da_make_new_array(); char *da_get(dynamicArray *da,int index); void da_set(dynamicArray *da, int index, char *val); #endif

25. File I/O void printFile(char *filename) { char buf[256]; // step 1: open the file FILE *file = fopen(filename,”r”); // step 2: make sure it opened properly if (file == NULL) { printf(“Could not open %s\n”,filename);exit(1);} // step 3: read in each line of the file and print it out to the screen fgets(buf,256,file); while(!feof(buf)) { printf(“%s”,buf); fgets(buf,256,file); // step 4: stop when we reach the end of the file } // step 5: close the file fclose(file); }

26. Linking pointers Typdefstruct _link{ int value; struct _link *next; } link; link l1, l2, l3, l4; link *p1, *p2, *p3, *p4; p1 = &l1; p2 = &l2; p3 = &l3; p4 = &l4; l1.value = 1; l2.value = 2; l3.value = 4; l4.value = 8; p1->next = p2; p2->next = p3; p3->next = p4; p4->next = p1; p1-value = 5; p1->next->value = 3; p1->next->next->value = 7; for(i=0;i<4;i++) { printf(“%d, “,p2->value); p2 = p2->next; } p2 = null; p3 = null; p4 = null;

27. Let’s remove l3 from the list w/out using l1, l2, l3, l4

28. Function calls / pointersPass int by value void swap(intx, int y) int temp = x; x = y; y = temp; } int main () { int x=100, y = 50; swap(x, y); cout << ”x=" <<x << ", y=" << y; return 0; }

29. Function calls / pointersPass pointer (reference) by value void swap(int*x, int *y) int temp = *x; *x = *y; *y = temp; } int main () { int x=100, y = 50; swap(&x, &y); cout << ”x=" <<x << ", y=" << y; return 0; }

30. Function calls / pointersPass int by reference – C++ Syntax of passing an int, Actions of passing a pointer to int void swap(int& x, int& y) int temp = x; x = y; y = temp; } int main () { int x=100, y = 50; swap(x, y); cout << ”x=" <<x << ", y=" << y; return 0; }

31. Crazy Pointers char *c[]={ "ENTER", "NEW", "POINT", "FIRST" }; char **cp[]={&(c[3]),&(c[2]),&(c[1]),c}; char ***cpp=&(cp[1]); printf(“%s”,**cpp); printf(“%s “, &((*(cpp[2]))[3])); printf(“%c\n”,cpp[0][1][3]);