CS 223 - Spring 2017. 2/6

Welcome to CS 223!

Logical problem of the day

int main(){
  int n=1;
  printf("%d %d\n", n++, n++);
}

What happens above?

Lecture 7: Introduction to C: hw3 + structs, linked lists, hash tables

Administrivia

Office hours:

The office hours are Sun/Tue/Thu 8-11 PM at Hillhouse 17 Rm 111.

My office hours this week are Wednesday 4-5pm.

Reminder to speaker: use the microphone in Davies.

Michael Ljung's presentation demonstrated Brook's Law: The Mythical Man Month.

Accenture Case Study for Healthcare.gov

Per piazza: hw1 grade reports have been emailed

I have sent out the grade reports for hw1.
 
Here are the statistics:
 
Grade		Hours
Max	36		31
Min	0		0
Avg	30.9		9.6
Median	33		9
Stdev	7.8		5.7
 
The min and average data are not very meaningful.  Some hw's did not
compile and some students neglected to include their total time in
their log files -- or did not submit a log file.
 
If your code did not compile, you should fix it and resubmit, without
penalty.
 
If you lost points otherwise, you too may resubmit and get 50%
incremental credit.  For example, if you got 30/36 and you resubmit
for 36/36, you will get an additional 3 points (36-30 * 50%).
 
I will review this in class today as well.
 
--SS

p.s. I will post hw grades to classes v2 once the resubmit process concludes.

The trajectory of the course.

By the time you finish hw3, you will have learned almost all the C you need to know.
Midterm exam Wednesday February 22nd in class. topics
After the exam, we will focus on the following data structures and programming techniques: stacks, queues, trees, binary search, graphs, heaps, and dynamic programming.
You will continue to use pointers, malloc, valgrind, recursion, strings, and the other bag of tricks. It is like you have learned how to use various tools (hammers, saws, drills, measuring tapes) and materials (lumber, nails, screws, glue) and now we will build stuff: walls, bookshelves, dining room tables, and houses.

Assignment 3

Problem set 3

Piazza note about pointers

Private question from piazza:

  Why are struct elt and struct hash commented out in hash.h? If I want
  them to be available to both hash.c and Callme.c, shouldn't they be
  uncommented?  

Callme.c should not reference struct elt or struct hash directly.
Those are encapsulated in hash.c.
 
I defined them as comments in hash.h so you can define them in hash.c
 
--SS

Lecture Starts Here

Pointers

Aspnes: Pointers

Computer memory addresses are positive integers. Actually, 0 is also a memory address, but generally something went wrong if you are trying to access memory location 0. Negative addresses are also trouble.

Use a * to declare a variable a pointer.

int *pointerToInt;
double *pointerToDouble;
char *pointerToChar;
char **pointerToPointerToChar;

Use a & to access the address of a variable.

int n;              /* an int variable */
int *p;             /* a pointer to an int */
p = &n;             /* p now points to n */

Let's revisit the getLine program from above. mygetLine.c We have added wrappers for malloc, realloc, and free. The new functions, mymalloc, myrealloc, and myfree, report the addresses that they hand out or recycle in memory, as long as the debugflag is set.
We can also use valgrind to detect memory leaks.
There are different sections of memory. The Basic Service Set (BSS) hold statically allocated variables and code. The heap is used for dynamically allocated variables, e.g., from malloc. The stack is used for local variables and function calls. See lookingAtPointers.c
Pointers allow you to get around the C convention of copying function arguments. See badDoubler.c and goodDoubler.c

structs

Aspnes: structs

struct.c Basic date struct, and recursive person struct, with and without pointers and malloc.

You can use structs to make new data types. Here is a string data type that includes the length of the string.

myString.h

myString.c

Linked Lists

Aspnes: Linked Lists

Structs are just fine for creating single data type instances, like strings. It is usually more interesting to create data structures that actually produce structures. The simplest structure is a linked list. We will later see more complex structures, like trees and graphs, which are variations on linked lists.

lists.c

Hash Tables

Aspnes: Hash Tables

Hash Functions:

Determinism - consider a random number generator as a hash function random.c
Uniformity - Maintain constant load factor

exhash.c integer keys, linear probing

exhash2.c string keys, linear probing

examplehash.c integer keys, linear probing, dummy item for deletes

examplehash2.c string keys, linear probing, dummy item for deletes

hw3/hash.h

hw3/hashtest.c uses hash.h, string keys, linked lists

hw3/bighashtest.c uses hash.h,

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