Data Structures, Midterm Answers
Comp-206 : Introduction to Software Systems Lecture 14 Alexandre Denault Computer Science McGill University Fall 2006
Data Structures in C
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To better understand the importance of pointers, let's take a look at two data structures.
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Linked List Binary Trees
The source code for these examples is on the web, in the Supplemental Notes section.
Linked List
Head
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6
10
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Link List Node
/* Node for the link list */ typedef struct ll_node { int value; struct ll_node* next; } llnode, linkedlist;
Creating a node /* Create a link list */ linkedlist* createLinkedList() { llnode* head; // Our implementation of linklist has a // dummy node at the head. head = (llnode *)malloc(sizeof(llnode)); head->value = 0; head->next = NULL; return head; }
Adding a Node /* Add a value to the linklist. */ void addToLinkedList(linkedlist* list, int value) { llnode* freeSpot; llnode* newNode; // Find a free spot at the end to add the value freeSpot = list; while(freeSpot->next != NULL) { freeSpot = freeSpot->next; } newNode = (llnode *)malloc(sizeof(llnode)); newNode->value = value; newNode->next = NULL; freeSpot->next = newNode; }
Pretty Print /* Pretty print the list. */ void printLinkedList(linkedlist* list) { if (list->next != NULL) { printf("Content of list is :"); printLLNode(list->next); } else { printf("List is empty."); } printf("\n"); }
Printing A Node
void printLLNode(llnode* my_node) { printf(" %i ", my_node->value); if (my_node->next != NULL) { printLLNode(my_node->next); } }
Binary Tree 8
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3
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Binary Tree Node
/* Node in our binary tree */ typedef struct bt_node { char* value; struct bt_node* left; struct bt_node* right; } btnode, binarytree;
Creating the Tree
binarytree* createBinaryTree(char* value) { return createBTNode(value); }
Creating a Node
binarytree* createBTNode(char* value) { btnode* new_node; new_node = (btnode*)malloc(sizeof(btnode)); new_node->value = (char *)malloc(strlen(value) + 1); new_node->left = NULL; new_node->right = NULL; strcpy(new_node->value, value); return new_node; }
Adding a Node void addToBinaryTree(binarytree* tree, char* value) { // In order travel of the tree, until we hit a left. if (strcmp(tree->value, value) > 0) { if (tree->left != NULL) { addToBinaryTree(tree->left, value); } else { tree->left = createBTNode(value); } } else { if (tree->right != NULL) { addToBinaryTree(tree->right, value); } else { tree->right = createBTNode(value); } } }
Freeing the Tree void unallocateBinaryTree(binarytree* tree) { if (tree == NULL) return; // First dellocate the child nodes. unallocateBinaryTree(tree->left); unallocateBinaryTree(tree->right); // Deallocate the node's content // and the node itself free(tree->value); free(tree); }
Midterm
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Average 72.3%
54 students took the midterm 4 students got 90% or above 20 students got 80% or above 34 students got 70% or above 44 students got 60% or above
Question 1
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Average: 7.57 / 8 : 95% GUI: Provides a visual interface with ready-made components (buttons, text boxes, etc / widgets) to interact with the computer.
Question 2
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Average : 3.83 / 4 : 96% Time Sharing / Time Slicing
All the processes share the CPU in turn. Each turn is called a time slice
Question 3
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Average : 3.54 / 4 : 88% Close to 20% of the class answered all 4 questions. a) An OS is closed (or proprietary) when it owned by a single company.
It is often designed to work on a single kind of hardware
Question 4
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Average : 3.24 / 4 : 81% a) /home/bob/homework/main.c b) ../../tmp/assignment.log
Question 5
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Average : 5.94 / 8 : 74% The “>” and “>>” symbols enable redirection of output on STDOUT to a file. If a file already exists, “>” will overwrite the file while “>>” will append the new output to it. The “<” enables redirection of a text file to STDIN (input).
Question 6 ■ ■
Average : 13.59 / 24 : 57% Key elements in the answer included:
#!/bin/sh at the start of the script Testing to see if a second argument was passed Finding the user (either using $USER or whoami) ➔ Note : whoami and who am i are two different commands Finding the date (either using set `date` or date +%d-%m-%Y) Building the proper name for the output Looping over the files in the directory Testing the files to see if binary (either with test or file) ➔ With test, automatically get points, regardless of option ➔ With file, needed to test for both “text” and ASCII Properly using Tar
Question 7
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Average : 3.22 / 4 : 80% a) A line will match if it contains the word “Tea” or “tea”. b) A line will match if it starts with the letter b or a word that start with the letter b.
Question 8
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Average : 2.06 / 8 : 26% a) ^(([a-zA-Z]+ ){9}[a-zA-Z]+)$
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Does your solution require 10 spaces to work? Did you include the ^ and $
b) [^.-][1-9][0-9][0-9]
Did you consider 069? Did you consider 0.999? Did you conisder -999?
Question 9
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Average : 4.31 / 5 : 86% In Python, scope is defined thought indentation. if (x > 10) : if (x > 20) : print “Larger than 20” else print “Larger that 10”
Question 10
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Average : 2.93 / 5 : 59 % When coding in Python, you don't need to give variables a type. Types are only checked at runtime.
Question 11 ■ ■
Average : 7.44 / 8 : 93 % Many types to choose from :
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char : ASCII character, 1 byte byte : natural number, 1 byte short : natural number, 2 bytes int : natural number, size depends on platform ➔ Bad example of definition : Int hold integers long : natural number, 4 bytes float : real number, 4 bytes double : real number, 8 bytes
Signed variable can be +/-, unsigned variable can only be +, but have a larger range.
Question 12
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Average : 8.39 / 10 : 84% The C Programming languages uses a single-pass compiler. This means variables and functions must be defined before being used.
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Not before necessarily before main.
Function prototyping it the declaration of those functions. To declare a function, simply include the signature of that function at the top of your file.
Question 13
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Average : 3.67 / 4 : 92% Through the malloc function. Because used memory is never automatically freed and you can eventually run out of memory.
Memory leak are a consequence of not freeing up memory. By themselves, are not catastrophic. However, too allocating too much memory over time without freeing it, that crashes the process.
Question 14
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Average : 2.65 / 4 : 66% Only the value of d was important.
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All or nothing.
a 6 b 8 c 6 d -1075896576
