Midterm

Question 1

why is this code “dangerous”?

1. int test1 = 0;
2. char line[20];
3. int test2 = 0;
4. scanf(“%s”, line);

Answer 1

scanf does not honour the size of the target address. it doesn’t let you tell it how much to read

reading input that goes beyond the end of the buffer will overwrite values of other variables (in this case, test1 and test2

Question 2

why is this code “dangerous”?

int readData(char *string, int max) {
int read = -1;
FILE *in;
in = fopen( “in.txt”, “r” );
if (in != NULL) {
fgets( string, max, in );
read = strlen( string );
}
return read;
}

Answer 2

file is not closed using fclose.
-> the code leaks

could potentially cause the code to crash
- eg. someone could write code to call this function repeatedly

Question 3

Here is a physical memory that is pointed by char *str.

How long is this string? (i.e. what will strlen(str) return?)

str -> [a][v][4][0][\0][E][\0][h][\0]

Answer 3

4

The string is terminated by the null terminator at str[4].

extra:
- The null terminator is essential because C does not store the length of a string explicitly
-> functions that work with strings rely on the \0 to know where the string ends.

Question 4

explain why you would not test for NULL when writing test cases, even as an edge case. (3)

Answer 4

1. testing is meant to simulate user input. users can’t input NULL
2. Applying the principles of design by contract should catch programming errors (like passing NULL to a
   function)
3. Passing NULL to a function would cause a Segmentation fault or Abort.
   - A Segmentation fault or Abort isn’t a test failure, it’s a crash.

Question 5

passing NULL to a function would cause ________________ or _________

Answer 5

segmentation fault or abort

Question 6

what is the main purpose of testing code?

Answer 6

to simulate human interaction with our code

(user input)

Question 7

what are the 3 general classifications of inputs we need to use to test our code?

Answer 7

1. general cases
   - valid, expected input
2. edge cases
   - input is valid but looks weird
3. memory leaks
   - use loops to run code repeatedly
   - observation with tools like top (see man top) or profilers

Question 8

how do you check for memory leaks in your code?

Answer 8

• use loops to run code repeatedly
• observation with tools like top or profilers

Question 9

list three edge cases for the following in-place sorting routine:

void sort( int *array, int size );

Answer 9

1. array is empty
   - [], 0
2. array has one value
   - [1]
3. array has repeated values
   [2, 2, 2]

Question 10

the only output you receive when you run a program is Segmentation fault. Explain what tool you would use to identify the problem, and what the tool is helping you understand about the program.

Answer 10

asserts, debugger, print statements can all help but for this the best option is a debugger

a debugger helps you:

1. step through your program line-by-line (observe the flow)
   - helps you find where the code is crashing
2. watch variables as they change (observe the state)
   - determine which variables have the wrong values that are causing the crash

Question 11

Given the following function, what should be the pre and postconditions for the function, and what
should the loop invariant(s) be? Write your answer as a series of assertions. Give at least two examples of
each type

void substr(char *in, int start, int end, char *out) {
int i;
for (i = start; i < end; i++)
out[i - start] = in[i];
out[i] = ‘\0’;
}

Answer 11

// preconditions:
assert(in != NULL);
assert(out != NULL);
assert(start < end);
assert(start >= 0);
assert(end < strlen(in));

// loop invariant
assert(i - start >= 0);
assert(i - start < end);

// postconditions
assert(out[0] == in[start]);
assert(out[strlen(out)] == ‘\0’);

Question 12

Below is a structure definition for a string data structure in C:

typedef struct STRING {
char *content;
int length;
} string;

Write a function that will return the index of a character in an instance of this string structure. For complete points, your code must apply the principles of design by contract (you must define pre and postconditions for this function). No invariant function is provided, you should write your own.

Here’s the prototype:
// return the index at the specified character in the string (similar to // Java’s String#indexOf method). Returns -1 if not found.

int index_of(string *, char);

Answer 12

see notes

Question 13

The degree of reliance on other modules/functions
is called ________

Answer 13

coupling

Question 14

Higher coupling means higher __________ and we are more likely to be affected [positively/negatively] by changes

Answer 14

dependency

negatively

Question 15

The degree to which a function adheres to one task is called __________

Answer 15

cohesion

Question 16

_________ cohesion means doing many independent
tasks. is this good?

Answer 16

low

no. we want high cohesion. we dont want our functions doing many independent tasks

Question 17

well designed programs should have:

______ cohesion. The elements of each module
should be closely related to one another.
– makes modules easier to use and makes the entire program easier to understand.

_______coupling. Modules should be as independent
of each other as possible.
– makes it easier to modify the program and reuse modules.

Answer 17

high

low

Question 18

high cohesion means the elements of each module are what? how does this help?

Answer 18

closely related to each other

makes modules easier to use and makes the program easier to understand

Question 19

low coupling means modules should be…

how does this help?

Answer 19

as independent of each other as possible

makes it easier to modify the program and reuse modules

Question 20

The assembler translates the assembly to…

Answer 20

binary/machine code

Question 21

The von Neumann Architecture

A “stored-program” computer architecture consists of: (4)

Answer 21

• input device
• output device
• memory unit
• central processing unit

Question 22

what is piping for?

Answer 22

piping redirects standard output from one program to another to be processed as standard input

transfers standard output to some other destination

Question 23

what are the key differences between C and Java (7/8)

Answer 23

1. C doesn’t have objects
   - no String class. char arrays are strings
2. C doesn’t garbage collect
   - Java destroys unused arrays and frees memory for you. C does not do this
3. C doesn’t have exceptions
   - because it doesn’t have objects
   - you must handle exceptional cases yourself
4. C does not check bounds for you
   - will allow you to overflow and overwrite other blocks of memory
5. C has no concept of information hiding
   - everything is public
6. variables can be used without being initialized
7. arrays
   - not bounds checked
   - array size must be initialized. C doesn’t do this for you. eg. char name[]; // ERROR
   - array data isn’t initialized with 0s (depends)
   - you should initialize them yourself
8. C gives you direct access to memory
   - there are types but they are just representation of the bits & bytes in memory
   - in Java, bits & bytes are abstracted away into types

Question 24

what is important about C not having objects

Answer 24

• no string class. char arrays are strings
• no exceptions. must do that shit yourself

Question 25

explain the statement that "C does not garbage collect"

Answer 25

- as variables go out of scope, their resources are not automatically reclaimed - the OS has no idea it can use those resources again (leaks) - this is also a problem with dynamic memory allocation - C doesn't destroy things for you after you finish using them like Java does - this is important because to make efficient use of memory, you must do it yourself

Question 26

cmd 1 | cmd 2 explain whats happening here

Answer 26

piping cmd 1 output is being fed as input to cmd 2

Question 27

if the output is going by too quickly to see, what can you do?

Answer 27

you can pipe it somewhere (to a pager, like more or less) to view it easier

Question 28

why is it important that C doesn't have exceptions

Answer 28

no try/catch. no bounds checking - C will write outside of the bounds and overwrite other information

Question 29

why is it important that C does not check bounds for you?

Answer 29

because it wont cause an error. even if the input has too much data for the target address, it will keep going it will overflow and write over other memory, potentially replacing other variables in your program

Question 30

memory can be thought of as...

Answer 30

a long piece of paper separated into segments that have an address each segment can store info

Question 31

What can function as a Boolean in C?

Answer 31

numbers. 0 = false anything else = true int i = 10; while (i){ i-- }

Question 32

what are some important things to remember for arrays in C

Answer 32

not bounds checked must be passed using pointers you must initialize the size yourself - say how much space you'll need data in arrays aren’t initialized like in Java - int array wont necessarily be filled with 0s - depends on OS

Question 33

how is C compiled?

Answer 33

1. preprocessor 2. code generator 3. assembler 4. linker

Question 34

what is the preprocessor? what does it do?

Answer 34

# define a text replacement engine - finds lines that start with # - Replace those lines with file contents (#include) - Uses these lines to replace other text in the file (#define)

Question 35

what does the code generator do? where does it take output from?

Answer 35

- generates abstract syntax tree, - optimizes it, - converts to assembly - takes output from preprocessor

Question 36

what does the assembler do where does it take output from

Answer 36

translates assembly to an object file (machine code) takes output from code generator

Question 37

what does the linker do where does it take output from what does it produce?

Answer 37

links the functions - prototypes are important for this - will get errors from the linker otherwise takes output from the assembler Produces an executable binary file

Question 38

how does an array look in memory?

Answer 38

contiguous blocks of memory - the size you set for an array will reserve that many blocks of memory

Question 39

define exceptions

Answer 39

unexpected behaviour happens during code execution interrupts flow of execution

Question 40

what are some causes of exceptions?

Answer 40

1. file handling 2. null pointers 3. division by 0

Question 41

explain what happens when a function is called

Answer 41

a stack frame is created with the memory necessary for the variables in the function when the function has finished executing, the frame is popped off, and the memory allocated becomes available to the rest of the program

Question 42

explain why stack memory cannot be returned from a function

Answer 42

the memory only becomes available after the function is finished executing, and the stack frame gets popped off any variables held within the stack frame become available and can be rewritten

Question 43

struct vs class in Java

Answer 43

struct just has variable declarations struct does not have methods

Question 44

explain what happens when resources aren't released after use

Answer 44

leaks. if a function leaks, and is called repeatedly, can cause program to crash

Question 45

what are some programming errors (not at run time)

Answer 45

syntax indexing out of bounds

Question 46

explain design by contract what does it contain?

Answer 46

helps programmer ensure code is running the way they expect it to contains preconditions, invariants, postconditions essentially a debugging tool

Question 47

classifications of test data (3)

Answer 47

1. general cases - valid and expected input 2. edge cases - valid input that may require special handling 3. leaks - run code repeatedly

Question 48

explain the purpose of automated testing

Answer 48

makes it easier for testing throughout changing our program create template - saves time in the long run, especially for larger programs

Question 49

strategies for debugging

Answer 49

- printf statements - assertions - spelunking - debugger

Question 50

whats the problem with using printf for debugging? what can be done about this?

Answer 50

printf is buffered - may not print near the crash use fflush to ensure buffer is emptied

Question 51

what does debugging reveal

Answer 51

flow - how the program is running. where the crash is state - state of variables

Question 52

explain the difference between interface and implementation what does an interface include? what does implementation include?

Answer 52

interface - contains min amount of info for a piece of data - tells you what you can do - includes function prototypes - data type declaration - header files implementation - data type definitions - function implementations

Question 53

classify public interface and private behaviour

Answer 53

private behaviour hides implementation - user doesnt need to know whats going on behind the scenes

Question 54

whats the purpose of a build tool

Answer 54

helps us compile our code - useful for large programs that contain many .c and .h files - compiling those ourself each time we make a change to our code is a pain in the ass example: make

Question 55

explain why information/implementation hiding is important

Answer 55

hiding implementation allows for the use of programs/code/tools without needing to understand the inner workings

Question 56

is sharing data across scopes ok? why or why not

Answer 56

its bad! - it exposes implementation - it increases coupling

Question 57

identify code that can be separated into modules

Answer 57

abstract data types, data structures, and subsystems each get their own module

Question 58

examples of abstract data types

Answer 58

lists, queues, stacks

Question 59

examples of data structures

Answer 59

linked lists, hash tables

Question 60

what is coupling

Answer 60

measurement of how much a class/module/function/concept depends on others how interdependent * coupling is unavoidable but we want to keep it as low as possible

Question 61

what is cohesion

Answer 61

measurement of how much code and concepts belong together - classes/modules implement ONE concept

Question 62

do we want high or low coupling? why? how can this be helped?

Answer 62

we want low coupling high coupling makes it hard to make changes to our code - our code is interdependent. changing one thing affects many others modular design helps avoid coupling

Question 63

how do we reduce coupling?

Answer 63

modular design (what is that?)

Question 64

what is modular design?

Answer 64

subdividing a system into smaller parts (modules) modules can be independently created, modified, replaced

Question 65

what is the purpose of modular design(4)

Answer 65

- breaks code down and makes it easier to read and understand - provides abstractable concepts that are easier to use - allows multiple ppl to work on same thing - allows for dynamic implementation which can be changed and not affect the rest of the program

Question 66

T or F C gives you the choice for arrays to be placed in "stack memory" or "heap memory"

Answer 66

true

Question 67

how can you prevent exceptions from occurring

Answer 67

- check that variables have valid values before you use them - for now: writing a lot of if statements & checking error codes (invariants) - assertions

Question 68

how do we return an array in C

Answer 68

using a pointer

Question 69

what is a pointer

Answer 69

a type that stores an address

Question 70

what is the basic idea of Output Parameters

Answer 70

each function should be responsible for allocating the memory it needs, even if that memory is populated by another function

Question 71

int * x_ptr, y_ptr; what will these be declared as

Answer 71

x_ptr = a pointer y_ptr = an int

Question 72

Practice: consider the following program. draw it out int x = 1; int y = 2; int z = 3; int *a, *b; a = &y; b = &x; *b = z; x = *a;

Answer 72

see notes sep 17/19

Question 73

scanf returns one of three things:

Answer 73

1. number of tokens parsed (success) 2. 0 for a formatting error 3. EOF for the end of the file

Question 74

how does fgets get input safely? (unlike _______)

Answer 74

unlike scanf, fgets inserts a null terminator at the end of the desired length you set

Question 75

what are the steps to opening a file

Answer 75

1. fopen 1.5 check if file opened - fopen returns NULL if file doesn't exist 2. read file 3. fclose the file - only if it opened successfully

Question 76

T or F C treats stdout as a file

Answer 76

True

Question 77

what is a class?

Answer 77

a "blueprint" for what data and methods go together

Question 78

what is a struct

Answer 78

a named grouping of related heterogenous data like a class but only variables. no methods

Question 79

what is typedef for

Answer 79

so that every time we declare a variable of type struct FOO, we DONT have to write struct FOO - defines foo as a type

Question 80

what happens if you pass a struct as a parameter or return a struct

Answer 80

a copy will be made also, if you assign structs to each other

Question 81

can you compare two structs?

Answer 81

no but you could do a field-by-field comparison to check for duplicates

Question 82

what should you do if you want to modify a struct in a function

Answer 82

pass it as a pointer otherwise you will be working with a copy and wont change the original struct

Question 83

what does malloc do what does it return

Answer 83

requests allocation of heap memory a pointer to the space in memory it allocated the amount of space you requested (amount of bytes) - or NULL if it failed

Question 84

malloc takes a size in bytes whats important to know for this?

Answer 84

how many things we want to store multiplied by: how many bytes each thing takes up (what type) - use sizeof

Question 85

different platforms use different amounts of space for the same type. how do we deal with this when using malloc?

Answer 85

sizeof - unary operator that tells us how many bytes a specific data type requires char * arr = malloc(10 * sizeof(char)); // gives us 10 characters worth of space

Question 86

what should you always remember to do when using malloc?

Answer 86

garbage collect! free memory when we're done using it eg. free(array)

Question 87

when we design a data structure, our responsibility is to provide… (2) hint, one is unique to C

Answer 87

1. a constructor 2. a destructor

Question 88

what is design by contract a tool for? what is it not a tool for?

Answer 88

- prevent programming errors - exception prevention NOT for: - preventing user errors

Question 89

what is the state of a program?

Answer 89

the value of all variables that currently exist

Question 90

what kind of programming errors can a programmer write?

Answer 90

- index and bounds errors - forget to check inputs (data type, NULL, format) - syntax errors (handled by the compiler)

Question 91

exceptions usually come from a violation of our ____________ 3 places this can happen:

Answer 91

assumptions 1. before we start processing data 2. after we finish processing data 3. the state throughout execution (while we're processing data)

Question 92

char char_at(String* str, int loc){ return str.contents[loc]; } what are some assumptions we may have about this code?

Answer 92

- loc <= strlen(str) - loc >= 0 - str != null - str.contents != null - str.contents has a null terminator at position str.length (and this is the first null terminator)

Question 93

the preconditions, postconditions, and invariants of a function/block/thing are its ___________

Answer 93

contracts

Question 94

how do we check for leaks?

Answer 94

run code repeatedly and see if it crashes

Question 95

what are some edge cases for a sorting algorithm (5)

Answer 95

edge cases: ○ a list of length 1 ○ a sorted list ○ a reverse sorted list ○ an empty list (length 0) ○ n identical elements

Question 96

preventing exceptions... what kind of assumptions might we have

Answer 96

- provided arguments are not NULL - variables are non-negative - values are within bounds of array - a counter actually reflects the contents of an object

Question 97

what would some edge cases for a split function be? (split words separated by spaces)

Answer 97

* “hello” -> no spaces – 1 element {“hello”} program expects 0 elements * “hello, world” -> 2 spaces in a row – 2 elements {“hello,”, “world”} prgrm expects: 3 elements * “ “ only space characters – 1 element {“”}(empty string) program expects n elements * “ hello “ -> space at the beginning and the end – 1 element {“hello”} program expects 3 elements * “” -> empty string – 0 element

Question 98

organizing tests: 1. Each test function should be as ______ as possible – Test _____ thing. (and test it well.) 2. The data for the test belongs in the test ________. 3. The main function should only call other __________

Answer 98

atomic. test one thing function functions

Question 99

advantages of typedefs?

Answer 99

- make our program easier to understand - make our program easier to modify

Question 100

An __________ type is a type whose values are listed (“____________”) by the programmer (same word fits both blanks)

Answer 100

enumerated

Question 101

enumerations are a tool for

Answer 101

creating types that can only take on a small number of values eg. enum suit { heart, spade, club, diamond }; enum suit card = heart;

Question 102

whats one thing enumerations are useful for

Answer 102

creating a boolean typedef enum { false, // 0 true // 1 } boolean;

Question 103

general strategy for code that crashes

Answer 103

key words: flow and state 1. run the code in the debugger with no breakpoints (run or r) 2. inspect the stack trace. (bt) ○ tell you where the crash is happening 3. add a break point near the crash. (b or break, followed by the line number) 4. run program again, stepping through each line after the breakpoint. (n or next) - observe state step by step

Question 104

goals of modular design (3)

Answer 104

1. smaller, easier to understand components 2. ability for multiple ppl to work on program 3. extract reusable structures and concepts

Question 105

modular design has 2 main parts (what do we want to do with these two things)

Answer 105

1. the interface of the software 2. the implementation of the software - our main goal is to separate these two things

Question 106

what does a header file contain? (3) + (1) extra detail

Answer 106

- function prototypes (public function declarations) - struct definitions (as necessary) - enum definitions (as necessary) -> should not expose internal implementation details

Question 107

running the compiler without the linker -c produces __________ files

Answer 107

object (.o)

Question 108

what are header guards

Answer 108

for when we include the same header more than once

Question 109

Problem: what happens if we include the same header twice? what can we use to help this?

Answer 109

redefinitions -> compilation fails header guards

Question 110

a makefile consists of a list of ______ that describe how to build a program a ____ has 3 parts:

Answer 110

rules/rule 1. a target ○ the name of the thing this rule generates 2. a list of prerequisites ○ things that must exist before you build this thing 3. a list of recipes ○ the commands that will be run to generate the target eg. string.c is a prereq for string.o

Question 111

problem: in C, everything is effectively public by default how do we prevent calling private functions?

Answer 111

use "static" modifier - in C, static means per-file private - ie, this function/data is only available in the file where its declared

Question 112

what is the single responsibility principle

Answer 112

each piece should do one thing

Question 113

re: rules a target is

Answer 113

the name of the thing this rule generates

Question 114

re: rules a list of prerequisites is… example ?

Answer 114

things that must exist before you build this thing eg. string.c is prereq for string.o

Question 115

re: rules a list of recipes is…

Answer 115

the commands that will be run to generate the target

Question 116

Questions to ask when identifying functions to manipulate your data model: (4)

Answer 116

- what kind of input do you have? - how do you get that input? - how do you transform it to/from your data model? - what kind of operations might someone else need to do? (what are the public operations?)