Week 2 - Arrays, Strings, Random Numbers and Functions

Question 1

In C, how do you declare a 1d array of 5 characters, initialised with 5 values?

Answer 1

char array[5] = {‘A’, ‘B’, ‘C’, ‘D’, ‘E’};

Question 2

In C, how do you declare an initialised 2d 2x3 array of ints?

Answer 2

int array[2][3] = { {1, 2, 3}, {4, 5, 6} };

Question 3

Where is it guaranteed that arrays are stored in C?

Answer 3

In contiguous memory.

Question 4

How do arrays reserve an appropriate amount of memory in C? Use example int array[10].

Answer 4

int array[10] reserved 10 x sizeof(int) = 10 * 4 bytes = 40 bytes saved for array.

Question 5

What is another name for an array of chars in C?

Answer 5

String

Question 6

In C, what variable type uses single quotes (‘’) and what uses double quotes (“”)?

Answer 6

A char uses single, a string uses double

Question 7

In C, what does the null terminator do?

Answer 7

The null terminator is ‘\0’. It tells printf when to stop printing out characters

Question 8

In C, how does memory get allocated for a string?

Answer 8

Memory allocated is number of characters plus one for the null terminator

Question 9

What happens if your string doesn’t have a null terminator?

Answer 9

printf keeps going through memory and printing out characters until it encounters a null terminator (which is one reason arrays are stored contiguously)

Question 10

How would you print out 3 characters of the string variable char animal[21]=”elephant” in C?

Answer 10

printf(“0.3s\n”, name);

Question 11

How would you write this C code with width and precision? printf(“%20.10s\n”, “My elephants”);

Answer 11

int wid = 20;
int prec = 10;
char str[] = “My elephants”;
printf(“%.s\n”, wid,prec, str);

Question 12

What do these special characters mean?
\n
\t
\v
\b
\r
\f

'
"
%%

Answer 12

\n - line-feed (new line)
\t - horizontal tab
\v - vertical tab
\b - backspace
\r - carriage return
\f - form-feed (new page)
\ - backslash
' - single quote
" - double quote
%% - percent sign

Question 13

What do these C string functions do?
strlen(a)
strcpy(a, b)
strcat(a, b)
strcmp(a, “hello”)
sprintf(buffer, “%s %d”, a, b)

Answer 13

strlen(a) - returns the number of characters in variable name, not including the null terminator
strcpy() - copies the string a into b, including the null terminator
strcat(a, b) - concatenates b onto the end of a, overwriting a from first null terminator
strcmp(a, “hello”) - returns 0 if the two strings are the same, otherwise some other int value
sprintf(buffer, “%s %d”, a, b) - like printf, but places the result in buffer instead of printing it out

Question 14

Give 3 features of pseudo-random numbers.

Answer 14

Generated by an algorithm
Repeats over a (hopefully) very long period
Look random but aren’t

Question 15

Describe these random number distributions:
Uniform
Normal (Gaussian)
Poisson
Bernoulli
Gamma

Answer 15

Uniform - a distribution where all values in a defined range are equally likely
Normal (Gaussian) - the bell curve, a result of the Central Limit Theorem
Poisson - the distribution of a count
Bernoulli - the distribution of two mutually exclusive events (call them 0 and 1)
Gamma - a distribution of positive values only

Question 16

What header file in C is needed to include random numbers?

Answer 16

stdlib.h

Question 17

What is the command to generate a random number in C?

Answer 17

rand()

Question 18

What is a “good enough” sequence length for PRNGs (Pseudo Random Number Generators)?

Answer 18

2^26 = (1.8 x 10^19)

Question 19

What is the general formula for how pseudo random numbers are generated?

Answer 19

j := (j x a + c)%
Where a, c and m determine the period and complexity of the sequence

Question 20

Does integer division in C round up or down?

Answer 20

Down

Question 21

How would you cast an integer to a float in C?

Answer 21

Preceding the integer value with (float)

Question 22

How do you set the random number seed in C?

Answer 22

srand(num)

Question 23

What is GSL?

Answer 23

The GNU Scientific Library, used for producing high quality randomness in random numbers

Question 24

What link flags need to be included for the GSL library?

Answer 24

-lgsl -lgslcblas

Question 25

In C, are functions and methods the same?

Answer 25

No

Question 26

What is a function in C?

Answer 26

A uniquely named group of program statements (so no overloading), which accepts 0 or more arguments or parameters, and returns 0 or 1 value to the calling code.

Question 27

How is a function declared in C?

Answer 27

<return> myFunction (<parameter>)
{
/* zero or more program statements*/
return <value>;
}
</value></parameter></return>

Question 28

What is the form of the main function in C?

Answer 28

int main()
{
/zero or more statements/
return 0;
}

Question 29

In C, what is a procedure?

Answer 29

A function with no return value (void)

Question 30

Functions need to be declared in C (like variables) because…

Answer 30

1. The (one-pass) compiler needs to know a function’s definition before it is called
2. Functions may appear in any order in the program file (main first, by strong convention)
3. The functions may call each other
4. The code for the function might not be in this program file

Question 31

Where are prototypes for standard functions in C often supplied?

Answer 31

In .h header files

Question 32

What is function prototyping?

Answer 32

Declaring functions at the start of the program, e.g. int calculateAge();

Question 33

Should global variables be used?

Answer 33

No - it is very bad practice!

Question 34

In C, what are static variables and how are they declared?

Answer 34

Declared using the “static” key word, e.g. static int num = 2;

They are variables initialised once when the function is first called, and retain their value when they go out of scope.