Week 8: Structure types and structures in memory

Question 1

What is a structure in essence?

Answer 1

A construct to group together dissimilar

variables under one name

Question 2

What is the syntax for making a structure?

Answer 2

struct person {
char first[32]; /* 1st field is array of char /
char last[32]; / 2nd field is array of char /
int year; / 3rd field is int /
double ppg; / 4th field is double */
};

Question 3

If a structure is created whereas the first variable in the structure is called “first” how can you access the memory location of a structure that has been called “ex”?

Answer 3

ex

ex.first

&ex.first[0]

Question 4

What function can be used to add a string to a character array that has already been created?

Answer 4

strcpy();

ex:
strcpy(teacher.first,”Sam”);

Question 5

Can you create an array of structures?

Answer 5

YES

ex:

struct fraction {
int wP;
int fP;
};
struct fraction f[3], *g;

Question 6

With the following code in mind, how what is happening?

struct fraction {
int wP;
int fP;
};
struct fraction f[3], *g;

g = &(f[0]);
g++;
(*g).wP = 5;
g->fP = 11;

Answer 6

Creating an array of 3 fraction structures indexed at 0, 1, and 2.

Creating a pointer to type fraction called g and assigning it the memory location of the first element of the array f.

Increment g by the size of what is points to (fraction structure) which is 2 x the size of an int.

Go to the memory location now stored in g and make the wP variable of that array structure element assigned to 5

Go to the memory location now stored in g and make the fP variable of that array structure element assigned to 11

Question 7

What are the two ways of accessing a variable inside a structure?

Answer 7

(*structure).variable

or

structure->variable

Question 8

How do you declare the structure while also making a variable type of the structure in the same line?

Answer 8

struct fraction {
int wP;
int fP;
} fract;

Question 9

What is the difference between the following:

struct fraction {
int wP;
int fP;
} fract;

typedef struct fraction {
int wP;
int fP;
} fract;

Answer 9

The first creates a structure type and then creates a variable called fract that is of type fraction

The second creates an alias for struct fraction called fract that can be used in place of “struct fraction” when creating variables, etc.

Question 10

What is the difference between a UNION and a structure?

Answer 10

A struct is a block of memory that stores several data objects, where those objects don’t overlap. A union is a block of memory that stores several data objects, but has only storage for the largest of these, and thus can only store one of the data objects at any one time.

Question 11

Union sets aside how much space in memory?

Answer 11

Enough space to store the largest of the data types in the union

Question 12

The address of any of the different variables in a union all return ___

Answer 12

The same address

Question 13

What does realloc do EXACTLY

Answer 13

Is used to resize allocated memory without losing existing data. However, it will only increase or decrease to the given size. The given size is not the size to be added or subtracted, it is the size that will be the final size.

Ex:

double *a;
a = (double *)malloc(5 *sizeof(double));
a = (double *) realloc (a, 9*sizeof(double));
9 x 8 bytes
increase to 9 double variables
72 byes
// could have used
a = (double *) realloc (a, 3*sizeof(double));
- decrease to 3 double variables
24 bytes.