Lecture 7 - Arrays, Memory Layout of Data

Question 1

arrays of structures

Answer 1

• we can create arrays of structures just as we can create arrays of anything else
• ex. struct person people[4] = { {“hey”, “TA”, {1, 2, 3, 4} }, {“Jon”, “TA”, {2, 3, 4, 5} }, …..

Question 2

array initialization

Answer 2

• you can partially initialize an array!
• ex. int my_numbers [200] = {5, 5, 3, 4, 5};
• the first five are explicitly initialized, there rest are zero
• this is not true only for global arrays but for arrays in functions

Question 3

array auto-sizing

Answer 3

• you can define and initialize an array without explicitly saying what its size is
• ex. int my_array[] = {1, 1, 2, 2, 3, 3, 7};
• there are no zero elements at the end of the array since we’re letting the compiler figure out how large it is

Question 4

data layout in memory

Answer 4

• everything that contains a value uses memory
• memory space looks like a long, continuous stream of bytes
• everything that contains a value occupies one or more bytes of memory

Question 5

variables and memory storage

Answer 5

• when we define a variable, the compiler creates a space for it in memory somewhere
• some types of variables consume several bytes of memory
• ex. an int is usually four bytes long

Question 6

size of int in memory

Answer 6

• takes up 4 bytes

Question 7

size of float in memory

Answer 7

• takes up 4 bytes

Question 8

arrays in memory

Answer 8

• each element takes up the number of bytes the array type is
• ex. for int arr[] the total storage is n elements times 4 bytes
• arrays of items are guaranteed to be packed together in memory

Question 9

strings in memory

Answer 9

• a string in C is an array of characters
• all strings delimited by “” characters are said to be null terminated (terminate by a zero byte)
• each char takes up a byte, with ‘\0’ at the end

Question 10

two dimensional arrays in memory

Answer 10

• ex. char array2d[2][3] = { {1, 2, 3} {4, 5, 6} }
• each element takes its array data type number of bytes stored right text to each other in order

Question 11

structures in memory

Answer 11

• members are placed in memory just like arrays (guaranteed to be packed next to each other)
• ex. struct my_stuff {
  int i;
  float f;
  char c[8];
  } my_var = {0, 0, “fife”}
  each one is right next to each ohter

Question 12

how do we know the size of variables and types

Answer 12

• a variable of a certain size may have different allocated size on different machines/compilers
• we don’t want our software to misbehave when compiled on a different system
• with sizeof() you don’t have to remember

Question 13

sizeof()

Answer 13

• sizeof() operator can tell us the size (number of bytes) of any
• variable definition
• type declaration

Question 14

example of how to access a character in a struct array

Answer 14

• struct crowd[100], int codes[4] inside of struct
• crowd[0].codes[0]