CODEWITHSUNDEEP
c
user4729852
user4729852

Reputation:

Confused about meaning of C statment relating to structs

Im having trouble understanding what this statement means, could someone explain it to me.

This is the struct I have:

struct dataT
{
    int m;
};

struct stack
{
    int top;
    struct dataT items[STACKSIZE];
} st;

This is the statment i am confused about. I dont quite understand what it means:

st.items[st.top].m

Upvotes: 2

Views: 83

Answers (4)

Mohit Jain
Mohit Jain

Reputation: 30489

st: An object of type struct stack
st.top: Top of stack
st.items: Array of stack items
st.items[st.top]: Top item of stack
st.items[st.top].m: member m of top item of stack

              st
  +-----------------------+
  |           top         |  st.top
  +-----------------------+
  | +-------------------+ |
  | |                   |m|  st.item[0]
  | +-------------------+ |
  +-----------------------+
  | +-------------------+ |
  | |                   |m|  st.item[1]
  | +-------------------+ |
  +-----------------------+
  .                       .
  .                       .
  .                       .
  +-----------------------+
  | +-------------------+ |
  | |                   |m| st.item[STACKSIZE - 1]
  | +-------------------+ |
  +-----------------------+

if you push 3 and 5 into the stack, it would looks like (Stack is growing downward)

              st
  +-----------------------+
  |            1          |  st.top = 1
  +-----------------------+
  |            3          |  st.item[0].m = 3
  +-----------------------+
  |            5          |  st.item[1].m = st.item[st.top].m = 5
  +-----------------------+
  .                       .
  .                       .
  .                       .
  +-----------------------+
  | xxxxxxxxxxxxxxxxxxx   |
  +-----------------------+

Upvotes: 4

Manthan Tilva
Manthan Tilva

Reputation: 3277

This statement retrieves the value of the st.topth element from struct stack. In other words if your implementation increments in decrements the top variable of struct stack then it will retrieve the last pushed element.

Upvotes: 1

Vlad from Moscow
Vlad from Moscow

Reputation: 310910

In this statement

st.items[st.top].m

st denotes an object of type struct stack because it was defined in this way

struct stack
{
   //...
} st;

As st is defined as having type of the structure then it has data member with name items Here is the structure definition itself.

struct stack
{
    int top;
    struct dataT items[STACKSIZE];
};

So record st.items means access to data member items of object st. This data member of the structure is defined as an array

struct dataT items[STACKSIZE];

with STACKSIZE elements. Thus record st.items[st.top] means access to the element with index st.top of the array items. Each element of the array in turn has type struct dataT This structure has data member m

struct dataT
{
    int m;
};

Thus record 

st.items[st.top].m

means access to data member m of element with index st.top of array itemsthat is in tirn a data member of object st

Upvotes: 2

mushfek0001
mushfek0001

Reputation: 3935

Dot (.) is used to access elements of a struct.

st.items - accesses items[STACKSIZE] array

st.top - accesses the top of st and used as the index

And items array contains dataT and you can access it's only element using

st.items[st.top].m

Upvotes: 3

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