can you help me explain this code?

Question

code link below:
https://code.sololearn.com/ca28a10a170A

#include <stdio.h>
#include <stdlib.h>

typedef struct {
int *elements;
int size;
int cap;
} dyn_array;

int main() {
dyn_array arr;
int i;

/* initialize array */
arr.size = 0;
arr.elements = calloc(1, sizeof(*arr.elements));
arr.cap = 1; /* room for 1 element */

/* expand by 5 more elements */
arr.elements = realloc(arr.elements, (5 + arr.cap)*sizeof(*arr.elements));
if (arr.elements != NULL)
arr.cap += 5; /* increase capacity */

/* add an element and increase size */
if (arr.size < arr.cap) {
arr.elements[arr.size] = 50; /* add element to array */
arr.size++;
}
else
printf("Need to expand array.");

/* display array elements */
for (i = 0; i < arr.cap; i++)
printf("Element %d: %d\n", i, arr.elements[ i ]);

return 0;
}

Answer 1

the
/*expand by 5 more elements/*
and
/*add an element and increase size*/
this 2 section

Answer 2

Let's begin by understanding calloc and realloc.

http://www.cplusplus.com/reference/cstdlib/calloc/
http://www.cplusplus.com/reference/cstdlib/realloc/

I'm not sure what to explain for now, the code is pretty straight forward. If you still have doubt after reading the references, then you're welcome to ask further.

Answer 3

Ipang thanks 🙌

Answer 4

1.In this line,
arr.elements = realloc(arr.elements, (5 + arr.cap)*sizeof(*arr.elements));
here realloc re allocates the memory of arr.element.the value of arr.cap is now 1.
so it adds 5+1=6*sizeof(*arr.elements) here the value of sizeof(*arr.elements)=4 as *arr.elements are declared int*.
so it reallocates 6*4=24 byte memory,for each integer it takes 4 bytes.So the total int elements are 6.and all the elements value becomes 0 by default .
If you don't know the use of realloc ,have a look at this:
https://www.tutorialspoint.com/c_standard_library/c_function_realloc.htm
2.In this line,
if (arr.size < arr.cap) {
arr.elements[arr.size] = 50; /* add element to array */
arr.size++;
here the value of arr.size is 0 as decalred at the beginning .
The value of arr.cap=5+1=6 as the expression if (arr.elements != NULL) is true.
so the condition if (arr.size < arr.cap) becomes true and it adds arr.element[0]=50
Hope you understood.

Answer 5

The future is now thanks to science thanks for full explanation ⭐⭐

Answer 6

Which part in particular you need explanation for?