Setting values in arrays

Accessing array values is generally done through square brackets:

int val;
int array[10];

/* Setting the value of the fifth element to 5: */
array[4] = 5;

/* The above is equal to: */
*(array + 4) = 5;

/* Reading the value of the fifth element: */
val = array[4];

As a side effect of the operands to the \+ operator being exchangeable (–> commutative law) the following is equivalent:

*(array + 4) = 5;
*(4 + array) = 5;

so as well the next statements are equivalent:

array[4] = 5;
4[array] = 5; /* Weird but valid C ... */

and those two as well:

val = array[4];
val = 4[array]; /* Weird but valid C ... */

C doesn’t perform any boundary checks, accessing contents outside of the declared array is undefined (http://stackoverflow.com/documentation/c/364/undefined-behavior/2144/accessing-memory-beyond-allocated-chunk#t=201701141217382606962 ):

int val;
int array[10];

array[4] = 5;    /* ok */
val = array[4];  /* ok */
array[19] = 20;  /* undefined behavior */
val = array[15]; /* undefined behavior */

Found a mistake? Have a question or improvement idea? Let me know.

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