How are my arrays modified when C passes by value?

Question

I made a simple program in C to check if two words are anagrams. My question is that if I\'m passing word_one and word_two as parameters, doesn\'t that mean that I\'m not modify

Answer 1

Except when it is the operand of the sizeof or unary & operator, or is a string literal used to initialize a character array in a declaration, an expression of type "N-element array of T" will be converted ("decay") to an expression of type "pointer to T", and the value of the expression will be the address of the first element of the array.

When you call

read_word(word_one); 

The expression word_one is converted from type "26-element array of int" to "pointer to int", so what actually gets passed to read_word is the address of the first element of word_one.

In the context of a function parameter declaration, T a[N] and T a[] are both interpreted as T *a - a is declared as a pointer to T, not an array of T. Thus, the prototype

void read_word(int counts[26])

is interpreted as

void read_word(int *counts)

Thus, read_word does indeed modify the contents of word_one and word_two.

You'll want to pass the size of the array as a separate parameter:

read_word( word_one, sizeof word_one );
...
void read_word( int *counts, size_t size )
{
  ...
}

since you won't know how big the target array is from the pointer alone.