Have you seen the following C code sample:
char x = 1; char c = x["monkey"];
Do you know what it the value of c is? Don’t read on unless you want to know the answer and why. The value of c is ‘o’. Why? Well, I wrote some code to start playing around with this. The answer seemed simple, but here were some suggestions about why the answer is ‘o’:
- 1 is the index into the string “monkey”
- There is some magic with math of memory on the stack for the compiler used
- Something else is happening
Okay, it seems relatively trivial now, but when I looked at it it wasn’t. Other people were putting up ideas so I tested them out. Here is my silly test code:
#include
void initial_test()
{
char x = 1;
char c = x["monkey"];
printf("What is c:%cn", c);
}
void different_index()
{
char x = 2;
char c = x["monkey"];
printf("What is c:%cn", c);
}
void space_allocation()
{
char x = 1;
char v = 'd';
char c = x["monkey"];
printf("What is c:%cn", c);
}
int main(int argc, char** argv)
{
initial_test();
different_index();
space_allocation();
return 0;
}
/* Output:
What is c:o
What is c:n
What is c:o
*/
What is actually going on here is really just the associative property of addition. I was telling a friend that I would understand “monkey”[x], but not the other way around. This is the quote from my friend (who wishes to remain nameless):
I mean, technically it’s base_address + sizeof(datatype)*index. since sizeof(char) == 1, it’s just base_address+index. 1+addressof(“monkey”) or addressof(“monkey”) + 1.. they both work
In the end it was just a fun little exercise.
