Reputation: 27133

c++ array index calculation issues

Sorry for this simple question, but I can't solve it...

There is an example:

int a[10] = {6, 7.8, 8.0, -6, -5.7, 5, 5.7};

std::cout << a[6 % 8 / 4 + 21 % 9];

In response I get -5, but I don't understand why.

Upvotes: 1

Answers (3)

Reputation: 3930

6 % 8 / 4 => (6%8) / 4 => 6 / 4 => (int)1.5 => 1 (6/4 is int -- num and denom are ints)
21 % 9 => 3
1 + 3 == 4

a[4] == (int)(-5.7) == 5

a is an int array and so the numbers there are int's and not floats.
index 4 is chosen because of C++ operator precedence

Upvotes: 2

Reputation: 3825

http://en.cppreference.com/w/cpp/language/operator_precedence Operators have their priority so in this case: 6 % 8 / 4 + 21 % 9 = (((6%8)/4)+(21%9))=(6/4)+(3)=4

a[0]=6,
a[1]=(int)7.8 = 7, (correct me if i'm wrong)
a[2] = (int)8.0 = 8,
a[3] = -6,
a[4] = (int)-5.7 = -5,
a[5] = 5,
a[6] = (int)5.7 = 5

So element with index 4 it's -5.7 casted to integer gives -5 (the rest is just cut off)

Upvotes: 3

Reputation: 4589

because your array is a integer array.

you should use at first

float a[10]

Upvotes: 7