please explain the reason for the "weird" false condition coming out from for conditional checking.

Question

In this program the TOTAL_ELEMENTS calculates properly when not used in for loop. And the first printf prints properly.

But why the 2nd printf is not working even if the condition in the loop is true. TOTAL_ELEMENTS returns 7.

And -1<7-2 i.e -1<5 is true. So what is wrong here?

#include<stdio.h>

#define TOTAL_ELEMENTS (sizeof(array) / sizeof(array[0]))
int array[] = {23,34,12,17,204,99,16};

int main()
{
int d;
printf("Total= %d\n", TOTAL_ELEMENTS);
for(d=-1;d <= (TOTAL_ELEMENTS-2);d++)
printf("%d\n",array[d+1]);
return 0;
}

Answer 1

Due to implicit type casting.

When we operate on two different data types, the smaller one is implicitly casted to bigger one. And between signed and unsigned, unsigned is ranked higher. 

http://gatecse.in/wiki/Chapter_2:_Data_Types_and_Operators_in_C#Implicit_Type_Conversion

In the definition of TOTAL_ELEMENTS, sizeof, returns unsigned int, and unsigned int divided by unsigned int returns unsigned int. When compared with d, an integer, d is promoted to unsigned and becomes  2ⁿ-1, where n is the number of bits used for storing an int. So, the comparison here returns false. 

Comments

nice and correct!!!