Why the already set value is being reset to the new value in the

ghz 8months ago ⋅ 136 views

Why the already set value is being reset to the new value in the for loop condition in C?

I am studying for my programming course and one of the questions on the last test has this code that has to be "calculated".

#include <stdio.h>

int main(void) {
    short a[4] = {66, 066, 0x66};
    int i = 0, s = 0, p = 15;

    for (++a[i]; !i, p = a[i]; s += a[i++]) {
        p = a[i] >> 2 | p;
        a[i] &= p;
    }

    printf("%d", s);
}

The original question is to find the value of s, but I have a problem understanding why p = 67 is' before the first iteration' if it was declared as p = 15. Isn't p = a[i] the loop condition and not a way to declare a value?

I tried "calculating" for p = 15 where I didn't get the right result and for p = 67 where I did get the right result.

Answers

Let's break down the code to understand why p becomes 67 before the first iteration of the loop.

  1. Array Initialization:

    short a[4] = {66, 066, 0x66};

    Here, a is an array of 4 shorts. The elements of the array are initialized as follows:

    • a[0] = 66 (Decimal representation)
    • a[1] = 066 (Octal representation, equivalent to 54 in decimal)
    • a[2] = 0x66 (Hexadecimal representation, equivalent to 102 in decimal)

  2. Variable Initialization:

    int i = 0, s = 0, p = 15;

    Here, i, s, and p are initialized. p is initialized to 15.

  3. Loop Condition:

    for (++a[i]; !i, p = a[i]; s += a[i++]) {

    In the loop condition, the following actions are performed:

    • ++a[i]: The value of a[0] is pre-incremented to 67. So, a[0] becomes 67.
    • !i: The value of i is negated, which is initially 0, so it becomes 1 (indicating true).
    • p = a[i]: The value of a[0] (which is now 67) is assigned to p. So, p becomes 67. The loop will continue as long as the condition p = a[i] is true.

  4. Loop Body:

    • Inside the loop, the bitwise right shift operation >> is applied to a[i], and then the result is bitwise OR-ed with p. The result is then stored back in p.
    • Also, a[i] is bitwise AND-ed with p, and the result is stored back in a[i].

Based on the above analysis, before the first iteration of the loop, p becomes 67 because of the initialization inside the loop condition (p = a[i]). The loop condition itself acts as an assignment operation in this case, causing p to take the value of a[0] (which was pre-incremented to 67) before the loop starts.

#c