C语言equilibrium函数返回-1是否为笔误？求技术解析

Hey there! Let's break down this code and why returning -1 isn't a typo at all—it's intentional, correct behavior for a valid edge case.

First, let's clarify what this function does: it's searching for an equilibrium index in an array. An equilibrium index is a position where the sum of all elements to the left of the index equals the sum of all elements to the right.

Here's the code for reference:

int equilibrium(int array[], int size) { 
    int sum = 0; 
    int left_sum = 0; 
    int i; 
    for (i = 0; i < size; i++) { 
        sum += array[i]; 
    } 
    for (i = 0; i < size; i++) { 
        if (array[i] == sum - 2 * left_sum) { 
            return i; 
        } 
        left_sum += array[i]; 
    } 
    return -1; 
}

Let's walk through the logic #

1. The first loop calculates the total sum of the entire array and stores it in sum.
2. The second loop iterates through each element, maintaining left_sum—the cumulative sum of elements to the left of the current index.
3. The condition array[i] == sum - 2 * left_sum comes from rearranging the equilibrium check:
   • The sum of elements to the right of index i is sum - left_sum - array[i] (total sum minus left sum minus the current element).
   • We want left sum = right sum: left_sum = sum - left_sum - array[i]
   • Rearranging gives us the code's condition—this is a clever way to avoid recalculating the right sum every time.
4. If an equilibrium index is found, the function returns it right away.

Why return -1? #

Not every array has an equilibrium index! For example, take the array [1, 2, 3, 4]—no position exists where the left sum equals the right sum.

In cases like this, after checking every element and finding no match, the function returns -1 to signal that no equilibrium index exists in the input array. This is a standard convention in C for search-style functions: return -1 when the target isn't found (you'll see this pattern in standard library functions like strchr too).

So that -1 isn't a mistake—it's the proper way to handle the scenario where the array has no equilibrium point.

内容的提问来源于stack exchange，提问作者SummerGorilla