请求解释给定C语言代码的运行逻辑及代码中[]的语法概念

Let’s start by looking at the full code snippet:

#include<stdio.h>
int f(int t[]){ return t[0] + t[2]; }
int main(void){ int i,a[] = {-2,-1,0,1,2}; i = f(a+2); printf("%d",i); return 0; }

Step-by-Step Running Logic #

1. Array Initialization in main:
   The array a is initialized with values {-2,-1,0,1,2}. In C, arrays are zero-indexed, so:

   • a[0] = -2, a[1] = -1, a[2] = 0, a[3] = 1, a[4] = 2

2. Passing the Array to Function f:
   The line i = f(a+2) does two key things:

   • a is the base address of the array (points directly to a[0]). Adding 2 to this pointer uses pointer arithmetic to move it forward by 2 int-sized steps, so a+2 now points to a[2] (the value 0).
   • When you pass an array to a function in C, the array name decays to a pointer. Even though f is declared to take int t[], this is actually equivalent to int *t (a pointer to an integer). The [] here is just syntax that hints the parameter is meant to be used like an array, but it’s treated as a pointer under the hood.

3. Execution Inside Function f:
   The function returns t[0] + t[2]. Let’s unpack this:

   • t[0] is the value at the pointer t (which is a[2], so 0).
   • t[2] is the value 2 positions ahead of t in memory. Since t points to a[2], t[2] translates to a[2+2] = a[4] (value 2).
   • Summing these gives 0 + 2 = 2, which is returned to main and stored in i.

4. Output:
   printf("%d",i) prints the value 2, which is the final output of the program.

What’s the [] Syntax in C? #

The square bracket operator [] is primarily used for array indexing, but it’s actually syntactic sugar for pointer arithmetic. Here’s the core rule:

• For any pointer or array x and integer y, x[y] is exactly equivalent to *(x + y).

Let’s confirm this with our code:

• t[0] → *(t + 0) → *t (the value at the pointer t).
• t[2] → *(t + 2) (the value 2 integers ahead of t).

A fun quirk of this equivalence is that since addition is commutative, y[x] works the same as x[y] (e.g., 2[t] would be identical to t[2] in our function), though this is rarely used in practice because it hurts readability.

When you see int t[] in a function parameter list, remember this is just another way to write int *t—C doesn’t pass entire arrays to functions, only pointers to their first element. The [] syntax is a readability hint that the parameter is intended to be accessed like an array.

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