按值返回结构每次都会给出相同的错误答案

Question

I'm trying to return a struct by value to find the position of a node in a tree. However using a wrapper to simplify the function call returns the incorrect value.

Relevant code:

typedef struct {
    uint16_t x;
    uint16_t y;
} coordinate_t;

coordinate_t node_pos_(uint16_t x, uint16_t y, node_t *node, node_t *find) {
    printf("%u, %u\n", x, y);

    if (node == find) {
        printf("found node at %u, %u\n", x, y);
        coordinate_t coords;
        coords.x = x;
        coords.y = y;
        return coords;
    }

    for (uint16_t i = 0; i < node->child_count; i++) {
        node_pos_(x + i, y + 1, node->children[i], find);
    }
}

coordinate_t node_pos(node_t *root, node_t *node) { 
    return node_pos_(0, 0, root, node);
}

int main() {
    coordinate_t coords = node_pos(root, child2);

    printf("coordinates of %s: %u, %u\n", child2->name, coords.x, coords.y);

    return 0;
}

The output:

0, 0
0, 1
0, 2
1, 2
1, 1
found node at 1, 1
coordinates of child2: 2, 0

Answer 1

Currently, your node_pos_ function does not return a value in all execution paths, and has no way to indicate to the caller whether the node was found or not. Both of those are essential for a recursive algorithm that searches for a node in a tree.

In keeping with the spirit of returning a coordinate_t by value, I have reserved the coordinate pair ( UINT16_MAX , UINT16_MAX ) to represent the "not found" condition.

The modified function is below:

coordinate_t node_pos_(uint16_t x, uint16_t y, node_t *node, node_t *find) {
    coordinate_t coords;

    printf("%u, %u\n", x, y);

    if (node == find) {
        printf("found node at %u, %u\n", x, y);
        coords.x = x;
        coords.y = y;
        return coords;
    }

    // look for node in children
    for (uint16_t i = 0; i < node->child_count; i++) {
        coords = node_pos_(x + i, y + 1, node->children[i], find);
        if (!(coords.x == UINT16_MAX && coords.y == UINT16_MAX)) {
            // found
            return coords;
        }
    }

    // not found
    coords.x = UINT16_MAX;
    coords.y = UINT16_MAX;
    return coords;
}

---

As pointed out by @yano, the use of the %u printf format specifier to print a uint16_t value is not portable. A simple fix is to cast the values to unsigned int as below:

        printf("found node at %u, %u\n", (unsigned)x, (unsigned)y);

The "proper" way to fix it, avoiding the type cast, is to use the printf format specifier macros from #include <inttypes.h> as below:

        printf("found node at %" PRIu16 " , %" PRIu16 "\n", x, y);