使用链表实现堆栈的意外结果

Question

我正在编写这个程序，我想了解为什么我得到了错误的打印（应该是 1、2、3、4、5）给出了一些地址。我的stackIsEmpty() 甚至没有像它的意思那样在堆栈为空时停止打印值。这是我的代码：

#include <stdlib.h>
#include <stdio.h>

//Stacks using Structures and linked list and pointerws.

typedef struct Stack{
    //Type of the elements of the stack. Here integers
    int elem;
    //To of the stack
    struct Stack* previous;
} Stack;

void initialize(Stack *st);
void push(Stack *st, int element);
int pop(Stack *st);
int stackIsEmpty(Stack *st);

int main(){
    int i;
    Stack st;
    Stack *pt_st = &st;
    initialize(pt_st);

    //Test phase...
    for(i=0; i<5; i++)
        push(pt_st, i+1);

    for(i=0; i<5; i++)
        printf("here is one element: %d\n", pop(pt_st));
    //End of test with error!
    return 0;
}

void initialize(Stack *st){
    st = NULL;
}

void push(Stack *st, int element){
    Stack *sta = (Stack*)malloc(sizeof(Stack));
    if (sta != NULL)
    {
        sta->previous = st;
        sta->elem = element;
        st = sta;
        free(sta);
    }
    else
        exit(0);
}

int pop(Stack *st){
    if(!stackIsEmpty(st))
    {
        printf("stack is empty cannot pop\n");
        return 0;
    }
    else
    {
        int number = st->elem;
        Stack* copy = st->previous;
        free(st);
        st = copy;
        return number;
    }
}

int stackIsEmpty(Stack *st){
    if(st == NULL)
        return 0;
    else
        return 1;
}

Answer 1

您创建的堆栈指针pt_st 是按值传递给函数的，因此函数只修改它的本地副本。

您可以创建指向指针的指针，或将 pt_st 设为全局变量。

你的push函数也有问题。

我已经修改了你的代码，使 pt_st 成为全局的，所以它现在可以工作了：

#include <stdlib.h>
#include <stdio.h>

//Stacks using Structures and linked list and pointerws.

typedef struct Stack{
    //Type of the elements of the stack. Here integers
    int elem;
    //To of the stack
    struct Stack* previous;
} Stack;

void initialize();
void push(int element);
int pop();
int stackIsEmpty();

Stack *pt_st; // made pt_st global, so functions can modify it

int main(){
    int i;
    initialize();

    //Test phase...
    for(i=0; i<5; i++)
        push(i+1);

    for(i=0; i<5; i++)
        printf("here is one element: %d\n", pop(pt_st));
    //End of test with error!
    return 0;
}

void initialize(){
    pt_st = NULL; // set it to NULL
}

void push(int element){
    Stack *sta = malloc(sizeof(Stack)); // don't need to cast malloc
    sta->elem = element;

    sta->previous = pt_st; // set sta's previous

    pt_st = sta; // point to the pushed node
}

int pop(){
    if(stackIsEmpty())
    {
        printf("pt_stack is empty cannot pop\n");
        return 0;
    }
    else
    {
        int number = pt_st->elem;
        Stack *temp = pt_st; // create a temp copy of the first node's address
        pt_st = pt_st->previous;
        free(temp);
        return number;
    }
}

int stackIsEmpty(){
    if(pt_st == NULL)
        return 1; // if pt_st is NULL, then the stack is empty
    else
        return 0;
}

Answer 2

我认为您需要阅读有关通过副本传递值的内容：What's the difference between passing by reference vs. passing by value?

initialize 什么都不做，你传递一个地址（按值），所以任何东西都会在你main 中报告。这对于每个功能几乎都是一样的。您还可以在分配结构后立即free。 您必须通过地址将指针传递给堆栈，因为堆栈的顶部由您的函数修改：

#include <stdlib.h>
#include <stdio.h>

//Stacks using Structures and linked list and pointerws.

typedef struct Stack{
    //Type of the elements of the stack. Here integers
    int elem;
    //To of the stack
    struct Stack* previous;
} Stack;

void initialize(Stack **st);
void push(Stack **st, int element);
int pop(Stack **st);
int stackIsEmpty(Stack *st);

int main(){
    int i;
    Stack *pt_st;
    initialize(pt_st);

    //Test phase...
    for(i=0; i<5; i++)
        push(&pt_st, i+1);

    for(i=0; i<5; i++)
        printf("here is one element: %d\n", pop(&pt_st));
    //End of test with error!
    return 0;
}

void initialize(Stack **st){
    *st = NULL;
}

void push(Stack **st, int element){
    Stack *sta = (Stack*)malloc(sizeof(Stack));
    if (sta != NULL)
    {
        sta->previous = *st;
        sta->elem = element;
        *st = sta; // new top of stack is sta            
    }
    else
        exit(0);
}

int pop(Stack **st){
    if(!stackIsEmpty(*st))
    {
        printf("stack is empty cannot pop\n");
        return 0;
    }
    else
    {
        int number = (*st)->elem;
        Stack* copy = (*st)->previous;
        free(*st);
        *st = copy;
        return number;
    }
}

int stackIsEmpty(Stack *st){
    if(st == NULL)
        return 0;
    else
        return 1;
}

也尽量避免在你的函数中过早地使用exit，最好返回一个错误代码并让调用者做出决定，比如：

//pseudo code
int push(...) {
   if alloc fails return -1
   else { do the trick; return 0;
}

...
if (push(...)==-1) { print "something's wrong"; exit 0; }

它应该与pop 略有不同，因为您需要在成功时返回一个值，在失败时返回一个错误代码：

int pop(Stack **st,int *pop) {
    if (staksIsEmpty(st)) return -1;
    *pop = *(st.elem);
    ...
    return 0;
}

...
int value;
if (pop(&pt_st,&value)==-1) //error