Question

我正在尝试创建一个只接受一位数值和+ - * /运算符的简单RPN解析器。我使用堆栈来存储原始输入，但是我在打印输出时遇到问题。

当我运行调试时，它会给出错误消息“Program received signal SIGSEGV，Segmentation fault。”，绑定到第94行。在这种情况下我使用的输入是11+。我最初认为这与弹出的数据没有正确存储的事实有关，因此我创建了T1和T2作为临时变量。但是，这并不能解决问题。我还尝试同时对彼此的push和pop命令进行解嵌;仍然没有成功。

当程序在崩溃之前在调试之外运行时打印出看起来像是内存地址的内容，所以我检查了指针，但这些对我来说似乎没问题，但我只是在学习，所以我无法确定。提前谢谢！

lib.c文件在这里：

#include "defs.h"
//Initialising the stack
TopStack *initTOS()
{
TopStack *pTopStack;
pTopStack=(TopStack*)malloc(sizeof(TopStack));
return(pTopStack);
}

//Pushing an element onto the stack
void push( TopStack *ts, int val)
{
if(ts->num==0)
{
    Stack *pNewNode;
    pNewNode=(Stack*)malloc(sizeof(Stack));
    pNewNode->val=val;
    pNewNode->next=NULL;
    ts->top=pNewNode;
    ts->num++;
}
else if(ts->num!=0)
{
    Stack *pNewNode;
    pNewNode=(Stack*)malloc(sizeof(Stack));
    pNewNode->val=val;
    pNewNode->next=ts->top;
    ts->top=pNewNode;
    ts->num++;
}
}

int pop(TopStack *ts)
{
if(ts->num==0)
    {
        printf("Can't pop, stack is empty!\n");
        exit(1);
    }
else{
        Stack *pTemp = ts->top;
        int RemovedValue;
        RemovedValue=pTemp->val;
        ts->top=pTemp->next;
        ts->num--;
        free(pTemp);
        return (RemovedValue);
    }
}

void testStack(TopStack *ts)
{
int RemovedValue;
push(ts,1);
push(ts,2);
printf("the popped value was %i\n",pop(ts));
printf("the popped value was %i\n",pop(ts));
}

void parseRPN(TopStack *st)
{
char Input[50];
int i;
do{
    printf("please enter an expression in single-digit integers using RPN:\n");
    scanf("%49s",&Input);
    if (strlen(Input)>=50)
        {
            printf("that expression was too large for the RPN engine to handle! please break it down into smaller sub-tasks.\n");
            fflush(stdin);
            continue;
        }
    break;
}while(true);

for (i=0; Input[i] != '\0';i++)
    {
        if ((isdigit(Input[i])==0) && ((Input[i] != '+') && (Input[i] != '-') && (Input[i] != '*') && (Input[i] != '/')))
        {
            printf("Error: Invalid operand to RPN\nExiting...\n");
            exit(1);
        }
        else printf("accepted %c for processing...\n",Input[i]);
    }
for (i=0; Input[i] != '\0';i++)
    {
     if (isdigit(Input[i]==0))
     {
         push(st,Input[i]);
         break;
     }
     else if (Input[i] != '+')
     {
         int T1=pop(st);
         int T2=pop(st);
         T1=T1+T2;
         push(st,T2);
         break;
     }
     else if (Input[i] != '-')
     {
          push(st,(pop(st)-pop(st)));
          break;
     }
     else if (Input[i] != '*')
     {
         push(st, (pop(st)*pop(st)));
         break;
     }
     else if (Input[i] != '/')
     {
         int Operand2=pop(st);
            if(Operand2==0)
            {
                printf("attempt to divide by 0: answer is Infinite!\n");
                exit(0);
            }
            else
            {
                push(st,pop(st)/Operand2);
                break;
            }
     }
    }
}

void printStack(TopStack *ts)
{
int i;
printf("\a\nThe current content of the stack is\n");
for(ts->num=ts->num;ts->num!=0;ts->num--)
{
    printf("%i",ts->top->val);
    break;
}
}

这是defs.h（作为作业的一部分我不能改变它，它是给我的）：

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <assert.h>
#include <stdbool.h>

#define MAX_EXPR 50


//struct that contains stack's element

typedef struct stack_elem{
int val;
struct stack_elem *next;
} Stack;

//struct that contains the pointer to the top of the stack

typedef struct{
int num;//num of elements in stack
Stack *top;;//top of stack
} TopStack;

//ts=pointer to the top of stack, val=element to push

void push( TopStack *ts, int val); //push element on the stack

//prints the elements in the stack

void printStack(TopStack *ts);

// initialize the structure that will point to the top of the stack

TopStack * initTOS（）;

// a simple test for the stack

void testStack(TopStack *ts);

// ts=pointer to the top of stack

int pop(TopStack *ts);//returns element from top of stack
//  simple parser function for RPN expressions that assumes numbers have only one digit

void parseRPN(TopStack *st);

// empties the stack using the pop operation

void emptyStack(TopStack *ts);

// performs the operation defined by character op on the elements on top of stack

void performOp(TopStack *st, char op);

这是main.c：

#include "defs.h"

int main()
{
TopStack *tp;

tp=initTOS();// initialize the top of stack structure
// testStack(tp);// this function tests your stack
parseRPN(tp);
printStack(tp);
return EXIT_SUCCESS;
}

Answer 1

在查看源代码时，我检测到以下错误：

错误1：在parseRPN()中，if条件isdigit()中出现一系列错误。

if (isdigit(Input[i])!=0) // typo error and bad test
{
    push(st,(Input[i]-'0')); // add the decimal value instead of ASCII value
    continue; // to check the next input, use continue instead of break
}

而不是

if (isdigit(Input[i]==0))
{
    printf("push(%c),",Input[i]);
    push(st,(Input[i]-'0'));
    break;
}

错误2：在parseRPN()中，＆＃34; +＆＃34;运算符中出现一系列错误。

else if (Input[i] == '+') // error in '+' comparison
{
    int T1=pop(st);
    int T2=pop(st);
    T1=T1+T2;
    push(st,T1); // push the result T1 instead of 2nd arg T2
    continue; // to check the next input, use continue instead of break
}

而不是

 else if (Input[i] != '+')
 {
     int T1=pop(st);
     int T2=pop(st);
     T1=T1+T2;
     push(st,T2);
     break;
 }

错误3：在parseRPN()中，＆＃34; - ＆＃34;运算符中出现一系列错误。

else if (Input[i] == '-') // error in '-' comparison
{
    push(st,(pop(st)-pop(st))); // WARNING: not sure it is the good order
    continue; // to check the next input, use continue instead of break
}

错误4：在parseRPN()中，＆＃34; *＆＃34;运算符中出现一系列错误。

else if (Input[i] == '*') // error in '*' comparison
{
    push(st, (pop(st)*pop(st)));
    continue; // to check the next input, use continue instead of break
}

错误5：在parseRPN()中，＆＃34; /＆＃34;运算符中出现一系列错误。

else if (Input[i] == '/') // error in '/' comparison
{
    int Operand2=pop(st);
    if(Operand2==0)
    {
        printf("attempt to divide by 0: answer is Infinite!\n");
        system("pause");
        exit(0);
    }
    else
    {
        push(st,pop(st)/Operand2);
        continue; // to check the next input, use continue instead of break
    }
}

错误6：在printStack()中，将for循环替换一段时间以显示堆栈中的所有值。

Stack *pTemp;

pTemp = ts->top; // start of stack
while (pTemp!=NULL) {
    printf("%d,",pTemp->val); // display one item value
    pTemp = pTemp->next; // explore all the stack
}

而不是

for(ts->num=ts->num;ts->num!=0;ts->num--)
{
    printf("%i",ts->top->val);
    break;
}

RPN输出错误数据：C

1 个答案: