Question

我正在研究的程序是一个下推自动机，它只接受^ nb ^ n的输入：$ ab，$ aabb，$ aaabbb等。关于它的一切似乎都有效，除了事实是在主while循环的4次迭代之后，它得到分段错误。我认为这与我在每个循环中将堆栈复制到另一个堆栈这一事实有关，但我不明白为什么它不会在每次迭代时都出错。非常感谢任何帮助！

#include <iostream>
#include <stack>
#include <string>

using namespace std;

int main()
{
    cout << "Enter an expression: ";
    string expression;
    getline(cin, expression);

    stack<char> expressionStack;
    stack<char> unreadStack;
    stack<char> expressionStackPOP;
    stack<char> unreadStackPOP;
    char expressionOutput;
    char readInput;
    char transitionState = 'p';
    int step=0;
    int counter;
    int trule = 0;
    int rrule = 0;
    char expressionO;
    char unreadO;

    //Enter expression into Stack<char> unreadInput
    for(unsigned int i = 0; i<expression.length(); i++)
    {
        unreadStack.push(expression[i]
            );
    }
    counter = expression.length();
    cout << "Got passed putting the expression in unreadStack" << endl;
    cout <<"Step"<< "\tState" << "\tUnread Input" << "\t\tStack" <<"\t\tΔ Rule used " << "\tR rule used "<< endl;
    while(!unreadStack.empty())
    {
        switch(transitionState)
        {
        case 'p':
            expressionStack.push('S');
            transitionState = 'q';
            trule = 1;
            //cout << "Changed state to P and pushed S into stack" << endl;
            break;
        case 'q':
            if(unreadStack.top() == 'a' && transitionState=='q')
            {
                readInput = unreadStack.top();
                unreadStack.pop();
                transitionState = 'a';
                trule = 2;
                //cout << "Changed state to qa and popped a from unread stack" << endl;
            }
            if(unreadStack.top() == 'b' && transitionState=='q')
            {
                readInput = unreadStack.top();
                unreadStack.pop();
                transitionState = 'b';
                trule = 3;
                //cout << "Changed state to qb and popped b from unread stack"<<endl;
            }
            if(unreadStack.top() == '$' && transitionState=='q')
            {
                readInput = unreadStack.top();
                unreadStack.pop();
                transitionState = '$';
                trule = 4;
                //cout << "This is the end" << endl;
            }
            break;
        case 'a':
            //cout << "in case a" << endl;
            if(readInput=='a' && transitionState=='a')
            {
                expressionStack.pop();
                transitionState='q';
                trule = 5;
                //cout << "Changed state to q and popped a from stack" << endl;
            }
            if(readInput=='S' && transitionState=='a')
            {
                expressionStack.pop();
                expressionStack.push('b');
                expressionStack.push('S');
                expressionStack.push('a');
                transitionState='q';
                trule = 6;
                //cout << "Changed state to q and popped S from stack and pushed aSb into stack" << endl;
            }
            break;
        case 'b':
            if(readInput=='b' && transitionState=='b')
            {
                expressionStack.pop();
                transitionState='q';
                trule = 7;
                //cout << "Changed state to q and popped b from stack" << endl;
            }
            if(readInput=='S' && transitionState=='b')
            {
                expressionStack.pop();
                transitionState='b';
                trule = 8;
                //cout << "Changed state to qb and popped S from stack" << endl;
            }
            break;
        case '$':
            cout<<"test"<<endl;
            break;
            trule = 9;
        }

        //Output RIGHT HERE THIS IS THE SPOT-----------------------------------------------------
        expressionStackPOP = expressionStack;
        unreadStackPOP = unreadStack;
        //--------------------------------------------------------------

        if (step<=9)
            cout<<" ";
        cout <<step<<"\t"<<transitionState<<"\t";
        while(!unreadStackPOP.empty())//copy unreadStackPOP stack to string
        {
            cout<<unreadO;
            unreadO = unreadStackPOP.top();
            unreadStackPOP.pop();
        }
        cout<<"\t\t\t";
        while(!expressionStackPOP.empty())//copy expressionStackPOP stack to string
        {
            cout<<expressionO;
            expressionO = expressionStackPOP.top();
            expressionStackPOP.pop();
        }
        cout<<"\t\t"<<trule<<"\t\t"<<rrule<<endl;
        step++;
    }
}

Answer 1

此代码：

        if(unreadStack.top() == 'a' && transitionState=='q')
        {
            readInput = unreadStack.top();
            unreadStack.pop();
            transitionState = 'a';
            trule = 2;
            //cout << "Changed state to qa and popped a from unread stack" << endl;
        }
        if(unreadStack.top() == 'b' && transitionState=='q')
        {

如果在此代码开头，unreadStack仅包含'a'，请考虑会发生什么。

第二个if语句将尝试做什么？

将堆栈复制到堆栈而不出现分段错误

1 个答案: