Reading user input gives Segmentation fault but not with same input when hardcoded

Question

Before posting this question, I checked the existing questions related to Segmentation fault and yet I am unable to resolve my error.
I am trying to run a c++ program for the following input but I am facing segmentation fault error.

Input:

4
gitohro qf ejvh i
3
gor
pih
sto

Expected output:

gitohro i

I get the expected output when I hardcode the input in my program. But when I take the input from user/console, I get Segmentation fault error.

Actual error message:

/bin/sh: line 1: 22094 Segmentation fault: 11 gtimeout 4s ./main < input.txt > output.txt [Finished in 1.3s with exit code 139] [cmd: ['g++-11 main.cpp -o main && gtimeout 4s ./main<input.txt>output.txt']]

I checked all the indexes to see if it is array out of bound issue but I am not sure what I am doing wrong. Please guide.

My complete code:

#include <bits/stdc++.h>
using namespace std;

    struct node{    //TrieNode
        char c;
        int ends;
        string word;
        node *child[26];
    };
    struct node *getNode(char c)    //get newnode
    {
        node *newnode = new node;
        newnode->c = c;
        newnode->ends = 0;
        newnode->word = "";
        for(int i=0;i<26;++i)
            newnode->child[i] = NULL;
        return newnode;
    }
    node *root = getNode('/');  //root
    
    //Trie INSERT
    void insert(string s)
    {
        node *curr=root;
        int index,i=0;
        while(s[i])
        {
            index = s[i]-'a';
            if(curr->child[index]==NULL)
                curr->child[index] = getNode(s[i]);
            
            curr=curr->child[index];
            i+=1;
        }
        curr->ends += 1;
        curr->word = s;
    }
    
    void solve(vector<vector<char>>& board,int i,int j,int r,int c,vector<string>& ans,node *curr)
    {
        //Base case
        //If the trie doesn't have the current char OR cell is Visited
        int index = board[i][j]-'a';
        if(board[i][j]=='$' || curr->child[index]==NULL)  
            return;
        
        curr = curr->child[index];
        if(curr->ends > 0)
        {
            ans.push_back(curr->word);
            curr->ends -=1;
        }
        
        //Body
        char ch = board[i][j];   //Store current char
        board[i][j] = '$';  //Mark current node visited
        
        if(i>0)     //TOP
            solve(board,i-1,j,r,c,ans,curr);
        if(i<r-1)   //DOWN
            solve(board,i+1,j,r,c,ans,curr);
        if(j>0)     //LEFT
            solve(board,i,j-1,r,c,ans,curr);
        if(j<c-1)   //RIGHT
            solve(board,i,j+1,r,c,ans,curr);
        if(i>0 && j>0)     //TOP LEFT
            solve(board,i-1,j-1,r,c,ans,curr);
        if(i<r-1 && j>0)   //DOWN LEFT
            solve(board,i+1,j-1,r,c,ans,curr);
        if(i<r-1 && j<c-1)     //DOWN RIGHT
            solve(board,i+1,j+1,r,c,ans,curr);
        if(i>0 && j<c-1)   //TOP RIGHT
            solve(board,i-1,j+1,r,c,ans,curr);
        
        board[i][j] = ch;    //Mark current node as Unvisited by restoring the value
    }
    
    vector<string> findWords(vector<vector<char>>& board, vector<string>& words) {        
        int r=board.size();
        int c=board[0].size();
        
        //Insert all words in TRIE
        for(int i=0;i<words.size();++i)
            insert(words[i]);
        
        //Now search words
        vector<string> ans;
        for(int i=0;i<r;++i)
        {
            for(int j=0;j<c;++j)
                solve(board,i,j,r,c,ans,root);
        }
        return ans;
    }

int main()
{
    #ifndef ONLINE_JUDGE
        freopen("input.txt",  "r",  stdin);
        freopen("output.txt", "w", stdout);
    #endif    
    vector<string> w ;    
    vector<vector<char>> x;   
    int o,p;
    cin>>o;
    for (int i = 0; i < o; ++i)
    {
        cin>>w[i];
    }
    cin>>p;

    for (int i = 0; i < p; ++i)
    {
        for (int j = 0; j < p; ++j)
        {
            cin>>x[i][j];
        }
    }    

    /*
    vector<string> w = {"gitohro", "qf", "ejvh", "i"};
    vector<vector<char>> x = {
                            {'g','o','r'},
                            {'p','i','h'},
                            {'s','t','o'}                           
                        };
    */
    vector<string> res = findWords(x,w);
    
    for(auto i:res)
        cout<<i<<" ";  

    return 0;
}


/*
Input 1:
4
gitohro qf ejvh i 
3
gor
pih
sto

Output 1:
gitohro i 

*/

Does this answer your question? [How to cin values into a vector](https://stackoverflow.com/questions/8377660/how-to-cin-values-into-a-vector). This [answer](https://stackoverflow.com/a/61731245/2089675) on that page should be most useful — smac89, Sep 25 '21 at 21:22
It works when you hard code the values because you already initialized the vector to hold the exact amount of values it needs. — smac89, Sep 25 '21 at 21:23
Another link to show you how to make the vector big enough for what you need: https://stackoverflow.com/questions/10559283/how-to-create-a-vector-of-user-defined-size-but-with-no-predefined-values — smac89, Sep 25 '21 at 21:25

Remy Lebeau · Accepted Answer · 2021-09-25T21:42:07.980

Your main() is reading user input into empty vectors. You are not adding any entries to them, so your use of vector::operator[] is causing undefined behavior (if you were to replace vector::operator[] with vector::at(), you would get std::out_of_range exceptions thrown).

When you hard-code the input, you are populating the vectors with actual entries correctly.

To fix this, try the following instead:

int main()
{
    #ifndef ONLINE_JUDGE
        freopen("input.txt",  "r",  stdin);
        freopen("output.txt", "w", stdout);
    #endif

    vector<string> w;
    vector<vector<char>> x;
    int o, p;

    cin >> o;
    w.resize(o); // <-- ADD THIS!
    for (int i = 0; i < o; ++i)
    {
        cin >> w[i];
    }

    cin >> p;
    x.resize(p, vector<char>(p)); // <-- ADD THIS!
    for (int i = 0; i < p; ++i)
    {
        for (int j = 0; j < p; ++j)
        {
            cin >> x[i][j];
        }
    }    

    vector<string> res = findWords(x, w);
    
    for(auto i : res)
        cout << i << " ";  

    return 0;
}

Alternatively:

int main()
{
    #ifndef ONLINE_JUDGE
        freopen("input.txt",  "r",  stdin);
        freopen("output.txt", "w", stdout);
    #endif

    int o, p;

    cin >> o;
    vector<string> w(o); // <-- INIT HERE!
    for (int i = 0; i < o; ++i)
    {
        cin >> w[i];
    }

    cin >> p;
    vector<vector<char>> x(p, vector<char>(p)); // <-- INIT HERE!
    for (int i = 0; i < p; ++i)
    {
        for (int j = 0; j < p; ++j)
        {
            cin >> x[i][j];
        }
    }    

    vector<string> res = findWords(x, w);
    
    for(auto i : res)
        cout << i << " ";  

    return 0;
}

Alternatively, use vector::push_back() instead of vector::operator[]:

int main()
{
    #ifndef ONLINE_JUDGE
        freopen("input.txt",  "r",  stdin);
        freopen("output.txt", "w", stdout);
    #endif

    vector<string> w;
    vector<vector<char>> x
    int o, p;

    cin >> o;
    w.reserve(o);
    for (int i = 0; i < o; ++i)
    {
        string s;
        cin >> s;
        w.push_back(s);
    }

    cin >> p;
    x.reserve(p);
    for (int i = 0; i < p; ++i)
    {
        vector<char> v;
        v.reserve(p);
        for (int j = 0; j < p; ++j)
        {
            char ch;
            cin >> ch;
            v.push_back(ch);
        }
        x.push_back(v):
    }    

    vector<string> res = findWords(x, w);
    
    for(auto i : res)
        cout << i << " ";  

    return 0;
}

Reading user input gives Segmentation fault but not with same input when hardcoded

1 Answers1