输入大小为 1 的字符串时出现分段错误

Question

I am getting segmentation fault (core dumped) when i pass string of size 1 as input. I tried searching for the reason but i could not find any , i was asked to debug my program . From debugging i have realised that it throws segmentation fault on reading string of size 1.

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

int main()
{
    ll t,x,y;
    string s;
    cin>>t;
    while(t--){
        cin>>s;
        int tilt=0;
        for(int i=s.length()-1,j=0;i>=s.length()/2;i--,j++){
            if(s[i]!=s[j]){
                tilt = s[i]>s[j]?-1:1;
                s[i]=s[j];
            }
        }
        if(tilt==-1 || tilt==0){
            if(s.length()%2==0){
                y=1;x=s.length()/2-1;
                while(y!=0 && x>=0){
                    if(s[x]=='9'){
                        s[x]='0';
                    }else{
                        s[x] += 1;
                        y=0;
                    }
                    x--;
                }
                if(y==0){
                    for(int i=0;i<s.length()/2;i++){
                        cout<<s[i];
                    }
                    for(int i=s.length()/2-1;i>=0;i--){
                        cout<<s[i];
                    }
                    cout<<endl;
                }else{
                    cout<<1;
                    for(int i=0;i<s.length()/2;i++){
                        cout<<s[i];
                    }
                    for(int i=s.length()/2-2;i>=0;i--){
                        cout<<s[i];
                    }
                    cout<<1<<endl;
                }
            }else{
                y=1;x=s.length()/2;
                while(y!=0 && x>=0){
                    if(s[x]=='9'){
                        s[x]='0';
                    }else{
                        s[x] += 1;
                        y=0;
                    }
                    x--;
                }
                if(y==0){
                    for(int i=0;i<s.length()/2;i++){
                        cout<<s[i];
                    }
                    for(int i=s.length()/2;i>=0;i--){
                        cout<<s[i];
                    }
                    cout<<endl;
                }else{
                    cout<<1;
                    for(int i=0;i<s.length()/2;i++){
                        cout<<s[i];
                    }
                    for(int i=s.length()/2-1;i>=0;i--){
                        cout<<s[i];
                    }
                    cout<<1<<endl;
                }
            }
        }else{
            cout<<s<<endl;
        }
    }
    return 0;
}

Input: 1 11 Output: 22

Input: 1 1 Output: Error

Question : https://www.spoj.com/problems/PALIN/

Answer 1

Here

for(int i=s.length()-1,j=0;i>=s.length()/2;i--,j++){

when s is of size 1, s.length()/2 is an unsigned quantity with value 0. When comparing an integer with an unsigned, the integer is converted to an unsigned before the comparison. Checking if an unsigned value is >= 0 is always true, so this is an infinite loop. That's why it crashes.

I would hope that your compiler gives you warnings about the dangers of signed/unsigned comparisons.

Here's one way to fix this particular loop, but I'd guess you have similar problems elsewhere in your code.

for (size_t i = s.length(), j = 0; i > s.length()/2; ++j) {
    --i;
    if (s[i] != s[j]) {
        tilt = s[i] > s[j] ? -1 : 1;
        s[i] = s[j];
    }
}

Some advice

• Use size_t for index variables.
• When looping backwards be very careful about the termination condition, the way I do it is to make the variable one bigger than it needs to be and decrement it at the beginning of the loop instead of the end. That way you compare with > not >= and avoid the problem your code has.
• Try to use some spaces in your code. It makes it easier to read for everyone, including yourself.