为什么我会出现段错误？

Question

I'm a beginner to C++, and am working on a class that grades a student's answer ("11 3 1133 22322314231432211231 34 2") by comparing it to an answer sheet ("112341423114322314231442314231223422").I must compare each value of the student's answer and if they match, add 2 points; if they don't, subtract 1. Now, if the answer is empty, I don't do anything to the student's grade.

Right now, the code is as follows:

//
#ifndef GRADER_H
#define GRADER_H

using namespace std;

#include <iostream>

class Grader {
  protected:
    int punt;
    string answers, id, stans;
  public:
    Grader(){
      answers = "112341423114322314231442314231223422";
      int i;

      char ans_arr[answers.length()];

      for (i = 1; answers.length(); i++){
        ans_arr[i] = answers[i];
      }
    }

    Grader(string ans) {
      answers = ans;
    }

    void Grade (string ans, string id, string stans) {
      int punt = 0;
      for (int i = 0; ans.length(); i++) {
        if (stans[i] == ans[i]){
          punt = punt + 2;
        } else if ((stans[i] != ans[i]) && (stans[i] != ' ')) {
          punt = punt - 1;
        }
      }

      cout << punt;
    }
};

#endif

//Main

#include <iostream>
#include "grader.h"
using namespace std;

int main() {

  string ans = "112341423114322314231442314231223422";

  Grader a(ans);

  string student_id = "12345";
  string student_ans = "11 3 1133 22322314231432211231 34 2";

  int punt = 0;

  a.Grade(ans,student_id,student_ans);

  return 0;
}

With this, I get a segfault. I understand I'm dealing with memory I shouldn't deal with, but I have no idea how to make this work.

Answer 1

Your for-loop conditions are not actually conditions.

From w3schools :

for (statement 1; statement 2; statement 3) {
  // code block to be executed
}

Statement 1 is executed (one time) before the execution of the code block.

Statement 2 defines the condition for executing the code block.

Statement 3 is executed (every time) after the code block has been executed.

Your conditions are, for example, ans.length() , when they should be i < ans.length() .

Since ans.length() will always have the same (positive) value, it will be interpreted as the loop needing to continue and i will continue to be incremented. Then something like ans[i] is actually attempting to look at memory after the end of the array, resulting in a segfault when that out-of-bounds memory is not allocated to your application.

Answer 2

ans.length is 36 and stans.length is 35 . The following code reads from the outside of stans .

for (int i = 0; ans.length(); i++) {
        if (stans[i] == ans[i]){

Here is the memory access error:

  Memory access error: reading from the outside of a memory block; abort execution.
  # Reading 1 bytes from 0x9df62c0 will read undefined values.
  #
  # The memory-block-to-be-read (start:0x9df6290, size:48 bytes) is allocated at
  #    unknown_location (report this ::244)
  #
  #  0x9df6290           0x9df62bf
  #  +--------------------------+
  #  | memory-block-to-be-read  |......
  #  +--------------------------+
  #                              ^~~~~~~~~~
  #        the read starts at 0x9df62c0 that is right after the memory block end.
  #
  # Stack trace (most recent call first) of the read.
  # [0]  file:/prog.cc::28, 13
  # [1]  file:/prog.cc::50, 3
  # [2]  [libc-start-main]

You can use this link to debug segfaults of your code in the future. Just click "Start" to build and run your code in the Terminal.