C++代码出现栈错误，求错误原因及修复方案

Hey fellow C++ beginner! Let's break down why your code crashes with longer strings and fix it step by step.

核心崩溃原因 #

• You initialized n1 as an empty string (length 0). When you directly assign values using n1[i] = '.', this is out-of-bounds memory access. The std::string operator[] doesn't automatically expand the string's length—short strings might not trigger a crash by chance, but longer ones will definitely mess with invalid memory and cause your program to crash.
• A minor side note: Using endl for every valid character will print each one on a new line. That's fine if it's what you want, but just wanted to call it out!

两种修复方案 #

方案一：直接生成目标字符串（推荐，更高效简洁） #

Instead of pre-filling with ., we can just append valid characters to n1 as we go. This completely avoids any out-of-bounds issues:

#include<iostream>
#include<string>
using namespace std;

int main(){
    string n;
    string n1;
    cin >> n;
    
    // Use range-based for loop for cleaner traversal
    for(char c : n) {
        int ascii_val = static_cast<int>(c);
        // Check if character is between 'd'(100) and 'n'(110)
        if(ascii_val > 99 && ascii_val < 111) {
            n1 += c;
        }
    }
    
    // Print the final filtered string (remove endl if you don't want a newline)
    cout << n1 << endl;
    // If you still need to print each valid character on a new line:
    // for(char valid_char : n1) {
    //     cout << valid_char << endl;
    // }
    
    return 0;
}

方案二：保留 original length, fill invalid positions with . #

If you specifically need n1 to match the length of n (with . for invalid characters), we just need to resize n1 first to make all indices valid:

#include<iostream>
#include<string>
using namespace std;

int main(){
    string n;
    string n1;
    cin >> n;
    int l = n.length();
    
    // Resize n1 to match n's length, fill all positions with '.'
    n1.resize(l, '.');
    
    for(int i=0; i<l; i++){
        int ascii_val = static_cast<int>(n[i]);
        if(ascii_val > 99 && ascii_val < 111){
            n1[i] = n[i];
        }
    }
    
    // Print all non '.' characters
    for(int i=0; i<l; i++){
        if(n1[i] != '.'){
            cout << n1[i] << endl;
        }
    }
    
    return 0;
}

Quick Tips #

• Using static_cast<int>(c) to convert a char to its ASCII value is more explicit and safer than direct assignment.
• Range-based for loops (for(char c : n)) make string traversal cleaner and less error-prone than manual index tracking.

内容的提问来源于stack exchange，提问作者whomanx