cout of string doesn't show anything

Question

The cout of encrypted string doesn't show anything and sometimes the program crashes. And when i do cout << encrypted[i] in for loop , i got the correct result. Also if i do a for loop to read string char by char for(char c:encrypted) , cout << c << endl; => it doesn't work also and got garbage.

#include <iostream> #include <string> using namespace std; int main() { string alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"; string key = "XZNLWEBGJHQDYVTKFUOMPCIASRxznlwebgjhqdyvtkfuompciasr"; string encrypted; string decrypted; string message; int pos{}; cout << "enter the message" << endl; getline(cin,message); //encrypting for (size_t i{} ;i<message.length();i++) { if (alphabet.find(message[i]) != string::npos) {pos = alphabet.find(message[i]); encrypted[i] = key[pos]; }else {encrypted[i]=message[i]; cout << encrypted[i]; } } cout << "the encrypted message is: "<< encrypted << endl;

encrypted is initialised as an empty string. encrypted[i] = key[pos] does not change that - in fact it gives undefined behaviour. Lack of output is only one possible symptom - your code may be writing to random memory. Either append characters needed to encrypted, or resize the string as needed BEFORE doing such assignments. — Peter
– Peter, Commented Jun 10, 2020 at 2:13

songyuanyao · Accepted Answer · 2020-06-10 01:43:54Z

After string encrypted; encrypted is default-initialized as empty std::string which contains no elements. Then any access to non-existent elements like encrypted[i] leads to UB.

You can use push_back (or operator+=) instead.

//encrypting for (size_t i{}; i<message.length(); i++) { if (alphabet.find(message[i]) != string::npos) { pos = alphabet.find(message[i]); encrypted.push_back(key[pos]); } else { encrypted.push_back(message[i]); cout << encrypted[i]; } }

Or initialize encrypted with message.length() elements in advance.

getline(cin,message); string encrypted(message.length(), '\0'); // initialize encrypted as containing message.length() elements with value '\0' //encrypting for (size_t i{}; i<message.length(); i++) { ...

pvc · Accepted Answer · 2020-06-10 04:25:41Z

from https://en.cppreference.com/w/cpp/string/basic_string/basic_string

Default constructor. Constructs empty string (zero size and unspecified capacity)

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