Why the string is not getting updated and doesn't print anything in the end

Question

CODE:

#include <iostream>
using namespace std;

int main() {

string message, rMessage;
getline(cin,message);

for(int i=0;i<message.length();i++) {

if( (message[i]>='a' && message[i]<='z') ||
    (message[i]>='A' && message[i]<='Z') ||
    (message[i]>='0' && message[i]<='9') ) {

  rMessage[i] = message[i];
}
else {
    continue;
     }
 }
    cout<<"Removed: "<<rMessage;
}

Problem:

It is showing a blank string in output and string length as 0. Is this code is correct to remove punctuation? What edit we must do resolve the problem?

Please don't spam with tags, especially with tags of unrelated languages. — Some programmer dude, Dec 20 '19 at 08:57
Strings do not magically expand when you assign to non-existent elements. Instead, the behaviour is undefined and anything can happen. — molbdnilo, Dec 20 '19 at 08:58
As for your problem, think about the size of `rMessage`... You never set its size anywhere, so any indexing of it will be *out of bounds* and lead to *undefined behavior*. — Some programmer dude, Dec 20 '19 at 08:58
Lastly, Please read about the [`std::isalnum`](https://en.cppreference.com/w/cpp/string/byte/isalnum) function. And C++ have many nice [algorithm](https://en.cppreference.com/w/cpp/algorithm) functions, for example to conditionally copy elements of a container to another container (with [`std::copy_if`](https://en.cppreference.com/w/cpp/algorithm/copy), which could be used instead of your current loop). — Some programmer dude, Dec 20 '19 at 08:58
Don't `operator[]` assign to `rmessage` because it doesn't yet have reserved capacity. Rather use [`operator+=`](https://en.cppreference.com/w/cpp/string/basic_string/operator%2B%3D) or [`append`](https://en.cppreference.com/w/cpp/string/basic_string/append) — acraig5075, Dec 20 '19 at 08:58
Does this answer your question? [Function to trim non, alphabetic, numeric characters from character array](https://stackoverflow.com/questions/13412667/function-to-trim-non-alphabetic-numeric-characters-from-character-array) — arsdever, Dec 20 '19 at 10:30
Please try to make your question title more informative and legible. Also a tip: try to avoid `using namespace std;`. Just use `std::cout` et cetera. — mand, Dec 20 '19 at 10:53

score 3 · Accepted Answer · answered Dec 20 '19 at 09:36

3

What edit we must do resolve the problem?

Change

rMessage[i] = message[i];

To

rMessage += message[i];

The reason is given in the comments section of your question.

answered Dec 20 '19 at 09:36

acraig5075

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Why the string is not getting updated and doesn't print anything in the end

1 Answers1