CODEWITHSUNDEEP
c++user-inputstdgetline
julius72
julius72

Reputation: 23

Why does getline behave weirdly after 3 newlines?

I'll preface this by saying I'm relatively new to posting questions, as well as C++ in general, my title is a little lame as it doesn't really specifically address the problem I am dealing with, however I couldn't really think of another way to word it, so any suggestions on improving the title is appreciated.

I am working on a relatively simple function which is supposed to get a string using getline, and read the spaces and/or newlines in the string so that it can output how many words have been entered. After reaching the character 'q' it's basically supposed to stop reading in characters.

void ReadStdIn2() {
    
    std::string userInput;
    const char *inputArray = userInput.c_str();
    int count = 0;
    
    getline(std::cin, userInput, 'q');
        
        for (int i = 0; i < strlen(inputArray); i++){
            if ((inputArray[i] == ' ') || (inputArray[i] == '\n')){
                count += 1;
            }
        }   
        
        
    std::cout << count << std::endl;
    
}

I want to be able to enter multiple words, followed by newlines, and have the function accurately display my number of words. I can't figure out why but for some reason after entering 3 newlines my count goes right back to 0.

For example, if I enter:

hello
jim
tim
q

the function works just fine, and returns 3 just like I expect it to. But if I enter

hello
jim
tim
bill
q

the count goes right to 0. I'm assuming this has something to do with my if statement but I'm really lost as to what is wrong, especially since it works fine up until the 3rd newline. Any help is appreciated

Upvotes: 1

Views: 52

Answers (1)

eerorika
eerorika

Reputation: 238391

The behaviour of the program is undefined. Reading input into std::string potentially causes its capacity to increase. This causes pointers into the string to become invalid. Pointers such as inputArray. You then later attempt to read through the invalid pointer.

P.S. calculating the length of the string with std::strlen in every iteration of the loop is not a good idea. It is possible to get the size without calculation by using userInput.size().

To fix both issues, simply don't use inputArray. You don't need it:

for (int i = 0; i < userInput.size(); i++){
    if ((userInput[i] == ' ') || (userInput[i] == '\n')){
        ...

Upvotes: 4

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