Is there a better way to format this while-loop?

Question

While searching for potential errors on a program that I am creating, I got "[Error] ISO C++ forbids comparison between pointer and integer [-fpermissive]."

The error is coming from my while loop, wherein I intended that the program accepts only the following inputs from the user. Right now, I am considering to use an array for this one.

How should I fix this? If anybody any more details, I am more than happy to provide them by showing what I am (as of late).

int main () // While using one of the three functions to make conversions, 
// the program will create the table based on following information. 
{
    char dimension;
    double u1, u2;  
    int starting_value, ending_value, increment, startingUnitDigits, 
    endingUnitDigits;
    cout << "-------------------------------------------------------------------------------------------------";
    cout << "Please answer the following questions as they appear.";
    cout << "Check spelling, use correct abbreviation for units of measurement, and avoid negative values.";
    cout << "-------------------------------------------------------------------------------------------------";
    cout << "Enter the dimensions from the following choices: length, mass and time. (i.e.: 'length,' 'Length').";
    while (!(dimension == "length" || dimension == "Length" || dimension == "mass" || dimension == "Mass" || dimension == "time" || dimension == "Time"))
    {
        cout << "----------------------------------------------------------------------------------------------------";
        cout << "Error! Input for Dimension, " << dimension << ", was either an invalid choice or typed incorrectly.";
        cout << "Please know that this programs accepts length, mass, and time only!";  
        cout << "----------------------------------------------------------------------------------------------------";
        cout << "Enter the dimensions from the following choices: length, mass and time. (i.e.: 'length,' 'Length').";
        cin >> dimension;

`char dimension` its 1 character and you are checking dimension == "length" .. etc — Samer Tufail, Oct 18 '17 at 19:45
`dimension == "length"` compares a `char` with a `const char[7]`, hence the error message. — user0042, Oct 18 '17 at 19:47
@Catholic_Ryanite Use std::string dimension; instead of char dimension; and initialize it before the loop — Vlad from Moscow, Oct 18 '17 at 19:47
btw your title is rather misleading and does not really relate to the question, formatting isnt really the problem (apart from poor intendation maybe) — 463035818_is_not_an_ai, Oct 18 '17 at 19:48
The first time through the loop you'll report an error, even though the user hasn't entered anything. You should put the check *after* the `cin` statement. — Barmar, Oct 18 '17 at 19:49
@Barmar actually the first time through the loop is UB, isnt it? — 463035818_is_not_an_ai, Oct 18 '17 at 19:50
Use `while (true) { cin >> dimension; if (...) { print error message } else {break;}}` — Barmar, Oct 18 '17 at 19:51
@tobi303 If he changes to `std::string`, it defaults to an empty string. — Barmar, Oct 18 '17 at 19:51
Maybe utilize the `switch` statement inside your `while` loop which could then depend on one simple flag whose value is determined in one of the `case` blocks. — Ron, Oct 18 '17 at 19:57

463035818_is_not_an_ai · Answer 1 · 2017-10-18T20:04:26.540

1

The comments are already quite exhaustive, so I just sum them up:

dimension is a single char that you cannot compare to string literals, hence the error. You should use a std::string instead.

Once you changed that the problem remains that you check user input before anything was read from cin. You should do it the other way around.

Your condition will be more readable if you use a container and try to find the user input in that container.

So it would be something like this:

std::vector<std::string> correct_input{"Foo","foo","f00"};
while(true) {
    std::string dimension;
    std::cin >> dimension;
    auto it = std::find(correct_input.begin(),correct_input.end(),dimension);
    if (std::cin.fail() || it==correct_input.end()) {
        // print error and continue to ask for input
        std::cin.clear();  // clear possible error flags
    } else {
       // do something with input
       break;
    }
}

edited Oct 18 '17 at 20:04

answered Oct 18 '17 at 20:00

463035818_is_not_an_ai

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Sorry, this wasn't what I was looking for. – Catholic_Ryanite Oct 18 '17 at 20:17
1

this wasn't what you were looking for? Come on, thats all you comment? What are you looking for?? – 463035818_is_not_an_ai Oct 18 '17 at 20:18
My intentions were to allow the user to ask whether they want length, mass or time for the dimensions. If the user, for whatever reason, did not type their inputs correctly, the program is supposed to let them know that they must use correct spelling. – Catholic_Ryanite Oct 18 '17 at 20:19
@Catholic_Ryanite and thats what my code does, you just have to fill in the blanks – 463035818_is_not_an_ai Oct 18 '17 at 20:19
I get the following errors: [Error] 'vector' is not a member of 'std' [Error] expected primary-expression before '>' token [Error] 'correct_input' was not declared in this scope [Error] 'it' does not name a type [Error] 'it' was not declared in this scope – Catholic_Ryanite Oct 18 '17 at 20:24
@Catholic_Ryanite c++11 enabled? – 463035818_is_not_an_ai Oct 18 '17 at 20:25
What do you mean? – Catholic_Ryanite Oct 18 '17 at 20:26
@Catholic_Ryanite I mean: did you enable c++11 ? – 463035818_is_not_an_ai Oct 18 '17 at 20:27
How do I enable it? Sorry, I am really new to C++ and I may not know what I am supposed to do, or rather, what to look for. – Catholic_Ryanite Oct 18 '17 at 20:28
add the -std=c++11 compiler flag – 463035818_is_not_an_ai Oct 18 '17 at 20:30
Oh, I see. Is that everything? – Catholic_Ryanite Oct 18 '17 at 20:30
1

I think that will quickly degrade to something discussionary-tutorial-ish, what is, to some extent, frowned upon here. @Catholic_Ryanite, if what you are looking for is some kind of interactive wizard/walkthrough/menu/guide-me-by-hand-error-handling/etc, please include the specifics in the question. And, frankly, better do it as a new question, because that one you opened here now is about conversion warning/error and this request you made now is a completely different story. – quetzalcoatl Oct 18 '17 at 20:50
I can only agree with quetzalcoatl and to be honest the best advice I can give you here is to [read a book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) – 463035818_is_not_an_ai Oct 18 '17 at 20:54

Is there a better way to format this while-loop?

1 Answers1