How to make sure nothing else is in array

Question

I'm doing an assignment for school and I seem to have a problem with other characters being in the array. (Amongst other problems...)

In one function, I declared the array..

const int MAXSIZE = 100;
char inFix[MAXSIZE];

And this code is used to put chars into the array

 //loop to store user input until enter is pressed
    while((inputChar = static_cast<char>(cin.get()))!= '\n')
    {
        //if function to decide if the input should be stored or not
        if(isOperator(inputChar) || isdigit(static_cast<int>(inputChar)) || inputChar == '(' || inputChar == ')')
        {
            inFix[a] = inputChar; //stores input
            a++;
        }

    }

At the end of this, I append the null character to the array though I wasn't sure if I should do this:

 inFix[MAXSIZE] = '\0';

Or if I should've used strcat.. either way... in my next function, I use strcat to append a parenthesis to the end.

But I've been having problems with the code, so I ran a for loop to print what is inside the infix array at the beginning of my next function, just to see...

And I get this annoying beeping sound, and a string of weird characters like hearts, and music signs... and.. a whole list of odd characters. What could be the problem? Thanks.

EDIT: by the way, I input 9*4, and I run the for loop after i append the parenthesis, so at the beginning of the output, I get:

9*4) and then the string of odd characters...

Answer 1

You declared the array with:

char inFix[MAXSIZE];

At this point in time, the content of the array is not initialized. Printing out these uninitialized characters will lead to the behavior you see (strange characters, sounds, etc.).

However, the contents of that array are not initialized. If you want to initialize this array, change this line to:

char inFix[MAXSIZE] = {0};

This initialization syntax causes the elements in the array to be initialized. The first elements are initialized to the values provided in the brackets, while any values not in the brackets will be initialized to 0. In the case above, all of the array elements will be initialized to 0 (which happens to correspond to the NULL character \0).

Since all the characters are initialized to NULL in this case, you don't need to append the null character after your input.

That said, a better solution would be not to use a C-Style array at all, but a std::vector<char>, or simply a std::string, as pointed out above.

Answer 2

so I ran a for loop to print what is inside the infix array at the beginning of my next function, just to see...

And I get this annoying beeping sound, and a string of weird characters like hearts, and music signs... and.. a whole list of odd characters. What could be the problem?

The problem is that you're printing out array elements which you never initialized. The answer you have currently accepted advises you to initialize all those elements. Although this will not cause errors it is a mistake to let this answer prevent you from fully understanding the problem you encountered.

Reconsider your code where you inserted a null character at the end of the array:

inFix[MAXSIZE] = '\0';

You apparently know that a null character has something to do with marking the end of the string, but you've mistaken how to do that correctly. Everything from the beginning of the array until a null character will be treated as part of your string. If you copy three characters from the input 9*4 into your array then you should only want those three characters to be seen as part of your string. What you do not want is for everything in the array past those three characters, up to MAXSIZE to also be treated as part of your string. So you need to put the end-of-string marker, the '\0', right after the characters you care about.

(BTW, inFix[MAXSIZE] = '\0'; not only puts the end-of-string marker at the end of the array, it puts it outside the array, which you are not allowed to do. The program will behave unpredictably.)

inFix[0] = '9';
inFix[1] = '*';
inFix[2] = '4';
inFix[3] = '\0'; // <-- this is where you need to put the end-of-string marker, because this is the end of the characters you care about.

Putting the end-of-string marker at the end of the array effectively does this:

inFix[0] = '9';
inFix[1] = '*';
inFix[2] = '4';

inFix[3] = ???
inFix[4] = ???
   .
   .
   .
inFix[98] = ???
inFix[99] = ??? // annoying bell noise? musical note symbol?

inFix[100] = '\0'; // Yes, Please!

The reason initializing the array to all zeros (which can also be done like this char inFix[MAXSIZE] = {};, empty braces instead of a 0) worked for you is because that means that no matter where you stop writing characters you care about, the next character will be a '\0'. That position, right after the characters you care about, is the only place it matters.

Since the loop that's copying the characters knows exactly where it stops it also knows exactly where to insert the end-of-string marker. It would be easy to just insert a single '\0' in the correct place.

Answer 3

You say you null terminate string using inFix[MAXSIZE] = '\0' as you currently understand this should converted into inFix[MAXSIZE - 1] = '\0' but in this case you say that you have an string that ended in last possible character!! so how you should add something( for example a parenthesis ) to it?? because it is full and it can't accept any more characters.

So may be this is better to use inFix[a] = '\0' because you know that a point to end of string, and now if a < MAXSIZE then you have enough space to add more items to the string otherwise you may increase MAXSIZE (using strcat for example) or show some error to the user.

Answer 4

Try appending the '\0' to position a in the array instead - that is, exactly after the last char you've read. Otherwise you're putting your characters in the array, then a random sequence of what was in the array before, and then after that is the \0 (or, in this case, it's one after the end of the array, which is even worse).