getword that properly handles undersores, string constants, comments, or preprocessor control lines

Question

Our version of getword does not properly handle underscores, string constants, comments, or preprocessor control lines. Write a better version.

This is the exercise 6-1 and can be foud on K&R 2 at page 150. http://net.pku.edu.cn/~course/cs101/2008/resource/The_C_Programming_Language.pdf

My solution:

static int isValidKeyWord(char c) {
    if(isalnum(c) || c == '_') {
        return 1;
    }
    return 0;
}

int getword(char *word, int lim) {
    int c;
    char *w = word;

    int ordinaryKeyWord = 0;
    int comment         = 0;
    int stringConstant  = 0;

    while(isspace((c = getch())))
        ;

    if(c == '#' || c == '_' || isalpha(c)) {
        ordinaryKeyWord = 1;
        *w++ = c;
    }
    else if(c == '/') {
        *w++ = c;
        c = getch();
        if(c == '*') {
            *w++ = c;
            comment = 1;
        }
        else {
            *w = '\0';
            return *--w;
        }
    }
    else if(c == '\"') {
        *w++ = c;
        stringConstant = 1;
    }
    else {
        *w++ = c;
        *w = '\0';
        return c;
    }

    for(; --lim; w++) {
        *w = getch();
        if(ordinaryKeyWord && (!isValidKeyWord(*w))) {
            ungetch(*w);
            break;
        }
        else if(stringConstant && *w == '\"') {
            w++;
            break;
        }
        else if(comment && *w == '*') {
            *++w = getch();
            if(*w == '/') {
                w++;
                break;
            }
            else {
                ungetch(*w);
                w--;
            }
        }
    }

    *w = '\0';
    return word[0];
}

There are 3 main cases:

case 1: comments, if the first two characters are / and *. In this case the function should return when the corresponding * and / are met.
case 2: string constants, if the first character is a ", then the function should return when the closing " is met.
case 3: words that begin with #, _ and letters. In this case, the program should return when a character different that _ or an alphanumeric character is met.

First, the program analyzes which case is. Based on that, "it knows" when to stop, then it returns the first character of that word.

Community · Accepted Answer · 2017-04-13 12:40:52Z

I ~~don't~~ didn't immediately see a bug in it and you're doing a lot of things well; so some minor comments.

while(isspace((c = getch())))
    ;

This will often cause a compiler warning "possible missing or empty statement". If you use a compound statement instead that may suppress that warning:

while(isspace((c = getch()))) { }

int ordinaryKeyWord = 0;
int comment         = 0;
int stringConstant  = 0;

These are mutually exclusive, so a 3-valued enum might be better.

for(; --lim; w++)

I don't think your lim testing is strict enough: for example if lim is 8 then /*this*/ would overrun (write past the end of) the input buffer; even an ordinaryKeyWord will write its last '\0' past the end of the buffer.

return *--w;

That's a bit tricky. It would be clearer to return word[0]; everywhere consistently.

    if(ordinaryKeyWord && (!isValidKeyWord(*w))) {
        ungetch(*w);
        break;
    }

That's compact (few lines) but could be expanded to make the logic clearer for a tired reviewer:

    if(ordinaryKeyWord) {
        if (isValidKeyWord(*w)) {
            continue;
        }
        ungetch(*w);
        break;
    }
    if (stringConstant) {
        ... break or continue ...

else if(c == '/') {
    *w++ = c;
    c = getch();
    if(c == '*') {
        *w++ = c;
        comment = 1;
    }
    else {
        *w = '\0';
        return *--w;
    }

You're missing ungetch in the else case.

Is there any way to show end-of-input: pressing <Ctrl>-D for EOF for example? If so, how does getword signal that?

I don't know an easy way to automate code which reads from the keyboard using getch and ungetch.

That's a pity because I'd like to see the automated unit tests which define how well your code works.

For example, this question includes its unit tests: 6 different tests of the function being coded. I was able to identify one or two bugs in the function, not by reading the function but by reviewing the set of unit tests, to find a condition (a set of input data) that wasn't being tested.

My boss wasn't a programmer but used to user-acceptance-test the software before shipping it: he said, not 'you get what you expect' but "You get what you INspect".

Especially when you're beginning you should learn to do unit testing, and take pride in constructing a good/complete set of test cases, which is able to detect bugs in the earlier versions of your software.

Stack Exchange Network

getword that properly handles undersores, string constants, comments, or preprocessor control lines

1 Answer 1

You must log in to answer this question.

Linked

Hot Network Questions

getword that properly handles undersores, string constants, comments, or preprocessor control lines

1 Answer 1

You must log in to answer this question.

Linked

Related

Hot Network Questions