CODEWITHSUNDEEP
c
sphere
sphere

Reputation: 43

K&R programming exercise 8

Alright, so I'm doing exercise 8 in K&R second edition. Upon looking up the answer after my attempt at doing the exercise didn't print anything but the newlines (the other ints for tabs and empty spaces remained 0 despite running loops to count - I later found out that I used the wrong character for blank space which is just a blank space but it still neglected to count '\t' correctly), I found this:

#include <stdio.h>

int main(void)
{
    int blanks, tabs, newlines;
    int c;
    int done = 0;
    int lastchar = 0;

    blanks = 0;
    tabs = 0;
    newlines = 0;

    while(done == 0)
    {
        c = getchar();

        if(c == ' ')
            ++blanks;
        if(c == '\t')
            ++tabs;
        if(c == '\n')
            ++newlines;

        if(c == EOF)
        {
            if(lastchar != '\n')
            {
                ++newlines;


            }
            done = 1;
        }
        lastchar = c;
    }

    printf("Blanks: %d\nTabs: %d\nLines: %d\n", blanks, tabs, newlines);
    return 0;
}

Now this works fine. K&R is interesting in that it uses ideas not taught to you in the actual text, for instance I tried to run my "while" loop with multiple IFs the same way this one does, except my WHILE loop ran only when getchar was != EOF. I want to know why it didn't work that way.

I found that what they did is a much better idea, creating the int done and then assigning it a 1 instead of 0 at the end of the program was a much better idea, but mine still ran somewhat correctly. (sorry I don't have my own original code this time).

Where I am stumped is what is the purpose of main(void) and return 0;? Before starting this book I found criticism on this but readers claimed it was only in the 1st edition. Here I find that the 2nd edition doesn't teach that but then puts it in the solutions text.

Also, what is the purpose of the int "lastchar"? If getchar(c) is the input and lastchar is always defined as 0, then how could lastchar possibly be changed by any input whatsoever to make it meaningful to the program at all by running a loop to count newlines with it? I see that lastchar is defined as 'c' at the end of the program, but how does that pertain to it being called previously?

Sorry if any of my questions are complicated. Please just answer whatever you can and let me know if you need any further clarification. Just to reiterate I'm very curious why the program can't run a while loop using getchar(c) != EOF, with the same IF statements. Rather than using while done == 0. I feel as if it could be a little shorter/concise (definitely can't say simpler) that way.

Upvotes: 0

Views: 195

Answers (1)

tomlogic
tomlogic

Reputation: 11694

Where I am stumped is what is the purpose of main(void) and return 0;?

In standard C programs, main(0) should return an int, and 0 indicates successful program completion. One could argue that main should have two parameters -- the command-line argument count and an array of arguments, but if your program doesn't make use of arguments then it isn't necessary.

Also, what is the purpose of the int "lastchar"?

And the end of the while loop, the program stores a copy of the current character in the lastchar variable. As you can see in the EOF-handling code, it makes use of lastchar when determining whether the input text ended in a partial line.

I'm very curious why the program can't run a while loop using getchar(c) != EOF, with the same IF statements.

You could code it that way, but the conditional for the while can appear confusing to someone who doesn't have a lot of experience with C: while ((c = getchar()) != EOF). You would also have to move the if (lastchar != '\n') ++newlines; to just outside of the while loop.

Maybe you should make that change to the program and compare it's output to the original for various types of input (empty file, file ending with a newline, file not ending with a newline). Do both programs show the same output? If not, why? Does the modified version still seem more concise? Which would be easier to make changes to in the future?

Many decisions go into a choice of how to structure a program. Even one as simple as this K&R example.

Upvotes: 2

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