1. On the first execution of the inner while loop it accesses line[MAX_LINE] which is an illegal array index
2. It accesses uninitialized array elements - fgets only assigns to the array elements necessary to hold the characters of a line plus a '\0'
3. There may not be a '\n' in the array. fgets won't put a '\n' in the array if the line is too long to fit in the array. If there is no '\n' in the array the code will access an non-existent array element (line[-1]).

#include <stdio.h>

#define MAX_LINE 4096

int
main(void) {
    char line[MAX_LINE];
    int  i;

    while (fgets(line, MAX_LINE, stdin) != NULL) {
        i = 0;
        while (line[i] != '\n' && line[i] != '\0') {
            i = i + 1;
        }
        printf("line %d characters long\n", i);
    }
    return 0;
}

// reads lines from its input
// and prints how many characters each line contains
// only use fgets and printf
#include <stdio.h>
#include <stdlib.h>

#define MAX 4096	// max number of characters in a line

int main (int argc, char* argv[]) {

	int numC;			// counter for number of characters in line
	char line[MAX];	// array to store the input

	// keep on reading until EOF or error
	while (fgets(line, MAX, stdin) != NULL) {
		// count the number of characters
		numC = 0;
		// each elt in the array line is a character (alphanumeric or whitespace)
		// unless the elt. is a newline or null terminating character
		// in which case the line has stopped
		// so numC is also the index for your line array
		while (line[numC] != '\n' && line[numC] != '\0') {
			// this is a character
			numC++;	// now go test the next one
		}

		// print your result
		printf("line %d characters long\n", numC);

	}


	return EXIT_SUCCESS;
}

#include <stdio.h>

#define MAX_LINE 4096

int
main(void) {
    char line[MAX_LINE];
    int  i;

    while (fgets(line, MAX_LINE, stdin) != NULL) {

        // strip // style comments from input

        i = 0;
        while (line[i] != '\n' && line[i] != '\0') {

            // safe to look at line[i+1] because
            // we know here line[i] != '\0'

            if (line[i] == '/' && line[i + 1] == '/') {

                // replace // with end of -line
                line[i] = '\n';
                line[i + 1] = '\0';

                // could break here but loop will stop anyway
            }

            i = i + 1;
        }

        // write possibly-modified line

        printf("%s", line);
    }
    return 0;
}

// reads lines from its input
// prints them after removing an // style comments
// so DOES NOT PRINT // and everything after it
#include <stdio.h>
#include <stdlib.h>

#define MAX 4096

int main (int argc, char* argv[]) {

	char line[MAX];		// to store input
	int i;				// counter to iterate through each line you read in

	// get input until EOF or error
	while (fgets(line, MAX, stdin) != NULL) {
		// iterate through line and see if there is a comment, action appropriately
		// comments start with '/' and are followed by '/'
		// note the "base case" for the input for line is a 1 elt. array with '\n'
		// i.e. when you just hit enter
		// so lines arrays have at least 1 elt. initialized
		i = 0;
		while (line[i] != '\n' && line[i] != '\0') {
			// check if this is a comment
			// first elt. not newline or null character, so we have at least one other char.
			// i.e. line[i + 1] is legal
			if (line[i] == '/' && line[i+1] == '/') {
				// we have the start of a comment
				// printf stops printing strings at a null-terminating character '\0'
				// so "remove" comment and everything after by changing to '\0'
				
				line[i] = '\n';			// helps the user so lines print on a new line, 
				// could go without this line as long as you print later like this
				// printf("%s\n", line);
				
				// this is the necessary one to stop printing
				line[i + 1] = '\0';

				// break;

				// or loop will stop on next iteration since line[i] == '\n'
			}

			i++;
		}

		// write line (modified if it has comments)
		printf("%s", line);

	}


	return EXIT_SUCCESS;
}

#include <stdio.h>

#define MAX_LINE 4096

int
main(void) {
    char line[MAX_LINE];
    int  i;
    int whiteSpaceCount;

    while (fgets(line, MAX_LINE, stdin) != NULL) {

        // print lines iff they contain a non white-space character

        i = 0;
        whiteSpaceCount = 0;
        while (line[i] != '\0') {
            // test for white space (isspace would be better here)

            if (line[i] != ' ' && line[i] != '\t' && line[i] != '\n') {
                whiteSpaceCount = whiteSpaceCount + 1;
                // could break here
            }

            i = i + 1;
        }

        if (whiteSpaceCount > 0) {
            printf("%s", line);
        }
    }
    return 0;
}

// reads lines from input
// prints ONLY IF they contain a non-whitespace character
// i.e. remove lines that are empty or contain ONLY whitespace
#include <stdio.h>
#include <stdlib.h>

#define MAX 4096
#define TRUE 1
#define FALSE 0

int main (int argc, char* argv[]) {

	char line[MAX];			// to store input
	int i;					// counter for the line
	int notWhite;			// counter for number of notwhitespace characters we have

	// read until EOF or error
	while (fgets(line, MAX, stdin) != NULL) {

		// now iterate through line to see if it has whitespace
		// strings are terminated by a null terminating character
		i = 0;
		notWhite = 0;	// assume this is an empty line, prove false
		while (line[i] != '\0') {
			// test for whitespace
			if (line[i] != ' ' && line[i] != '\t' && line[i] != '\n') {
				// this character isn't whitespace
				notWhite++;	// increment number of non-whitespace
				break;
				// can do this without a break
				// would just go count all the non-whitespace chars
				// but really now you have all the info you need, slight optimization
			}
			i++;
		}

		// print line only if no whitespace
		if (notWhite > 0) {
			// found non-whitespace characters
			printf("%s", line);
		}

	}

	return EXIT_SUCCESS;
}

#include <stdio.h>

#define MAX_LINE 4096

int
main(void) {
    char line[MAX_LINE];
    int  i;

    while (fgets(line, MAX_LINE, stdin) != NULL) {

        i = 0;
        while (line[i] != '\n' && line[i] != '\0') {
            i = i + 1;
        }

        i = i - 1;

        while (i >= 0) {
            printf("%c", line[i]);
            i = i - 1;
        }
        printf("\n");

    }
    return 0;
}

// reads lines and writes them out
// with the chars of each line in reverse ordder
#include <stdio.h>
#include <stdlib.h>

#define MAX 4096

int main (int argc, char* argv[]) {

	char line[MAX];
	int i;

	while (fgets(line, MAX, stdin) != NULL) {
		// now print the line in reverse order

		// so first figure out what your HIGHEST index 
		// that's been initialized is
		i = 0;
		while (line[i] != '\n' && line[i] != '\0') {
			// this char isn't the last one
			i++;
		}
		// when you exit, i is one greater than the last char
		// since line[i] == '\n' or line[i] == '\0'
		i--;		// so minus one to get to the index of the last char

		// now print char by char until the first element at index 0
		while (i >= 0) {
			printf("%c", line[i]);
			i--;
		}
		// end this line
		printf("\n");
	}


	return EXIT_SUCCESS;
}