What is the difference between IF and ELSE IF clauses? - c

Question

I'm on the path of learning C with K&R. Besides the exercises in the book, I'm also doing some by myself. I wrote the following code, what it does is, it counts your input, gives you feedback how many words are left to reach the "goal" of 10 words and congratulates you as soon as you reach the 10 words.
#include <stdio.h>
main()
{
/* This programm will read your input, check the number of words,
and will congratulate when you reach a value of 10 words*/
int c, nw, counter;
nw = 0;
counter = 0;
while (nw < 10)
{
c = getchar();
if (c == ' ' || c == '\t' || c == '\n')
{
counter = 0;
}
else if (c != ' ' || c != '\t' || c != '\n')
{
if (counter == 0)
{
counter = 1;
++nw;
}
printf("Only %d words left\n", 10-nw );
}
}
}
Ok, in this version the code will not count blanks as words, resulting in the correct output of words left.
At first I wrote the code with only "if" instead of "else if". What it did was it also counted the blanks as words.
The question I am asking is why ? Wheres the difference in using if or else if.
As I understand is, that compiler will check whether a condition is met or not met. This should also be the case when only using if. As everything is the same expect else if instead of if, I can't figure out what the problem is.

Answer 1

It looks like this might be a case of overthinking a problem. The logic you've ended up with, aside from being wrong, is overly complicated. Your question of the difference between if and else if is fair, and I promise I will address it in my answer.
First, let me restate what you are trying to do:
Read input, count number of words, and congratulate you when you reach 10 words
From your code, I believe your intention is to split words based on spaces, tabs, and newlines. There are many ways to split words, but for the purposes of this question, your intended method is fine.
The problem, of course, is that your logic doesn't work. You have a condition that can never be false:
else if (c != ' ' || c != '\t' || c != '\n')
Think about it (hint: the else doesn't change the condition itself). Say it out loud if that helps: You are looking for a condition where c isn't a space, or c isn't a tab, or c isn't a newline. Remember that logical or (||) is an inclusive or, in other words, the expression is true if any of the conditions are true. For example, let's suppose c is a space. The first condition (c != ' ') fails, but the 2nd one, c != '\t' is true because a space is not a tab. Thus, the entire expression is true. In fact, that expression will be true for any value of c.
But what about else?
As I said, the else part of the else if doesn't make a difference here. The only thing else if does differently is essentially tack itself on as a new condition to your if statement. Let's look at a simpler example:
if (a == 1) {
/* a is 1 */
}
if (a != 1 && b == 2) {
/* a isn't 1, but b == 2 */
}
That's an example of two completely independent if statements. It's a perfect example of where to use else, because as you probably noticed, the 2nd if statement tests for the inverse of the 1st. (a != 1). So, the above can be simplified as follows:
if (a == 1) {
/* a is 1 */
else if (b == 2) {
/* a isn't 1 and b is 2 */
}
In the else if block, we needn't test for a != 1, as that's implied because we only evaluate the else statement if the 1st if conditional was false.
Note also that else if is actually a combination of two separate keywords. It is equivalent to:
else {
if (b == 2) { ... }
}
However, by convention we omit the optional braces and write it as:
else if (b == 2) { ... }
In fact, in some cases we don't need that 2nd if at all:
if (a == 1) {
printf("a is 1!\n");
} else {
printf("a isn't 1. In fact, it's %d.\n", a);
}
Simplified Version
So, now there's no need to get caught up in else if. Focus on your logic, and do your best to simplify it. I will simplify it for you, however I encourage you to skip over this part and try it on your own first:
char c;
int in_word = 0;
while (nw < 10) {
c = getchar();
if (c == ' ' || c == '\t' || c == '\n') {
/* If we were in a word, then count that word! */
if (in_word) {
nw++;
printf("You only have %d words to go!", 10 - nw);
}
in_word = 0; /* We are not in a word now */
} else {
in_word = 1; /* Now we're in a word
}
}

Answer 2

Not sure if this is the answer you are looking for. But two separate if statements are both evaluated no matter what. With "if" and "else if", first if is evaluated if it is true, else if is skipped. When "if" is not true "else if" is evaluated.

Answer 3

else if (c != ' ' || c != '\t' || c != '\n')
This line of code is probably not doing what you intended. It works fine as an else if, but in fact it doesn't really check anything (you could actually replace it with a simple else)! For any conceivable character, it's will always be either not a space, or not a tab, or not a newline. What you really want is the conjunction of those statements, rather than the current disjunction:
else if (c != ' ' && c != '\t' && c != '\n')
This checks that the character is neither a space, a tab, nor a newline. It would work even with else removed, as a separate if-statement. However, you really should just go with a simple else.

Answer 4

In your if condition, if you use simple if .. else construct, it will be equivalent to
if (c == ' ' || c == '\t' || c == '\n')
{
...
}
else if (c != ' ' && c != '\t' && c != '\n') //Notice &&.
//Apply De-morgan's law over if conditio.
{
...
}
In the above case else if is not required, if else is suffice to use. Using if .. else would be more efficient than the given code.
However, logically it may have same effect as your given code.
As per logic, else if is not required.
The difference you will get in terms of number of comparison operation as explained below.
Current code:
For any non-space character such as 'a', it will perform 4 comparisons (|| and && are short-circuited).
For any space character, there will be max 3 comparisons.
In if-else code, max of 3-comparison will be performed.
This difference is not so significant. And can only be noticed when code is run billions of times.
if else has extra advantage of maintenance. In the above code in OP, if there is any change in if part, it should be reflected in else if part also.
Hence, to me it seems it will be beneficial to go with just if else rather if else if.