if and else if

[Ken Austin]

Hello, again...

I've realized that I really don't understand the difference between if, else and else-if. As I am working through the exercises in my textbook, I seem to apply them more or less interchangeably, and have been lucky so far that no logic error has popped up.

For instance...

for (j = 0; j < str.length(); j++) {
    char ch = str.charAt(j);
    if (Character.isLetter(ch) && !inWord) {
        inWord = true;
        numWords++;
    }
    else if (Character.isSpaceChar(ch)) {
        inWord = false;
    }
    else if (ch == '\n') {
        inWord = false;
    }

}

The else-if parts work, but I can't explain to you why.

Quick site searches here and at Stack Overflow didn't turn up a clear and thorough explanation in one place. Can someone point me to a good thread or web page that has some clear examples of when and how to use these two similar but different constructions?

Thanks,
Ken

[Jesper de Jong]

What exactly do you want to know? These keywords in Java mean more or less exactly what they mean in plain English. You could replace the word "else" by "otherwise", I don't know if that helps in understanding it:

Line 3: If the character ch is a letter and inWord is not true
Line 7: Otherwise, if the character ch is a whitespace character
Line 10: Otherwise, if the character ch is a newline

[Wendy L Gibbons]

Don't think code for now think English. if I said to you "if you have the keys can you throw them over here, else can you help me find them"

You would know what I meant. Well now write that in java

Now for the extension


if you have the keys can you throw them over here, else if you have seen them where are they, else can you help me find them".

Hope this helps

[Jelle Klap]

A simple if-condition, the if body block may or may not execute:

if(someConditionIsTrue) {
  // Execute the code in this body block
}

An if-else construct, it's guarenteed that one and only one of the body blocks will execute, which one depens on the evaluation of the conditon:

if(someConditionIsTrue) {
  // Execute the code in this body block
} else {
  // Execute the code in this body if previous condition wasn't true.
}

An if-elseif construct, only one of the body blocks may execute, or none may execute depending on the evaluation of the conditons:

if(someConditionIsTrue) {
  // Execute the code in this body block
} else if(someOtherConditionIsTrue) {
  // Execute the code in this body block
}

An if-elseif-else construct, it's guarenteed that one and only one of the body blocks will execute, which one depends on the evaluation of the conditons:

if(someConditionIsTrue) {
  // Execute the code in this body block
} else if(someOtherConditionIsTrue) {
  // Execute the code in this body block
} else {
  // Execute the code in this body block if neither of the previous conditions were true.
}

Then there's also the possibility of multiple if-conditions without else, one of the blocks may execute, they both may execute, or neither may execute, it depens on the evaluation of the conditons:

if(someConditionIsTrue) {
  // Execute the code in this body block
}
if(someOtherConditionIsTrue) {
  // Execute the code in this body block
}

Etc. etc.

Does that make sense?

Edit: Crud, too slow...

[Ken Austin]

Jelle, that is exactly the type of explanation I was looking for. Thank you very much!

[fred rosenberger]

Imagine you are a butler for someone. You are going to show up every day monday through friday to work for them, week after week. They give you a piece of paper that says:

If it is monday
   clean the office
else if it is Tuesday
   dust the chandeliers
else if it is Wednesday
   drain the pool
else if it is Thursday
   sweep the back porch
else if it is Friday
   defrost the refrigerator
else
   it's a weekend - go home!!!

That should be pretty clear.

Note that this COULD be written as a bunch of individual if statements that are independent of each other:

If it is monday
   clean the office

if it is Tuesday
   dust the chandeliers

if it is Wednesday
   drain the pool

if it is Thursday
   sweep the back porch

if it is Friday
   defrost the refrigerator

if it's a weekend 
   go home!!!

There is a subtle difference between the two ways of writing it. In the first way, ONLY ONE thing will be done. As soon as the first condition that is true is encountered, that block of code runs, and then execution jumps to after the 'else' block. In the second way, each and every case is tested, and it is possible more than one could run. In my second example above, that is unlikely to happen (Unless you start cleaning the office at 11:59 p.m. and then check at 12:01 a.m. to see if it is Tuesday...).

[Ken Austin]

Fred, that is helpful, as well. Thank you. That does make it a lot clearer.

[Campbell Ritchie]

The subtle difference Fred mentioned doesn’t show up for days of the week, because they are mutually exclusive. A common example is marks and grades:

if you got 80+
   then it’s an A+
else if you got 70+
   then it’s an A
else if you got 60+
   then it’s a B
else if you got 50+
   then it’s a C
else if you got 40+
   then it’s a D
else
   then it’s a fail

If you try:

if you got 80+
   then it’s an A+
if you got 70+
   then it’s an A
if you got 60+
   then it’s a B
if you got 50+
   then it’s a C
if you got 40+
   then it’s a D
else
   then it’s a fail

… without the elses, only Ds and fails will be recorded properly. If you get 80, then you get an A+, an A, a B, a C and a D all together. Or maybe each if will override the previous one and you will end up with a D for your 80. You could have got that for 40. Hardly seems worth the effort!

[Winston Gutkowski]

Ken Austin wrote:Fred, that is helpful, as well. Thank you. That does make it a lot clearer.

And just to add to what Fred and Campbell have said: this is one area where English and Java (or indeed, any computer language) differ.

In English, we will quite often leave out the 'else's because it's fairly obvious to anyone who's looking at it in written form; but when writing a logical expression, you MUST be explicit about everything.

HIH

Winston