How come this is not possible? I am getting illegal start of expression.
(s1.charAt(i) == ' ') ? i++ : break;
The thing to understand here is that the ?: operator is used to return a value. You're basically calling a function that looks like this in that line:
anonymous function:
if(s1.charAt(i) == ' '):
return i++;
else:
return break;
Makes no sense, right? The ?: operator was only designed as a shorthand for if/else return statements like the above, not a replacement of if/else altogether.
You cannot use break in part of a ternary conditional expression as break isn't an expression itself, but just a control flow statement.
Why not just use an if-else construct instead?
if (s1.charAt(i) == ' ') {
i++;
} else {
break;
}
The ternary operator is an expression, not a statement. Use if ... else ... for this.
Of course it works. But it's an operator. Since when was a statement such as 'break' an operand?
I recommend avoiding the ternary (?:) operator EXCEPT for simple assignments. In my career I have seen too many crazy nested ternary operators; they become a maintenance headache (more cognitive overload - "don't make me think!").
I don't ban them on my teams, but recommend they are used judiciously. Used carefully they are cleaner than a corresponding if/else construct: -
public int ifFoo() {
int i;
if( isSomethingTrue()) {
i = 5;
}
else {
i = 10;
}
return i;
}
Compared to the ternary alternative: -
public int ternaryFoo() {
final int i = isSomethingTrue()
? 5
: 10;
return i;
}
The ternary version is: -
Shorter
Easier to understand (my opinion, of course!)
Allows the variable to be "final"; which simplifies code comprehension; in a more complex method, someone reading the code knows no further code will try and modify the variable - one thing less to worry about.
Related
I'm a beginner in coding. I was recently working with to create a chatting programme where a user will chat with my computer. Here is a part of the code:
System.out.println("Hello, what's our name? My name is " + answer4);
String a = scanner1.nextLine();
System.out.println("Ok, Hello, " + a + ", how was your day, good or bad?");
String b = scanner2.nextLine();
**if (b.equals("good"))** { //1
System.out.println("Thank goodness");
} else **if (b.equals("it was good"))** { //2
System.out.println("Thank goodness");
} else **if (b.equals("bad"))** { //3
System.out.println("Why was it bad?");
String c = scanner3.nextLine();
System.out.println("Don't worry, everything will be ok, ok?");
String d= scanner10.nextLine();
} else **if (b.equals("it was bad"))**{ //4
System.out.println("Why was it bad?");
String c = scanner3.nextLine();
System.out.println("Don't worry, everything will be ok, ok?");
String d= scanner10.nextLine();
}
if(age<18){System.out.println("How was school?");}
else if (age>=18){System.out.println("How was work?");}
The conditions of the if statements are in Bold (surrounded with **). In case of first and the second condition I want my application to do same thing. Similarly third and fourth condition. I thought it was possible to somehow group them in if statement.
I tried with below code but it doesn't compile:
if (b.equals("good"), b.equals("it was good")) {
System.out.println("Thank goodness");
} else if (b.equals("bad"),(b.equals("it was bad"))) {
System.out.println("Why was it bad?");
String c = scanner3.nextLine();
System.out.println("Don't worry, everything will be ok, ok?");
String d= scanner10.nextLine();
}
Can someone correct it for me?
You can use logical operators to combine your boolean expressions.
&& is a logical and (both conditions need to be true)
|| is a logical or (at least one condition needs to be true)
^ is a xor (exactly one condition needs to be true)
(== compares objects by identity)
For example:
if (firstCondition && (secondCondition || thirdCondition)) {
...
}
There are also bitwise operators:
& is a bitwise and
| is a bitwise or
^ is a xor
They are mainly used when operating with bits and bytes. However there is another difference, let's take again a look at this expression:
firstCondition && (secondCondition || thirdCondition)
If you use the logical operators and firstCondition evaluates to false then Java will not compute the second or third condition as the result of the whole logical expression is already known to be false. However if you use the bitwise operators then Java will not stop and continue computing everything:
firstCondition & (secondCondition | thirdCondition)
Here are some common symbols used in everyday language and their programming analogues:
"," usually refers to "and" in everyday language. Thus, this would translate to the AND operator, &&, in Java.
"/" usually refers to "or" in everyday language. Thus, this would translate to the OR operator, ||, in Java.
"XOR" is simply "x || y but both cannot be true at the same time". This translates to x ^ y in Java.
In your code, you probably meant to use "or" (you just used the incorrect "incorrect solution" :p), so you should use "||" in the second code block for it to become identical to the first code block.
Hope this helped :)
You're looking for the "OR" operator - which is normally represented by a double pipe: ||
if (b.equals("good") || b.equals("it was good")) {
System.out.println("Thank goodness");
} else if (b.equals("bad") || b.equals("it was bad")) {
System.out.println("Why was it bad?");
String c = scanner3.nextLine();
System.out.println("Don't worry, everything will be ok, ok?");
String d= scanner10.nextLine();
}
This is probably more answer than you need at this point. But, as several others already point out, you need the OR operator "||". There are a couple of points that nobody else has mentioned:
1) If (b.equals("good") || b.equals("it was good")) <-- If "b" is null here, you'll get a null pointer exception (NPE). If you are genuinely looking at hard-coded values, like you are here, then you can reverse the comparison. E.g.
if ("good".equals(b) || "it was good".equals(b))
The advantage of doing it this way is that the logic is precisely the same, but you'll never get an NPE, and the logic will work just how you expect.
2) Java uses "short-circuit" testing. Which in lay-terms means that Java stops testing conditions once it's sure of the result, even if all the conditions have not yet been tested. E.g.:
if((b != null) && (b.equals("good") || b.equals("it was good")))
You will not get an NPE in the code above because of short-circuit nature. If "b" is null, Java can be assured that no matter what the results of the next conditions, the answer will always be false. So it doesn't bother performing those tests.
Again, that's probably more information than you're prepared to deal with at this stage, but at some point in the near future the NPE of your test will bite you. :)
You can have two conditions if you use the double bars(||). They mean "Or". That means only ONE of your conditions has to be true for the loop to execute.
Something like this:
if(condition || otherCondition || anotherCondition) {
//code here
If you want all of conditions to be true use &&. This means that ALL conditions must be true in order for the loop to execute. if any one of them is false the loop will not execute.
Something like this:
if(condition && otherCondition && anotherCondition) {
//code here
You can also group conditions, if you want certain pairs of them to be true. something like:
if(condition || (otherCondition && anotherCondition)) {
//code here
There is a simpler way.
if (b.contains("good")) {
...
}
else if (b.contains("bad")) {
...
}
I have a piece of code that uses a conditional operator to compare the value of the variable x to 5:
(x >=5 ? x : -x)
What benefit is there to using a conditional operator over a regular if else statement?
Note that is ?: is an expression - it returns a value which must be consumed
z = (x >=5 ? x : -x)
The if construct in Java is not an expression (there are languages where it is) and does not return a value so in this sense they are not equivalent.
An example where they are equivalent is when the if options perform an assignment:
if("Cheese".equals(x)) {
type = "Dairy";
} else {
type = "Maybe not dairy";
}
this is the same as
type = "Cheese".equals(x) ? "Dairy" : "Maybe not dairy";
You can nest ?: to arbitrary depth but really shouldn't - it becomes quite difficult to read:
List<String> cheeses = Arrays.asList("Gouda", "Edam");
String x= "Gouda";
String type = cheeses.contains(x) ? "Gouda".equals(x) ? "Yummy Gouda" : "Cheese - but not Gouda" : "Maybe not dairy";
Ternary operator is not the equivalent of if-else in EVERY possible case. This is because both of possible results of using ternary operator are return values (e. g. simple printing to the console is not a return value so it can't be one of possible results in ternary operator).
Look, you can do it with if-else, but it's not possible with ternary operator:
if (x > 5) {
System.out.println("Statement is true");
else {
System.out.println("Statement is false");
}
You can't do it with ternary operator:
x > 5 ? System.out.println("Statement is true") : System.out.println("Statement is false");
When both of results of using ternary operator are considered as return values, they are then an equivalent of if-else.
Yes, it can be used, but off course in that case using -x doesn't make sense so it depends on what do you want to return when String is an input. You can do this:
("Cheese".equals(x) ? <some object of expected type X> : <some other object of expected type X>)
where X can be String on any other type. And remember to do "string".equals(x) instead of x.equals("string"). This prevents NullPointerException.
For primitive values you can use ==, but for objects you need to use equals method. There are some edge cases when you can use == on String, but they are not relevant to your case I guess and you can read about it outside of this topic.
PS: Some answers talk about if else statement, but that's not what question is about.
Can this be used like an if else on all accounts? For example, can you
compare strings with it?
I am pretty sure that by saying like an if else he means conditional operator which has if else like behaviour, not if else instruction itself.
Yes it can be used in Java; however note that x =="Cheese" isn't the proper way to compare strings in Java, you want something like "Cheese".equals(x).
How does the Java compiler, and other compilers, handle the following scenario; where there is a nested if with no curly brackets?
// In Java, to which of the 'if's does this 'else' belong?
// If bOne=false and bTwo=true, will the println run?
boolean bOne = false;
boolean bTwo = true;
if (bOne)
if (bTwo)
bTwo = bTwo; // do nothing
else
System.out.println("bOne:" + bOne + ". bTwo:" + bTwo);
The else clause applies to the second if statement. At first, it should be read as:
if (bOne)
if (bTwo)
bTwo = bTwo; // do nothing
else
System.out.println("bOne:" + bOne + ". bTwo:" + bTwo);
And then the code should be rewritten to use curly braces.
This applies to many imperative programming languages, one notable exception being Python which has significant indentation instead of curly braces.
This is known as the dangling else problem. The rule that solves this is that the else belongs to the closest if. So in that example, the else belongs to if (bTwo).
It does, as some others have already said, belong to the "nearest preceding if". However, to AVOID this problem always use braces (and proper indentation), so that there's no need to try to figure this out by understanding how the compiler works.
By the way, the compiler solves this in this way (pseudo-code):
process_if()
{
expect_token("if");
next_token();
expect_token("(");
next_token();
condition = parse_expression();
expect_token(")");
next_token();
then_part = parse_statement_or_block();
if (current_token == "else")
else_part = parse_statement_or_block();
else
else_part = nothing;
}
parse_statement_or_block()
{
if (current_token() == "{")
do
{
statements += parse_statement();
} while(current_token() != "}");
else
statements = parse_statement();
}
parse_statement()
{
switch(current_token())
{
case "while":
process_while();
break;
case "for":
process_for();
break;
case "if":
process_if();
break;
....
}
}
The way this parser works is that it reads the first if, and the statement inside that is another if, so it goes to process_if, takes the condition, the then_part and then checks for an else and takes that as the else_part for that if. Then goes back to the first if-statement and finds "no else there" (because it has already been consumed).
Actual compiler code that does exactly this:
https://github.com/Leporacanthicus/lacsap/blob/master/parser.cpp#L1854
I'm trying to write a program that that will find if there's an equal number of odds and even numbers in a given one, it's working great but it want to use conditional operator instead of these 4 rows (the // rows),
I'm getting this:
Syntax error on token "%", invalid AssignmentOperator
Can someone tell me why? What's wrong?
while(number!=0) {
//if(number%2==0)
//even++;
//else
//odd++;
number%2==0 ? even++ : odd++;
number/=10;
}
number%2==0 ? even++ : odd++;
This is not a statement. The result of a ternary must be assigned to something:
int x = number % 2 == 0 ? even++ : odd++;
However, this is stylistically quite awkward. I would use an if-else (i.e., what you originally had) over this pattern. Here you've created a temporary variable that you're never going to reuse, for the sole purpose of using a ternary.
It requires a variable at the left side at where you can place the value after the condition.
int tmp = (number%2 == 0)?even++:odd++;
number%2==0 ? even++ : odd++;
This statement is not assigning any value to number.As per my knowledge its always recommended for use if-else instead of ternary operator.
Because, in ternary operator the false/else part is compulsory to write, where in if-else else/false part is optional.
while(number!=0) {
//if(number%2==0)
//even++;
//else
//odd++;
int temp= number%2==0 ? even++ : odd++;
number/=10;
}
Now, temp variable is holding the value .
I've written the following if-statement in Java:
if(methodName.equals("set" + this.name) ||
isBoolean() ? methodName.equals("is" + this.name) :
methodName.equals("get" + this.name)) {
...
}
Is this a good practice to write such expressions in if, to separate state from condition? And can this expression be simplified?
I would change it to
if (methodName.equals("set" + this.name)
|| methodName.equals( (isBoolean() ? "is" : "get") + this.name)) {
...
}
Is it good practice? It's good if it makes it easier to read. It makes it easier to read if (1) it does and (2) the sort of person who'd be confused by it won't be reading it. Who's going to read it?
Wouldn't something like the following work?
if (methodName.equals("set" + this.name)
|| methodName.equals("get" + this.name)
|| (isBoolean() && methodName.equals("is" + this.name))) {
...
}
It's more readable than the way in which you used the ternary operator and certainly easier to understand. It also has the advantage that it can avoid an unnecessary method call to the isBoolean method (it has either 1, 2 or 4 method calls whereas yours always has either 1 or 3; the performance gain/loss is probably too minute to notice).
Also there's a similar question here titled "Is this a reasonable use of the ternary operator?" One user had the following to say:
The ternary operator is meant to
return a value.
IMO, it should not mutate state, and
the return value should be used.
In the other case, use if statements.
If statements are meant to execute
code blocs.
Do note that I included parentheses around the expression containing '&&' for readability. They aren't necessary because x && y is evaluated before m || n.
Whether you choose to use it is up to you, but I tend to avoid it in favour of readability.
I would be inclined to change it to
if (methodName.equals(setterForThis())
|| methodName.equals(getterForThis())) {
...
}
with some functions extracted:
private String setterForThis() {
return "set" + this.name;
}
private String getterForThis() {
return (isBoolean() ? "is" : "get") + this.name;
}
It's longer of course, but I'm not really into golf anyway.