if/else and if/elseif
-
10-07-2019 - |
Question
If I have a statement block like this:
if (/*condition here*/){ }
else{ }
or like this:
if (/*condition here*/)
else if (/*condition here*/) {}
else if (/*condition here*/) {}
What is the difference?
It seems that with if/else, if part is for true state and the else part is for all other possible options (false). An else-if would be useful for a number of conditions. This is my understanding, is there anything more I should be aware of?
Solution
Situation a:
if( condition )
{
}
else
{
}
When the condition in the above statement is false, then the statements in the else block will always be executed.
Situation b:
if( condition )
{
}
else if( condition2 )
{
}
else
{
}
When 'condition' is false, then the statements in the else if block will only be executed when condition2 is true. The statements in the else block will be executed when condition2 is false.
OTHER TIPS
Without "elseif" syntax you would have to write chain if-statements for processing one of several possible outcomes this way:
if( str == "string1" ) {
//handle first case
} else {
if( str == "string2" ) {
//handle second case
} else {
if( str == "string3" ) {
//handle third case
} else {
//default case
}
}
}
instead you can write
if( str == "string1" ) {
//handle first case
} else if( str == "string2" ) {
//handle second case
} else if( str == "string3" ) {
//handle third case
} else {
//default case
}
which is completely the same as the previous one, but looks much nicer and is much easier to read.
Many languages have a grammer like this (here: ECMAScript Language Specification, so JavaScript):
IfStatement :
if (
Expression)
Statementelse
Statement
if (
Expression)
StatementStatement :
Block
VariableStatement
EmptyStatement
ExpressionStatement
IfStatement
IterationStatement
ContinueStatement
BreakStatement
ReturnStatement
WithStatement
LabelledStatement
SwitchStatement
ThrowStatement
TryStatementBlock :
{
StatementListopt}
StatementList :
Statement
StatementList Statement
So the branches of an ifStatement may contain a block of statements (Block) or one of the other statements (other than Block). That means this is valid:
if (expr)
someStatement;
else
otherStatement;
And as StatementList may just contain a single statement, these examples are equivalent to the previous:
if (expr) {
someStatement;
} else {
otherStatement;
}
if (expr)
someStatement;
else {
otherStatement;
}
if (expr) {
someStatement;
} else
otherStatement;
And when we replace otherStatement
by an additional IfStatement, we get this:
if (expr) {
someStatement;
} else
if (expr) {
someOtherStatement;
}
The rest is just code formatting:
if (expr) {
someStatement;
} else if (expr) {
someOtherStatement;
}
Emphasizing what Gumbo said.
Also, if a language has a real elif / elsif / elseif (say, a "real" else-if instruction, instead of a kind of nested chaining hidden away by formatting), then the compiler can easly emit a single node in an Abstract Syntax Tree (or similar, see http://en.wikipedia.org/wiki/Abstract_syntax_tree) instead of nesting them.
To give an example:
Say in C/C++ you have:
if (a) {
X
} else if (b) {
Y
} else if (c) {
Z
} else {
0
}
Then the compiler will build an AST-node like this:
a
/ \
X b
/ \
Y c
/ \
Z 0
But if the language of choice has a real if-else:
if (a) {
X
} elif (b) {
Y
} elif (c) {
Z
} else {
0
}
Then the AST could more easily look like this:
(a--b--c)
/ / / \
X Y Z 0
In such a language, an "if else" would only be possible if braces are not mandatory:
if (a) {
X
} elif (b) {
Y
} else if (c) { // syntax error "missing braces" if braces mandatory
Z
} else {
0
}
Corresponding AST (if braces are not mandatory):
(a--b)
/ / \
X Y c
/ \
Z 0
This could make CFG-Analysis (http://en.wikipedia.org/wiki/Control_flow_graph) easier to implement (though there might be no actual optimization benefit; so imho it'd just benefit the lazy programmer :D).
else if
basically means the else
part of if
is another if
statement.
**if/else**
if(condition)
statement;
else
statement;
if/ else if /else
if(condition)
{
if(condition)
statement;
else
statement;
}
else if(condition)
{
if(condition)
statement;
else
statement;
}
else
statement;
if/else and if/else if used also like this
Given a single variable, you will use the simple if-else
structure. When there are multiple variables and you have a different path to execute for the different possibilities, you will use if-else if-...-else
. Note, that the latter also ends with an else
statement.
You already gave the answer yourself. if/else is for true/false result, like is an int=2 or any other possible int value, and if/elseif is for more than 2 results, like an int=2, and int=3 and so on.
Also it groups the context of a variable. You could check every single result like
if (a == 2) { do one thing };
if (a == 3) { do another thing };
...
if (a != 2 && a != 3 ...) { do something else };
With if/else/elseif it's better readable.
if you want to check more condition we can use if..elseif. single condition then we can use if or if...else.
Here I can't able to upload the full explanation with example so please go through the following links.
if..else statement details
http://allinworld99.blogspot.in/2016/02/ifelse-flow-chart-with-easy-example.html
if...elseif details
http://allinworld99.blogspot.in/2016/02/flow-chart-with-example-for-if-then.html
import java.util.*;
public class JavaApplication21 {
public static void main(String[] args) {
Scanner obj = new Scanner(System.in);
System.out.println("You are watching an example of if & else if statements");
int choice, a, b, c, d;
System.out.println(" Enter 1-Addition & 2-Substraction");
int option = obj.nextInt();
switch (option) {
case (1):
System.out.println("how many numbers you want to add.... it can add up to 3 numbers only");
choice = obj.nextInt();
if (choice == 2) {
System.out.println("Enter 1st number");
a = obj.nextInt();
System.out.println("Enter 2nd number");
b = obj.nextInt();
c = a + b;
System.out.println("Answer of adding " + a + " & " + b + " is= " + c);
} else if (choice == 3) {
System.out.println("Enter 1st number");
a = obj.nextInt();
System.out.println("Enter 2nd number");
b = obj.nextInt();
System.out.println("Enter 3rd number");
c = obj.nextInt();
d = a + b + c;
System.out.println("Answer of adding " + a + " , " + b + " & " + c + " is= " + d);
}
case (2):
System.out.println("how many numbers you want to substract.... it can substract up to 3 numbers only");
choice = obj.nextInt();
if (choice == 2) {
System.out.println("Enter 1st number");
a = obj.nextInt();
System.out.println("Enter 2nd number");
b = obj.nextInt();
c = a - b;
System.out.println("Answer of substracting " + a + " & " + b + " is= " + c);
} else if (choice == 3) {
System.out.println("Enter 1st number");
a = obj.nextInt();
System.out.println("Enter 2nd number");
b = obj.nextInt();
System.out.println("Enter 3rd number");
c = obj.nextInt();
d = a - b - c;
System.out.println("Answer of substracting " + a + " , " + b + " & " + c + " is= " + d);
}
default:
System.out.println("no option you have chosen" + option);
}
}
}