Prerequisite: Loops in Java
The structure of basic for loop:
for(initialization; boolean expression; update statement) { //Body }
Let’s look at some basic examples of using for loop and the common pitfalls in using for loop
- Providing expression in for loop is must : For loop must consist a valid expression in the loop statement failing which can lead to an infinite loop. The statement
for ( ; ; ) is similar to while(true)
// Java program to illustrate
// infinite loop
public
class
Example1
{
public
static
void
main(String[] args)
{
for
( ; ; )
{
System.out.println(
"This is an infinite loop"
);
}
}
}
Output: This code prints the statement “This is an infinite loop” repeatedly.
- Initializing multiple variables : In Java, multiple variables can be initialized in initialization block of for loop regardless of whether you use it in the loop or not.
// Java program to illustrate
// Initializing multiple variables
// in initialization block
public
class
Example2
{
public
static
void
main(String[] args)
{
int
x =
2
;
for
(
long
y =
0
, z =
4
; x <
10
&& y <
10
; x++, y++)
{
System.out.println(y +
" "
);
}
System.out.println(x);
}
}
In the above code, there is simple variation in the for loop. Two variables are declared and initialized in the initialization block. The variable ‘z’ is not being used. Also, the other two components contain extra variable. So, it can be seen that the blocks may include extra variables which may not be referenced by each other.
- Redeclaration of a variable in initialization block : Suppose, an initialization vaiable is already declared as integer. Can we re-declare it in for loop with other data type? No, See the example:
// Java program to illustrate
// redeclaring a variable
// in initialization block
public
class
Example3
{
public
static
void
main(String[] args)
{
// x is integer
int
x =
0
;
// redeclaring x as long will not work
for
(
long
y =
0
, x =
1
; x <
5
; x++)
{
System.out.print(x +
" "
);
}
}
}
Example3.java:12: error: variable x is already defined in method main(String[]) for(long y = 0, x = 1; x < 5; x++)
Here, x was already initialized to zero as integer and is being re-declared in the loop with data type long.
But this problem can be fixed by slightly modifying the code. Here, the variables x and y are declared in a
different way.
// Java program to illustrate
// redeclaring a variable
// in initialization block
public
class
Example3
{
public
static
void
main(String[] args)
{
// x is integer
int
x =
0
;
long
y =
10
;
for
(y =
0
, x =
1
; x <
5
; x++)
{
System.out.print(x +
" "
);
}
}
}
Output:
1 2 3 4
- Variables declared in the initialization block must be of same type : It is just a common sense that when we declare a variable as
int x, y;
both variables are of same type. Its just the same in for loop initialization block too.
// Java program to illustrate
// declaring a variable
// in initialization block
public
class
Example4
{
public
static
void
main(String[] args)
{
// This will cause error;
// int x;
// redeclaring x as long will not work
for
(
long
y =
0
, x =
1
; x <
5
; x++)
{
System.out.print(x +
" "
);
}
}
}
- Variables in the loop are accessible only within: The variables that are declared in the initialization block can be accessed only within the loop. For more on scope of variables, Refer here
// Java program to illustrate
// scope of Initializing variables
// within the loop
public
class
Example5
{
public
static
void
main(String[] args)
{
// x and y scope is only
// within for loop
for
(
int
x =
0
, y =
0
; x <
3
&& y <
3
; x++, y++)
{
System.out.println(y +
" "
);
}
System.out.println(x);
}
}
Error
Example5.java:13: error: cannot find symbol System.out.println(x);
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
This article is attributed to GeeksforGeeks.org
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- Redeclaration of a variable in initialization block : Suppose, an initialization vaiable is already declared as integer. Can we re-declare it in for loop with other data type? No, See the example:
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