Table of Contents
I summarized for loop and for-each loop in Java.
For loop
for loop is a statement to repeat with some condition.
for loop is used with 3 kinds of statements. The first one is an initialization statement (statement 1), the second one is a condition statement for continuing the iteration (statement 2) and the third one is a statement executed after every iteration (statement 3). Each statement can be omitted if it is not needed. If the second statement is omitted, the condition is regarded as true.
Format
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1 2 3 |
for (statement_1; statement_2; statement_3) { // iteration part } |
If the iteration part can be one statement, { and } can be omitted, as follows.
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1 2 |
for (statement_1; statement_2; statement_3) iteration_statement; |
Example
Here are 2 examples of for loop which iterates with counting up number variable, i. i starts at 0 and after the iteration gets executed 3 times and i becomes 3, the for loop ends.
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for (int i = 0; i < 3; ++i) { System.out.println(i); } // Output: // 0 // 1 // 2 |
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1 2 3 4 5 6 |
for (int i = 0; i < 10; ++i) System.out.println(i); // Output: // 0 // 1 // 2 |
for-each loop
for-each loop is also used for traversing. It is available for arrays or objects with Iterable interface, and very efficient for them.
Format
Here are the formats of for-each loop.
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1 2 3 |
for (variable : expression) { // iteration part } |
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1 2 |
for (variable : expression) iteration_statement; |
Compare for loop and for-each loop, with examples
When traversing over an array, for loop requires a variable, the initialization statement, the condition for continuing and the statement executed after each iterating. But for-each loop requires only a variable to hold an element in the array. for-each loop is shorter than for loop.
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int[] intArray = {1, 2, 3}; for (int i = 0; i < intArray.length; ++i) { System.out.println(intArray[i]); } // Output: // 1 // 2 // 3 |
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1 2 3 4 5 6 7 8 |
int[] intArray = {1, 2, 3}; for (int i : intArray) { System.out.println(i); } // Output: // 1 // 2 // 3 |
As you see, for-each loop can traverse over one array (or one iterable object) in ordinary order easily. It is needless to write initialization statement, continuing condition or procedure after each operation. In that manner, for-each loop requires less description than for loop.
On the contrary, for loop can handle 2 or more arrays at the same time, and can traverse arrays/objects in extraordinary order such as reverse order. Here are examples.
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 |
2 arrays example int[] a = {1, 4, 5}, b = {1, 2, 6}; for (int i = 0, j = 0; i < a.length && j < b.length;) { if (a[i] == b[j]) { System.out.println(a[i]); ++i; ++j; continue; } if (a[i] < b[j]) { System.out.println(a[i]); ++i; continue; } if (b[j] < a[i]) { System.out.println(b[j]); ++j; } } // Output: // 1 // 2 // 4 // 5 |
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1 2 3 4 5 6 7 8 |
int[] intArray = {1, 2, 3}; for (int i = intArray.length - 1; 0 <= i; --i) { System.out.println(intArray[i]); } // Output: // 3 // 2 // 1 |
To do the same things with for-each loop, creating a new object with Iterable interface is required but that’s a heavier task.
Traverse enum
Come to think of an enum iteration, since enum class can produce an array of itself easily with values method, for-each loop is very efficient for traversing enum, too. Here are for and for-each loop examples over enum.
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 |
public class Discussion { private enum EnumForLoop { A, B, C } public static void main(String[] args) { for (EnumForLoop item : EnumForLoop.values()) { System.out.println(item); } for (int i = 0; i < EnumForLoop.values().length; ++i) { System.out.println(EnumForLoop.values()[i]); } } } // Output: // A // B // C // A // B // C |
Thus, using for-each loop for loops in ordinary order makes our program short. In other words, for-each loop decreases bugs, so it is better to use for-each loop as much as we can.
