When the loop may be a case where the loop condition repeatedly execute a section of code, the code is repeated is referred to as loop statement, when this loop is repeatedly performed, it is necessary at the right time cycle is determined to modify the conditions false, thus ending the cycle, otherwise the loop will always execute it, forming an endless loop.
for loop
Statement format:
Stepping expression: change the loop condition of expression
Implementation process:
The order of execution: ①②③④> ②③④> ②③④ ... ② so far satisfied.
① responsible for completing the loop variable initialization
② responsible for determining whether the loop condition is not satisfied out of the loop
statements execute specific ③
④ After cycling, cycling conditions change of variables involved
example:
Using a loop, calculates the even-numbered 1 to 100 and
public static void main (String [] args) { // 1. the definition of a variable initialization, recording cumulative sum, the initial value 0 int SUM = 0 ; // 2. use for obtaining the digital loop between 1-100 for ( int I =. 1; I <= 100; I ++ ) { // array 3. the judgment obtained is odd or even IF (I% 2 == 0 ) { // 4. If the sum is even increments, sum + = I; } } // after the 5 cycles, the accumulation result printed System.out.println ( "SUM:" + SUM); }
Note: for variable cycle can only be used inside the loop, the loop can not be used
while loop
Statement format:
Implementation process:
The order of execution: ①②③④> ②③④> ②③④ ... ② so far satisfied.
① responsible for completing the loop variable initialization.
② responsible for determining whether the loop condition is not satisfied out of the loop.
③ specific statements executed.
④ After the loop, the loop variable changes
example:
while loop and the calculated between 1-100
public static void main (String [] args) { // while loop implemented // define a variable record cumulative sum int SUM = 0 ; // defines the initialization expression int I =. 1 ; // use while loop initialization so value of the expression changes the while (I <= 100 ) { // accumulated sum SUM + = I; // step expression to change the value of the variable I ++ ; } // print summation variable System.out.println ( " and 1-100 are: "+ SUM); }
for and while small differences
Control statements that variable conditions controlled, at the end of the for loop, you can not be accessed, and
And while the end of the cycle can continue to use, if you want to continue using it with a while, or recommended for.
The reason is that for the end of the cycle, the variable will disappear from memory, can improve memory usage efficiency.
Recommended for use when the known number of cycles, when an unknown number of cycles is recommended while.
do ...... while loop
Statement format:
Implementation process:
The order of execution: ①③④> ②③④> ②③④ ... ② so far satisfied.
① responsible for completing the loop variable initialization.
② responsible for determining whether the loop condition is not satisfied out of the loop.
Specific executed statement ③
④ After the cycle, the loop variable changes
note:
do ... while loop characteristics: the unconditional implementation of a loop, even if we write directly to the loop condition false, it still will cycle once. This cycle has certain risks, and therefore not recommended for beginners to do ... while loop.