9. Loop in java:
loop in java
for loop:
Generally used when the number of loops is clearly known
① ② ③
for( initialization expression ; boolean expression ; increment expression ) {
④
// loop body
}
for(int i = 0 ; i<3;i++){
System.out.println(i);
}
10. Sorting of Java Arrays
[8,2,3,7,1] --->[1,2,3,7,8]
Bubble Sort
selection sort
Insertion sort
........
Arrays.sort();
Bubble Sort
[8,2,3,7,1]
Each sorting will pick out the largest number in this sorting and put it at the back
Compare two adjacent elements , the larger one is backward, the smaller one is forward .
First sort:
First comparison 8 <-> 2 [2,8,3,7,1]
Second comparison 8 <-> 3 [2,3,8,7,1]
Third comparison 8 <-> 7 [2,3,7,8,1]
Fourth comparison 8 <-> 1 [2,3,7,1,8]
Second sort :
First comparison 2 -> 3 [2,3,7,1,8]
Second comparison 3 -> 7 [2,3,7,1,8]
Third comparison 7 <-> 1 [2,3,1,7,8]
Third order :
First comparison 2 -> 3 [2,3,1,7,8]
Second comparison 3 <-> 1 [2,1,3,7,8]
fourth order
First comparison 2 <-> 1 [1,2,3,7,8]
There are a total of 5 numbers in the array :
How many times do you need to sort, and how many times for each sort to compare ?
At most arr.length-1 sorting is required
The first sort compares 4 times
The second sort compares 3 times
The third sort compares 2 times
The fourth sort compares 1 time
Code for bubble sort :
The outer for loop controls the number of times of sorting
The inner for loop controls the number of comparisons per sort
for(int i=0;i<arr.length-1;i++){
for(int j =0 ;j<arr.length-i-1;j++){
// Compare two by two, swap positions
if(arr[j] > arr[j+1]){
int temp = arr[j+1];
arr[j+1] = arr[j];
arr[j] = temp;
}
}
}
选择排序
将数组中的每个元素与第一个元素比较,
如果这个元素小于第一个元素，交换位置
每次排序找出一个最小的往前面放.
[8,2,3,7,1]
for(int i =0 ;i <arr.length-1;i++){
for(int j =i+1;j<arr.length;j++){
if(arr[i]>arr[j]){
int temp = arr[i];
arr[i] = arr[j];
arr[j] = temp ;
}
}
}
11方法的递归调用(了解为主，掌握为辅)
f(n){
f(n-1);
}
y =f(n)=1+3+5+7+9+....+n == f(n-2)+n ==f(n-4)+n-2+n
f(n) =1~n
f(n-2) =1~n-2
f(n-4) =1~n-4
1 3 5 7 9.... n-4 n-2 n
计算 9+7+5+3+1
n = 9
f(n){
1 +3+5+7+n
f(n-2)+n;
}
f(n-2){
f(n-4)+n-2;
}
f(1) == 1
publicstatic int f(int n){
if(n ==1){
return 1 ;
}
return f(n-2)+n;
}
f(9) =f(7)+9
f(7) =f(5)+7
f(5) =f(3)+5
f(3) =f(1)+3
f(1) =1
12.栈内存
栈是计算机中的一种数据存储方式,java进程启动时在内存中开辟的存储空间.
栈内存遵循LIFO(后进先出).java中的所有局部变量在栈中分配空间
局部变量:
写到方法中的变量
方法的形参
当离开局部变量的作用域，变量被回收，从栈中删除.
java中的方法调用就是使用栈实现的.