02-Array

• the size of an array cannot be changed once it’s created

Declaration and Initialization

   int[] arr = new int[5]; // creates an array of length 5
   int[] arr2 = {1, 2, 3, 4, 5}; // direct initialization

For-each loop

• used for array and arraylist
• aka enhanced for-loop
• 只读，修改的是参数的副本，不影响参数（这一性质只针对primitive types, 不针对reference types）

   for (typeName variableName : arrayName){
	//statements
   }

Common Algorithms

Maximum / Minimum

用maximum举例

int largest = values[0]; //initialize
 
//traverse the string to find max
for(int i = 1; i < values.length; i++){  
	if(values[i] > largest){
		largest = values[i];
	}
}

• 其中 line 1 有两种写法
  1. 一种如上：把初始值定为 array 里的一个数
  2. 另一种如下：取 Integer class 的最小值 （找 minimum 时取 MAX_VALUE）

	   int largest = Integer.MIN_VALUE;

Sum and Average

double sum = 0;
for(int element : values){
	sum += element;
}
double average = 0;
//the array should have a length greater than zero
if(values.length > 0){
	average = sum / values.length;
}

!! Check All Elements

for(int element : values){
	if(element < 60){
		return false;
	}
}
return true;

Count Numbers

int count = 0;
for(int element : values){
	if(element > 90){
		count++;
	}
}
return count;

!! Reverse Arrays

int[] values = {1, 2, 3, 4, 5, 6, 7, 8};
 
int temp;
for(int i = 0; i < values.length / 2; i++){
//swap
	temp = values[values.length - 1 - i];
	values[values.length - 1 - i] = values[i];  //互换元素的index相加等于length - 1
	values[i] = temp;
}
 
for(int e: values){
	System.out.print(e + " ");
}

题型： intended to return true if [condition], return false otherwise

• initialize: 两种 condition 中更难达到的那种
  • e.g. check if an int[] is decreasing ⇒ initialize to true
• 用循环检查，发现反例则赋值为相反 boolean value