Java中嵌套for循环的执行顺序
Java中嵌套for循环的执行顺序
提问于 2018-09-18 04:02:17
找到了一个有效的解决方案后，我试图理解为什么我的解决方案不起作用。我添加了最小工作示例(MWE)，首先显示工作解决方案，然后在1和2处显示尝试的解决方案。我手动打印数组的所有元素，然后在for循环中再次打印。
由此我得出结论，首先计算外部的for循环(具有计数器i)，然后计算内部的for循环(具有计数器q)。我很困惑，想知道我对Java for循环的计算顺序的理解是不正确的，还是我遗漏了一个语法/不同的错误。
问题：谁能为for循环的评估顺序提出一个原因？
public class LearnMultiDimensionalArrayLength {
public static void main(String[] args) {
// Generate array
double[][] a = {{1.5, -10.3, 0}, {-2.5, 8, 1.3}};
// call method that loops through the array
forloops(a);
}
public static void forloops(double[][] a) {
// Working solution:
//loops through all elements of the array and prints their values.
for (double[] x : a)
{
for (double y : x)
{
System.out.print(y + " ");
}
System.out.println();
}
// Now I tried my own solution:
System.out.println("The first dimension length: "+ a.length +
" The second dim. length: " + a[1].length);
System.out.println("first dim-1 ="+ (a.length -1));
System.out.println("Second dim-1 ="+ (a[1].length -1));
// Question to self: Why is the following for loop wrong?
// for (int i=0;i <= a.length;i++){
// for (int q=0;q <= a[1].length;i++){
// System.out.println("["+i+"]"+"["+q+"]"+"="+a[i][q]);
// }
// }
// 1. Attempted Answer: Because the lengths are the nr of rows
// and nr of columns in the matrix. so with n = 2, there are only the elements
// 0 and 1 in the first []
// Test hypotheses by looping through the array a with length-1:
// for (int i=0;i <= a.length-1;i++){
// for (int q=0;q <= a[1].length-1;i++){
// System.out.println("["+i+"]"+"["+q+"]"+"="+a[i][q]);
// }
// }
// Conclusion, that did solve the problem.
// 2. Attempt: Print all elements and their values, then compare
// with what the for loop does:
System.out.println("a[0][0]="+a[0][0]);
System.out.println("a[0][1]="+a[0][1]);
System.out.println("a[0][2]="+a[0][2]);
System.out.println("a[1][0]="+a[1][0]);
System.out.println("a[1][1]="+a[1][1]);
System.out.println("a[1][2]="+a[1][2]);
// System.out.println("a[2][0]="+a[2][0]);
// System.out.println("a[2][1]="+a[2][1]);
// System.out.println("a[2][2]="+a[2][2]);
// 2.a Troubleshooting: verify the upper bound of the for loops
// are not producing an error due to differences in types:
System.out.println(((Object)1).getClass().getName());
System.out.println(((Object)a[1].length).getClass().getName());
System.out.println(((Object)(a[1].length-1)).getClass().getName());
// All expressions are evaluated as integer, both i and q are integers
// hence the different types should not result in a false negative.
// The actual problematic for loop:
System.out.println("Boolean evaluation:" + (1 == (a.length-1)));
for (int i=0; (i <= (a.length-1));i++){
for (int q=0; (q <= (a[1].length-1));i++){
//System.out.println("retry" + "[i="+i+"]"+"[q="+q+"]"+"="+a[i][q]);
System.out.println("This is the wrong order of for loops" + "[q="+q+"]"+"[i="+i+"]"+"="+a[q][i]);
}
}
}
}回答 2
Stack Overflow用户
回答已采纳
发布于 2018-09-18 04:29:57
为什么你不运行这段代码，试着理解它是如何工作的……一旦你这样做了，尝试一下(改变值)，看看会发生什么
public static void main(String arg[]) {
for(int i=0;i<10;i++) {
System.out.println("Inside the Fist loop "+i);
for(int j=0;j<10;j++) {
System.out.println("Inside the second loop "+j);
}
}
}Stack Overflow用户
发布于 2018-09-18 04:40:47
感谢您的快速响应，正如@azurefrog指出的那样，我在q的计数器中有一个复制粘贴错误，因此，它不断增加内部循环的i，从而在外部循环应该迭代通过的行/列上产生越界错误。
这包含已更正的代码(在第二个for-loop中有越界错误，因为行和列的长度不同，并且与原始问题相反)。我很抱歉。
public class SolvedMultiDimensionalArrayLength {
public static void main(String[] args) {
// Generate array
double[][] a = {{1.5, -10.3, 0}, {-2.5, 8, 1.3}};
// call method that loops through the array
forloops(a);
}
public static void forloops(double[][] a) {
// Reversed with the intended order:
for (int i=0; (i <= (a.length-1));i++){
for (int q=0; (q <= (a[1].length-1));q++){
System.out.println("retry" + "[i="+i+"]"+"[q="+q+"]"+"="+a[i][q]);
//System.out.println("This is the wrong order of for loops" + "[q="+q+"]"+"[i="+i+"]"+"="+a[q][i]);
}
}
// The actual solution for loop:
for (int i=0; (i <= (a.length-1));i++){
for (int q=0; (q <= (a[1].length-1));q++){
//System.out.println("retry" + "[i="+i+"]"+"[q="+q+"]"+"="+a[i][q]);
System.out.println("This was the reversed order, with corrected counter" + "[q="+q+"]"+"[i="+i+"]"+"="+a[q][i]);
//error here, since a.length is shorter than a[1].length, and the columns and rows are reversed.
}
}
}
}页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/52374921
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