Java：for循环，不兼容的类型

Question

I'm trying to run this for loop;

        for (int col= 0; grid[0].length; col++)

However every time I try to compile I get an error stating 'incompatible types - found int but expected boolean'

I can't work out what I'm doing wrong!

Answer 1

the second statement: grid[0].length is an integer. The second statement in a for loop is a condition statement and needs to be a boolean.

If you're trying to loop while col is less than the length of grid[0], then you need this as your second statement:

col < grid[0].length;

Answer 2

for (int col= 0; col < grid[0].length; col++)   // See the typo

Answer 3

grid[0].length is the integer that the message refered to. A boolean value was expected there:

col < grid[0].length

Answer 4

您需要将代码更改为：
for (int col= 0; col<grid[0].length; col++)

Answer 5

For anyone interested in where this is specified: Check out docs.oracle.com for the Java Language Specification. Chapter 14 §14.1 states the rules for a basic for loop:

BasicForStatement:
  for ( [ForInit] ; [Expression] ; [ForUpdate] ) Statement

with the addendum that:

The type of the Expression must be boolean or Boolean, or a compile-time error occurs.

This particular citation was taken from the Java SE 17 specification. Since this post was 11+ years ago, (almost twelve at the time of this response) the question likely refers to Java SE 7, which has the same rule and almost the same verbiage verbatim.