C语言 for 循环中是否允许多个条件？
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Are multiple conditions allowed in a for loop?
提问by Nikunj Banka
The following code runs without giving any errors or warnings
以下代码运行时没有给出任何错误或警告
#include<stdio.h>
int main(){
int i, j;
int p = 0, q = 2;
for(i = 0, j = 0; i < p, j < q; i++, j++){
printf("HelloWorld\n");
}
return 0;
}
However, the book Let Us C(Yashwant Kanetkar)says that only one expression is allowed in the test expression of a for loop.(see pg 115 of the book).
然而，Let Us C(Yashwant Kanetkar) 一书说在 for 循环的测试表达式中只允许一个表达式。（参见本书的第 115 页）。
I am not sure of the standard. Are multiple expressions allowed in the test expression of a for loop ?
我不确定标准。for 循环的测试表达式中是否允许多个表达式？
EDIT : I surely can join the two expressions but I was dumbstruck when I found the above code on this website. My question is that is this valid C code or not?
编辑：我当然可以加入这两个表达式，但是当我在这个网站上找到上面的代码时我傻眼了。我的问题是这个有效的 C 代码是否有效？
回答by simonc
The condition
条件
i < p, j < q
is allowed but probably isn't what was intended as it discards the result of the first expression and returns the result of j < qonly. The comma operatorevaluates the expression on the left of the comma, discards it then evaluates the expression on the right and returns it.
是允许的，但可能不是预期的，因为它丢弃第一个表达式的结果并返回j < qonly的结果。该逗号操作评估的逗号左侧的表达，将其丢弃，然后计算右侧，并返回它的表达。
If you want to test for multiple conditions use the logical AND operator &&instead
如果你想测试多个条件使用逻辑运算符&&，而不是
i < p && j < q
回答by vogomatix
You can link them together with boolean and (&&)
您可以使用布尔值和 (&&) 将它们链接在一起
for(i = 0, j = 0; (i < p) && (j < q); i++, j++){
The above won't print out anything in the loop because the (i < p)condition fails immediately as i & p are both the same value (0).
以上不会在循环中打印出任何内容，因为(i < p)条件立即失败，因为 i & p 都是相同的值 (0)。
Update: Your example is valid (but silly) C because if you start i=30 your loop will still execute 2 times, because the first result in a comma separated list is ignored.
更新：您的示例是有效的（但很愚蠢）C，因为如果您开始 i=30，您的循环仍将执行 2 次，因为逗号分隔列表中的第一个结果被忽略。
回答by Nithin Bhaskar
If you want to test both conditions, use the &&operator.
如果要测试这两个条件，请使用&&运算符。
What is happening in your code is related to how the comma operator ,works.
您的代码中发生的事情与逗号运算符的,工作方式有关。
Both i < pand j < qare evaluated, but only the result of the 2nd expression j < qis checked by the for loop.
既i < p与j < q被评估，但只有第二表达式的结果j < q是由for循环检查。
回答by user2580624
for(i = 0, j = 0; i < p && j < q; i++, j++){
回答by Rohan Mundhey
Even I have read That Book by Mr Yashwant Kanetkar. It do says that only one condition is allowed in a for loop, however you can add multiple conditions in for loop by using logical operators to connect them. In other books that I have Read Along time Ago, said that only one condition is allowed.
甚至我也读过 Yashwant Kanetkar 先生的那本书。它确实说在 for 循环中只允许一个条件，但是您可以通过使用逻辑运算符将它们连接到 for 循环中添加多个条件。在我之前读过的其他书中，说只允许一个条件。
