why c for loop condition accepts initialization

Question

Yesterday I have attended an interview, there I saw a weird program snippets.

By initial glance I decide that Snippet has compilation error in it. But when came home and tried manually in C compiler I found I was totally wrong

See interview code

#include<stdio.h>

void main()
{
   for(int i=0;i=5;i=5)// here condition and increment are assigned 
                       //wrongly and where I thought it is compilation 
                       //error during interview which isn't wrong
   {
      printf("helloworld \n");
   }
}

Output:

 helloworld
 helloworld
 helloworld
 helloworld
 helloworld
 helloworld
 .
 .
 .
 .(goes on and on)

output in C++ is similar to C

But,

when we run this code in java compiler

public class Forhottest {

public static void main(String args[])
{
    for(int i=0;i=5;i=5)// here it throws compilation error
    {
        System.out.println("helloworld");
    }
}

}

similarly I tried in PHP ,same problem arise as in java. Why C and C++ allow this kind of weird condition statement inside "for loop". what is reason behind it

Answer 1

In both C and Java, the second component of the for loop is an expression, and assignments are expressions. This is why you can have (syntactically speaking) the expression

i = 5

as the loop condition in a for-loop in both languages.

However, Java, unlike C, adds the following line to the static semantic definition of the for-loop.

The Expression must have type boolean or Boolean, or a compile-time error occurs.

In Java, the type of the expression

i = 5

is int, not boolean, so the Java compiler gives the error.

C does not have this restriction, as it tends to be a lot more permissive with types, with integers and booleans being more or less "the same thing." Details are a little technical, perhaps, but the integer 5 is implicitly coerced to truthiness, so you see an infinite loop.

Answer 2

false (in C) is 0. In C, everything that isn't 0 is true. Java doesn't work that way (Java is strongly typed, and won't allow an int to be implicitly converted to a boolean). So, the equivalent Java, would be something like

public static void main(String args[]) {
    for (int i = 0; (i = 5) != 0; i = 5)
    {
        System.out.println("helloworld");
    }
}

Which also produces an infinite loop printing helloworld.