c - What does this not work for getchar() method? -

this own experiment understand goes under hood, program mean compiler?

main() {   int c;   printf("%d\n",c);   printf("%d ", getchar());     while ((c == getchar()) != eof){     putchar(c);   } } 

when c must equal getchar() (c == getchar()), not proceed through while loop? confused of own code, of c must mean!

also, in code:

main() { int c; c = getchar() while ((c = getchar()) != eof) putchar(c); } 

if modify int c int c = getchar(), why cannot somply write this:

while (c != eof)(     putchar(c);     c = getchar();     } 

the compiler should know previous statement c = getchar(), why have write statement again? sorry, if confused.

while ((c==getchar()) != eof) {   ... } 

is while loop. evaluates condition each iteration of loop , terminates if condition false.

in case, condition is:

(c==getchar()) != eof) 

which nonsensical expression, let's examine anyway:

first, program evaluate:

    getchar() 

this grabs keystroke standard input. value of expression value of key.

then:

 c==getchar() 

this takes result of getchar() , compares whatever in c. in first program, c uninitialized, value indeterminate. if c had defined value, c==getchar() evaluate either true or false. since c had no defined value, c==getchar() has no defined value.

now program evaluates:

(c==getchar()) 

which still true or false, except in case undefined.

the program next considers:

(c==getchar()) != eof 

that is, compares true-false value eof; makes no particular sense, , in case still have undefined behavior of uninitialized c.

in sum, if c initialized, expression fetch key standard input , compare either true or false eof. said, nonsensical expression.