1 Homework Solution Key 02/07/06 Each problems was graded to 10 points. The Brownie part of Q2 was graded to an extra 10 points, but the total score for the homework was capped at 50. 1. Show how the C compiler tokenizes the following by rewriting them, inserting natural whitespace between tokens. For each, state whether it is a syntactically correct expression. a. x+++y b. x+++++y c. x+y+-z++ d. x+y+-z+x Answer: The way the expressions are tokenized are: a. x+++y x ++ + y This is a correct expression should be: x++ + y, since in C, ++ has higher precedence than + b. x+++++y x ++ ++ + y This is an incorrect expression, since the order of evaluation would be as follows: x++ ++ + y. After evaluation of x++, the expression returns the value of x, not the variable x, hence (value of x) ++ does not make sense, and will return an error. c. x+y+-z++ x + y + z ++ This is a correct expression. The evaluation order is as follows: x + y + - z++
++ has highest precedence, so it is evaluated first and returns the value of z before increment. d. x+y+-z+x x + y + z + x This is a correct expression, the evaluation order is: x + y + -z + x 2. Show how the C compiler tokenizes the following complete program. In a brief sentence, state the effect the program has when run. int i=0;main(){for(;"]<i;++i){---i;}"[i];readchar(i+++'-'-'-', '/'/'/'));}readchar(_,__){__==_/_?putchar(" Hello, world!\n);"[_]):_;} Answer: Answer to Question 2-1 (tokenization): (10 points) Tokenized expression: int i = 0 ; main ( ) { for ( ; "]<i;++i){---i;}" [ i ] ; readchar
( i ++ + '-' '-' , '/' / '/' ) ) ; } readchar ( _ , __ ) { __ == _ / _ ? putchar ( " Hello, world!\n);" [ _ ] ) : _
; } Answer to Question 2-2 (brownie): (10 points) The code __ == _ / _ ? putchar(" Hello, world!\n);"[_]) : _; causes a "floating point exception error" when The run. code is supposed to print out "Hello World" character by character. But it fails on the first iteration of the for loop for loop, since the parameters to readchar are (_ = 0, __ = 1). so _/_ = 0/0 would give a floating point error. Some compilers may substitute _/_ by '1', hence eliminating this error, so the behavior varies with compilers used. 3. Go over Muldners coding style guidelines in Chapters 1-4 and list the most important ones that the program of Question 2 violates. Answer: There are many valid answers to this question, a few ones are: - use of meaningful variable/function names - use of indentation - use of whitespace for readability 4. Find the values of the following expressions: (4a - 4 points, 4b - 3 points, 4c - 3 points) a) 1+(3-2)*4 / 5 % 6 should be evaluated as: 1 + ((((3-2) * 4) / 5) % 6) = 1 Note that *,/ and % have the same precedence, so they are evaluated left to right. At the same time, + has less precedence than *,/,%, so the expression in the parenthesis is evaluated first. b) 9 % 2 % 3 % 4 % 5 should be evaluated as: ((((9 % 2) % 3) % 4) % 5) = 1 All the % operators have ...