of University Puerto Rico Mayagez Campus College of Engineering Department of Electrical and Computer Engineering ICOM4029 Compilers Professor: Bienvenido Vlez Technical Assistant: Ren D. Bada
Laboratory 4 PA's 1 & 2 I. Solution to Programming Assignment 1
Here's a sample solution to PA1: class Stack inherits IO{ (* Class Stack uses roughly the same code as class List in the samples except that the elements are Strings and some stack operations are added. It defines empty stacks. *) --isNil is used to check if the stack is empty isNil() : Bool {true}; --head returns the top element, which does not exist on empty --stacks so it is an error. top() : String { { abort(); ""; } }; --tail returns a Stack containing elements after the top element --This is an error on empty stacks. rest() : Stack { { abort(); self; } }; --push returns a new stack with argument "i" pushed. --If something is pushed, now the stack is not empty and is of --dynamic type NonEmptyStack. -i = the String to push into the stack push(i : String) : Stack { (new NonEmptyStack).init(i, self) }; --pop returns a new stack without the original top element. pop() : Stack{ rest() }; --print displays the contents of the stack. --It prints the top of the stack and then recursively prints --the rest. -l = the stack to be printed print(l : Stack) : Object { if l.isNil() then 0 else { out_string(l.top().concat("\n")); print(l.rest()); } fi
};
};
class NonEmptyStack inherits Stack { -- Class NonEmptyStack defines non-empty stacks. top : String; --the top element rest : Stack; --a stack containing the rest of the elements isNil() : Bool { false }; top() : String { top }; rest() : Stack { rest }; -- Initializes a NonEmptyStack object -i = the initial top element -s = the initial stack with the rest of the elements init(i : String, s : Stack) : Stack { { top <- i; rest <- s; self; } }; }; class Main inherits IO { theStack : Stack <- new Stack; --the stack object inStr : String; --for storing the command x : Bool <- false; --for exiting the program loop temp1 : String; --for storing an element : temp2 String; --for storing a 2nd element --switch switches the top 2 elements of the stack switch() : Object { { -- temporarily store and pop the next 2 elements temp1 <- theStack.top(); theStack <- theStack.pop(); temp2 <- theStack.top(); theStack <- theStack.pop(); -- push them in reverse order theStack <- theStack.push(temp1); theStack <- theStack.push(temp2);
} };
--sum adds the top 2 elements of the stack and pushes the result sum() : Object { let tempNum : Int, conv : A2I <- new A2I in { -- temporarily store and pop the the two operands temp1 <- theStack.top(); theStack <- theStack.pop(); temp2 <- theStack.top(); theStack <- theStack.pop(); -- calculate the sum
} };
tempNum <- conv.a2i(temp1) + conv.a2i(temp2); -- push the result theStack <- theStack.push(conv.i2a(tempNum));
--eval does the following: -- *if the top is 's' then switch the following 2 elements -- *if it is '+', add the 2 following elements and store the sum -- *otherwise, do nothing eval() : Object { if theStack.isNil() then 0 else { -- get the top of the stack temp1 <- theStack.top(); if temp1 = "s" then { theStack <- theStack.pop(); --switch the following 2 elements switch(); } else if temp1 = "+" then { -...