Midterm Examples

Expressions

Operator precedence and associativity

Q: Let’s suppose that in Language A every operator is left associative but in Language B every operator is right associative. Moreover, in language A + has higher precedence than - but in language B the situtation is the opposite. For other operators the usual rules apply. For every expression below give their value both in languages A and B.

Expression Language A Language B

10 - 5 + 3

100 / 20 / 5

19 + 5 - 2 + 7 % 3 / 10 * 10

Q: For expression (a+b)%c*d draw the corresponding expression tree.

Q: For the following expression tree, give the corresponding expression. Additionally, show step by step how this tree will be reduced to calculate the expression’s value.

Conditional Expressions

Q: Write the value of a after each line and give an equivalent Python conditional expression:

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4 var x=5, y=8, a;
a = x>y ? "yes" : (y%2 == 0 ? "no" : "may be")
a = x<y ? "yes" : (y%2 == 0 ? "no" : "may be")
a = x==y ? "yes" : (x%2 == 0 ? "no" : "may be")

Q: Give the JS equivalent of the following Python code:

1 a = 5 if x>y else (4 if x<y else 3)

Q: Rewrite the following function by replacing all conditional statements by conditional expressions:

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5 def f(x,y):
    if x>y: a = x
    else: a = y
    if a>0: return g(a*2)
    else: return g(a/4)

Types

Type Casting

Q: Sometimes PLs cast a value into another type implicitly (automatically, without programmer’s explicitly stating to do so) to make some expression’s type correct. Find such implicit type castings in the following JS code. Explain each like “in line 7 variable x was cast from string to int”.

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13 var a = 5, s = "123", x = [3,4,5];

function f(p1, p2) {
  p1 = p1 || 7;
  if (p2) {
    // do something
  }
}

alert("My array is: " + x);
f(a,x);
console.log(a*s);
console.log(s+a);

Q: Give values for variables a and b such that in JS a == b is true but a === b is false.

Q: Give two different values for a such that either the if part or else part will be executed. Explain shortly how the code runs in both cases.

var a = ...
var first = a && a[0]
if (first) console.log(first)
else console.log("array is empty")

Lazy Evaluation in Booleans

Q: For boolean expressions, Python and Javascript use lazy evaluation (or sometimes called short circuting in this context). See this discussion. Give the value of a after each line in the following Python code:

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6 a = 5 or (1/0)                       # a is now ...
a = (0 or "abc") and (0 or "")       # a is now ...
a = (0 or "") and (0 or "abc")       # a is now ...
a = ("" or []) and ("abc" or 0)      # a is now ...
a = 0 or "abc" or [1,2]              # a is now ...
a = [] or (5 and [1,2]) or "xyz"     # a is now ...

Scope

Q: Give the console output of the following code snippet. Recall that, in Javascript var keyword creates a new local variable in the current scope:

var x=1, y=2, z=3;
                                                       //
function f() {
  var y = 4;
  z = 5;
  console.log("f1", x, y, z)
  g(x,y)
  console.log("f2", x, y, z)
  h(11,12);
  console.log("f3", x, y, z)
}
                                                       //
function g(a,b) {
  var x = 6;
  console.log("g1", x, y, z);
  function h(y,z) {
    console.log("h1", x, y, z);
    x = 7;
    z = 8;
    console.log("h2", x, y, z);
  }
  h(y,z);
  console.log("g2", x, y, z);
}
                                                       //
function h(y,z) {
    console.log("h3", x, y, z);
    x = 9;
    z = 10;
    console.log("h4", x, y, z);
}
                                                       //
console.log("global", x, y, z)
f()
console.log("global", x, y, z)

Q: Try to achieve the same output of the previous question by writing an equivalent code in Python. Note that, unlike Javascript, in Python global variables are not automatically accessable in local scopes. You need to use the global keyword to make them available.

Assignment Semantics

Q: Give the values of each local variable in function f at the end of its execution (i.e. the output of the print statements at the end):

def f():
a = [ 3, {'x':3, 'y':4}, [1,2,3], (10,20), "xyz" ]
b = a
    c = b[:]              # shallow copy
    (a0,a1,a2,a3,a4) = a  # equivalent of a0=a[0], a1=a[1] ... a4=a[4]
    a5 = a[2][:]
    a2.append("new")
    c[0] = 30
    c[1]['x'] = 40
    a[4] = "hello"
    a[0] = 50
    a3 = "world"
    for (k,v) in locals().items():
    print("{0} = {1}".format(k,v))
    #
f()

Q: What will be the output of the following code:

a = [1,2,3]
b = a
c = [1,2,3]
print(a==c)
print(a is b)
print(a is c)
a.append(4)
print(a==c)
print(a is b)
b = [1,2,3,4]
print(a == b)
print(a is b)

Q: What will be the output of the following code:

a = [1,2]
b = [1,2]
c = a
console.log(a == b)
console.log(c == a)
d = {'x':0}
e = {'x':0}
console.log(d == e)
console.log(d.x == e.x)

Functions

Argument Passsing

Q: What will be the value of global variables x1,x2,x3,x4,x5 at the end of this code (i.e. after call to f returns):

function g(a40, a41, a,b,d,e) {
    a40.push(50);
    a41 += "ghi"
    a *= 10
    b.c *= 10
    b.f += "ghi"
    d.push(50);
    e += "ghi"
}
function f(a1,a2,a3,a4,a5) {
    a1 *= 10;
    a2 += "def";
    a3.push(40);
    a4[0].push(40);
    a4[1] += "def";
    a5.a *= 10;
    a5.b.c *= 10;
    a5.b.f += "def";
    a5.d.push(40);
    a5.e += "def";  
    g(a4[0],a4[1],a5.a, a5.b, a5.d, a5.e)
}
var x1 = 1
var x2 = "abc"
var x3 = [1,2,3]
var x4 = [ [1,2,3], "abc" ]
var x5 = { 'a':1, 'b': {'c': 2, 'f':"abc"}, 'd': [1,2,3], 'e': "abc"}
f(x1,x2,x3,x4,x5);
// print the values of x1,x2,x3,x4,x5 at this point

Default arguments

Q: What will be the output of the following JS code:

function f(a,b,c,d) {
    if (b == undefined) b = 5;
    c = c || 10;
    d = d || 20;
    console.log([a,b,c,d]);
}
f(1)
f(1,2)
f(1,2,3,4)
f(0,0,0,0)

Python *args, **kargs

Q: What will be the output of the following Python code:

def f(a, b=10, c=20, *args, **kargs):
    d = args[2] if len(args)>2 else 40
    # if key "x" exists in kargs dict returns its value (kargs["x"]), otherwise returns default value 40
    e = kargs.get("x",50)
    print("a={}, b={}, c={}, d={}, e={}".format(a,b,c,d,e))
    #
f(1,2)
f(1,2,3,4)
f(1,2,3,4,5,x="x")
f(1,2,3,4,5,6,7)

Q: Write the equivalent of the following Python function in Java and Javascript. Note that in Javascript arguments array includes all the arguments passed (i.e. will include k at index 0 and inc at index 1 if you include them in function parameters). In Java, you can pass variable length arguments as an array as well: example1, example2.

def f(k, inc, *args):
    sum = 0
    for num in args:
        sum += num * k
        k += inc
    return sum
    #
print(f(5,3,4,1,2))  # 50 = 4*5 + 1*(5+3) + 2*(5+3+3)

Functions as first class values

If functions are first class values in a PL you can do use them as other value types like integer. For instance:
- Store them in variables: var f = function(){}
- Pass them to other functions as arguments
- Return them from functions
- Construct a list of functions or use them as values in objects

Q: What will be the output of the following Javascript code:

var x = 30;
function f1(a) {
    return a+10;
}
var f2 = function (b) {
    return b+x;
}
function f3(a,b) {
    return function(x) {
        return x+a+b;
    }
}
funcs = [function(c) {return c+20}, f1, f3(3,4), f2]
var f4 = funcs[0];
funcs.push(f4);
for (i=0; i<funcs.length; i++) {
    console.log(funcs[i](3))
}

Closures

Let’s suppose that some function (let’s call it outer) defines a function (inner) and then returns it. If the inner function references a local variable in outer function then those variables should’t be destroyed as what normally happens. If they were destroyed, when the returned inner function is called its code will not be able to use them. So for such cases those local variables are kept (as a closure of the inner function) but they are only available to the inner function.

Q: What will be the output of the following Javascript code:

function outer(start) {
  var end = 20;
  var inner = function() {
    start++;
    end--;
    console.log("start=" +start + " end=" +end);
  }
  return inner;
}  
                                                    //
var f1 = outer(5)
f1();f1();f1()
var f2 = outer(1)
f2();f2();
f1();
f2();

Higher Order Functions (map, filter, reduce, sort)

Q: Give the output of the following Javascript code and write the corresponding code in Python.

var teams = [
  { name: "HataySpor", "wins": 3, "draws": 2, "losses": 1},
  { name: "HataySporrr", "wins": 3, "draws": 2, "losses": 1},
  { name: "Karabük", "wins": 4, "draws": 0, "losses": 1},
  { name: "Muğlaspor", "wins": 7, "draws": 1, "losses": 0},
  { name: "UlaGücü", "wins": 1, "draws": 3, "losses": 0},
  { name: "Kötekli", "wins": 1, "draws": 7, "losses": 0},
  { name: "Dalaman", "wins": 2, "draws": 0, "losses": 2}
]
teams.map(function(t) {
   t.points = 2*t.wins+t.draws;
   t.played=t.wins+t.draws+t.losses
})
teams
  .filter(function(t) { return t.played >= 5;})
  .sort(function (t1,t2) {
    if (t1.points != t2.points) return t2.points - t1.points;
    else if (t1.played != t2.played) return t1.played - t2.played;
    else return t2.name.length - t1.name.length;
  })
  .forEach(function(t) {
    t.flag = true;  // will this affect teams[4] output below?
    console.log(t.name + ": " + t.points + "-" + t.played);
  })
console.log(teams[4]);

Q: Give the output of the following Javascript code (i.e. result’s final value):

var nums = [3,7,2,6,11,4,9,4,1,3]
var result = nums.reduce(function(accumulated, nextValue) {
     if (nextValue % 2 == 0) accumulated[0].push(nextValue);
     else accumulated[1].push(nextValue);
     return accumulated;
  },
  [ [], [] ]  // initial value for accumulated
)
console.log(result);

Midterm Examples

Expressions

Types

Type Casting

Lazy Evaluation in Booleans

Scope

Assignment Semantics

Functions

That’s all folks. See you at the midterm…