15-110 Fall 2010 Quiz 5
10 Minutes

* No calculators, no notes, no books, no computers.
* Show your work.  Correct answers without supporting calculations will not receive full credit.

1.  Tracing (30 pts)
Indicate what the following code will print.

A) (15 pts)

a = [ 1, 3, 2, 4]
x = 0
while (x < a[0]):
    x += a.pop(0)
    print x, a

B) (15 pts)

b = [ "a", "bcd", "ef" ]
d = [ ]
for x in b:
    if (len(x) < len(b)):
        for c in x:
            d.append(c)
print d

2.  Mystery Method (20 pts)
State in just a few words of plain English what the following function does in general:

def mystery(a, b):
    # assume a and b are both lists of integers
    x = 0
    for i in range(max(len(a), len(b))):
        if (i < len(a)):
            x += a[i]
        if (i < len(b)):
            x -= b[i]
    return (x == 0)

3.       (50 pts) Write the function diagonal(matrix).  This function takes a matrix (a rectangular 2d list), and if it is a square matrix (same number of rows as columns), the function returns a 1d list composed of the values on the matrix’s top-left to bottom-right diagonal.  If the matrix is not square, the function returns None.  For example:
     diagonal([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
Returns:
     [1, 5, 9]

4.  Bonus/Optional (5 pts)
State in just a few words of plain English what the following function does in general:

def mystery(a):
    # assume a is a list of non-negative integers
    x = 0
    for i in range(max(a)):
        for z in a:
            if (z > i):
                x += z
    return x