Lab 0: intro to python

Comp 150, Dordal

Follow along with the following as I talk about them in class. The part you are to submit is at the bottom, under What you are to turn in. You should turn it in, by email, by Thursday, May 21.

1. To start python:

The file "foo.py" in your startup directory can be loaded with
    from foo import *
It is easiest to edit such a file with your preferred editor (wordpad?), and then import as needed. The idle program can be used to create and save a session, or just to run python, although python can also be run in a plain cmd window. Hint: it helps to start idle from a cmd window, because then you can cd to your working drive/directory first and then python will be in the right place.

2. Things to try:

How many digits can a number have?

3. Here is a simple function 
def greet(person):
	print "Hello", person
	print "How are you today?"
How does Python tell when you're done with def? 

4: ranges 

for i in range(1,10):
    print i

Tweak this to make a table of i, i*i.

Here's another example:

sum = 0
for i in range(1,100):
    sum += i

sum
5. What, by the way, is range(10), or range(20,40)? Can you type it by itself?
	z = range(1,10)
	z

6. The factorial function: 

def fact2(n):
	prod = 1
	for i in range(1,n+1):
		prod *= i
	return prod
7. That's the iterative version. But the standard mathematical definition of factorial, 
		1 if n=0
	n! =
		n*(n-1)! if n>0
tends to suggest the following recursive definition (this is also our first look at if-else):
def fact1(n):
	if n==0: return 1
	else:	return n*fact1(n-1)
8. fact1 gets into trouble at fact1(1000). What goes wrong? What is the largest value you can calculate?

9. More examples of printing. Note the middle line ends with a comma. What happens? You may have to put these in a function to see the difference.

print 3,4
print 5,6,
print 7,8

10. Here's a function with an error. What is it? How can it be fixed?

def addup(n):
    for i in range(n):
        sum += j
    return sum

11. Square roots. To find the square root of x, use math.sqrt(x). But note that the statement
    import math
must have been included (just once!) somewhere before using math.sqrt(x).

12. Some lists 

[1,2,2,3,4,5]
[[1], [1,2], "hello", 61]

z = [[1], [1,2], "hello", 61]

13. Now we can involve those lists in expressions: 

z[0], etc
len(z)

[z, [z,z]]
The in operator: 
61 in z
Concatenation: 
z+z
Slicing (maybe you need a longer z: try z=z+z to get one) 
z[1:3]
14. The quadratic formula. The formula for the roots of ax2+bx+c is x = (-b ± sqrt(b2-4ac))/2a. Implement a python function roots(a,b,c) to calculate this. Be careful of the denominator; it has to be in parentheses!You can calculate either just one of the two roots, or else return a list of both roots. If b2-4ac is negative, there are no (real) roots. Can you get the program to return the string "no roots" in this case?

What you are to turn in

Create a session file with Idle, containing the following:

1. An indication of the highest fact1(n) you can calculate, as per #8 above.

2. Your roots(a,b,c) function, and some successful runs (eg roots(1,3,2), roots(1,-3,2)). If you defined your function in a separate file and then imported it, be sure to turn that in too.

3. Turn in at least one non-numeric list expression with at least 50 elements in it. (If it weren't for the non-numeric part, range(0,50) would work.) Obviously, you can just type in a 50-element list. But that's tedious. Figure out how to make a longer list out of a short one!

Turn it in by emailing to pld@cs.luc.edu