cs2370 Notes: 04 Lists and Scripts

Reminder: HW01 #

Due tonight
Submit on Inkfish, just like the lab

Command Line #

Modern desktop / laptop style computers provide two distinct ways to interact with them:
- A Windows / Icons / Mouse / Pointer interface (or GUI)
- A text-based interface (or CLI)
Graphical interfaces are more discoverable in simple cases
Text-based interfaces are more expressive and flexibile

Windows has at least two different text-based shells, but we’ll focus on the traditional one for now: CMD.EXE

To start the shell, run CMD.EXE
Prompt: Drive Letter, Path, “>”
Basic commands: cd, dir
Running a Python script:
- No args: python foo.py
- Command line args: python foo.py arg1 arg2 arg3

Command Line Arguments #

import sys

print(sys.argv)

When we run a program, we have the option of passing it zero or more arguments. This is straightforward through a text interface, and possible but less straightforward through a GUI.

This can be a good way to create one program that can do slightly different things or operate on different stuff each time you run it.

import sys

_, name = sys.argv
print(f"Hello, {name}")

Lists #

Wait, that argv thing is one name with multiple values in it.

It’s a list, which is Python’s default data type for storing a collection of multiple items.

>>> xs = [1,2,3,4]
>>> len(xs)
>>> len("1234")

We can write list literals with square brackets.
The standard pattern for dealing with a list is a for loop.

xs = [1,2,3,4]

for x in xs:
    print("item:", x)

Let’s write a program that adds up a list of numbers.

xs = [1,2,3,4]
sum = 0;

for x in xs:
    # sum = sum + x
    sum += x
    
print("sum =", sum)

Command line args #

Let’s add up the command line args as integers:


import sys

sum = 0

for arg in sys.argv:
    if arg.isdecimal():
        sum += int(arg)

print("sum =", sum)


import sys

sum = 0

for arg in sys.argv[1:]:
    sum += int(arg)

print("sum =", sum)

Text Files #

nums.txt

read-file.py:

ff = open("nums.txt")

for line in ff:
    print("line:", line.trim())

sum-file.py:

ff = open("nums.txt")

sum = 0

for line in ff:
    sum += int(line)
    
print("sum =", sum)

Practice Scripts #

Print the even numbers from a text file
Print the longest command line argument
etc