COS 100: Introduction to Programming

Interim 2020

Lab 13: Bonus lab

Due to time constraint in this course, we've skipped over a lot of things. This optional bonus lab contains a few ideas that are very useful.

Lab 13A: Control flow in loops

In the body of a for or a while loop, you can use continue to skip the rest of the loop and start the next iteration or use break to get out of the loop.

Run this code to understand the effect of break and continue:

i = 0
while True:
    if i == 10:
        break
    i = i + 1
    print(i)
    if i % 2 != 0:
        continue
    print("even:", i)

Suppose gradebook.txt has the following format:
```
# max score
Max Perfect,6,10,20,20
# what follows are students
Rory Gilmore,5,9,13,7
Paris Geller,6,10,20,19
--
Paris, as always, did well.
Rory drifted off at the end.
```
where a line starting with # is a comment to be ignored and a line that is just -- marks the end of student records (what's below should be ignored).
Modify your load_file from the gradebook homework project to ignore comments (lines starting with #) and stop processing when (if) a line -- is reached.
See Zelle 8.5 for more details.

Lab 13B: Exceptions

When an error occurs, the program raises an exception.

A try block allows you to catch an exception (of a certain type) and do something in the except block.

try:
    num = int(input("Integer please: "))
except ValueError:
    print("That was not an integer!")

Modify your silo homework so that if the user provided bad input, you will repeatedly prompt the user until a good input is given.
Modify the animation from the graphics lab to exit gracefully (close the window, say bye to the user, and don't display error messages) when the user types Ctrl-C to quit.
See Zelle 7.4 for more details.

Lab 13C: String formatting

Strings have a format() method.

Run the code below and deduce its usage:

name = input('Name, please: ')
number = int(input('Number, please: '))
print('Hi {0}. You like {1}?'.format(name, number))

What's the curly braces { and } for? What happens if you swap the 0 and the 1? What if you omit the 0 and the 1?

You can make pretty columns with some special features of formatted strings.

people = ['Alice', 'Bob', 'Chrysanthemum', 'Donald']
hourlyWages = [15.456, 6.8, 254.25, 20]

for k in range(len(people)):
    print('{0:15}{1:f}'.format(people[k], hourlyWages[k]))

Look at the official documentation to see if you can figure out a way to force the hourly wages to be right-justified with exactly two digits beyond the decimal point.
See Zelle 5.8.2 for more details.

Lab 13D: Dictionaries

A dictionary can store key-value pairs.

wages = {'Alice': 15.456, 'Bob': 6.8, 'Donald': 20}
wages['Chrysanthemum'] = 254.25
print(wages)
print(wages['Alice'])
for name in wages:
    print(name, wages[name])
del wages['Bob']
print('Alice' in wages)
print('Bob' in wages)
print(20 in wages)
for (name, wage) in wages.items():
    print(name, wage)

See Zelle 11.7 for more details.