Lec 12, Wed 02/12

Wednesday Lecture Notes…

Using Loops With a Negative Increment

i = 3
while i >= 0: #Notice we use > rather than < because we are going backwards
    print(i)
    i = i - 1 #same as i -= 1

This will output:

3
2
1
0

The same while loop as a for loop using the range function looks like:

for i in range(3, -1, -1): #In order to include the 0, we must use -1 as the stop value
    print(i)

These are pretty straight-forward when the elements we want to print are integers, but what if we wanted to loop through a list or a string, accessing each element?

Looping through a String or List

text = "Hello"

i = 0
while i < len(text):
    print(text[i])
    i += 1

The same while loop as a for loop with the range function looks like:

text = "Hello"
for i in range(0, len(text),1):
    print(text[i])

The same while loop as a for loop using text as an iterable looks like:

text = "Hello"
for char in text:
    print(char)

Question: When should I use the range vs when should I use the other for loop? Answer: Use the iterable version when you only care about the value/element and you do not care about the index position. If the index position matters, use the range-based for loop. If you are unsure, use the range-based for loop since you can still access the elements of the string or list by using indexing, and you will also be able to know the index position.

Nested Loops

If you want to print a right triange of stars like so:

*   
**  
*** 
****

Such that the base and height are equal, how could you do that?
Notice that for this triangle with 4 rows and 4 columns,

when r=0 and c=0, there should be a *
when r=1 and c=0, there should be a *
when r=1 and c=1, there should be a *
when r=2 and c=0, there should be a *
when r=2 and c=1, there should be a *
when r=2 and c=2, there should be a *
when r=3 and c=0, there should be a *
when r=3 and c=1, there should be a *
when r=3 and c=2, there should be a *
when r=3 and c=3, there should be a *

num = 4
result = ''
for row in range(0, num, 1): # same as range(num)
    for col in range(num):
        if row >= col:
            result += '*'
    result += '\n'
    
print(result)

If you were given a list of strings like so:

l = ["one","two","three"]

How could you print each element of l backwards, like so:

e
n
o

o
w
t

e
e
r
h
t

l = ["one","two","three"]
for element in l: # we dont need the index to look at each element in order
    for i in range(len(element) - 1, -1, -1): # the index DOES matter for printing each word backwards, so we will use range
        print(element[i])
    print('\n')

Question: How could you print each letter on the same line?

l = ["one","two","three"]
for element in l: 
    for i in range(len(element) - 1, -1, -1):
        print(element[i], end = '')
    print('\n')