Making “smart” loops

Making “smart” loops

The trick is “knowing” something about the whole loop when you are stuck writing code that only sees one entry at a time

Set some variables to initial values
for thing in data:
		Look for something or do something to each entry separately, 
		updating a variable
Look at the variables

Finding the largest value

We make a variable that contains the largest value we have seen so far. If the current number we are looking at is larger, it is the new largest value we have seen so far.

largest_so_far = -1
print('Before', largest_so_far)

for the_num in [9, 41, 12, 3, 74, 15]:
	if the_num > largest_so_far:
			largest_so_far = the_num
	print(largest_so_far, the_num)

print('After', largest_so_far)

---

Counting in a Loop

zork = 0
print('Before', zork)

for thing in [9, 41, 12, 3, 74, 15]:
		zork = zork + 1
		print(zork, thing)

print('After', zork)

To count how many times we execute a loop, we introduce a counter variable that starts at 0 and we add one to it each time through the loop.

Summing in a Loop

zork = 0
print('Before', zork)

for thing in [9, 41, 12, 3, 74, 15]:
		zork = zork + thing
		print(zork, thing)

print('After', zork)

To add up a value we encounter in a loop, we introduce a sum variable that starts at 0 and we add the value to the sum each time through the loop.

Finding the Average in a Loop

count = 0
sum = 0
print('Before', count, sum)

for value in [9, 41, 12, 3, 74, 15]:
		count = count + 1
		sum = sum + value
		print(count, sum, value)

print('After', count, sum, sum/count)

An average just combines the counting and sum patterns and divides when the loop is done.

Filtering in a Loop

print('Before')
for value in [9, 41, 12, 3, 74, 15]:
		if value > 20:
				print('Large number', value)
print('After')

We use an if statement in the loop to catch/filter the values we are looking for.

Search Using a Boolean Variable

found = false
print('Before', found)

for value in [9, 41, 12, 3, 74, 15]:
		if value == 3:
				found = True
		print(found, True)

print('After', found)

If we just want to search and know if a value was found, we use a variable that starts at False and is set to True as soon as we find what we are looking for.

How to find the smallest value

How would we change this to make it find the smallest value in the list?

Finding the smallest value

smallest = None
print('Before')

for value in [9, 41, 12, 3, 74, 15]:
		if smallest is None:
				smallest = value
		elif value < smallest:
				smallest = value
		print(smallest, value)

print('After', smallest)
		

We still have a variable that is the smallest so far.

The first time through the loop smallest is None, so we take the first value to be the smallest.

None Types

• 단 하나의 값만 가지는 특별한 표식 (None)
• None을 저장했는지 확인할 수 있음
• 공백을 나타내기 위해 쓰임

The “is” and “is not” Operators

• Python has an is operator that can be used in logical expressions
• Implies “is the same as”
• Similar to, but stronger than ==
• is not also is a logical operator

ex) 0 == 0.0 → True

0 is 0.0 → False [different data types]