Loops

Ha Khanh Nguyen (hknguyen)

• 1. Smallest Even Problem
• 2. for Loops
  • 2.1 Use for loop for the smallest even problem
  • 2.2 for loop with strings
  • 2.3 range() function
• 3. while Loops

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1. Smallest Even Problem

• You are given 3 numbers x, y, z. Find the smallest even number among them and store it in a variable called result. If none of them is even, then result = -1. Note that they are all positive numbers.

even = []
if x % 2 == 0:
    even.append(x)
if y % 2 == 0:
    even.append(y)
if z % 2 == 0:
    even.append(z)

if even == []:
    result = -1
else:
    result = min(even)

• Now, let's say instead of 3 numbers, you are given 10 numbers! Well, the logic is the same, right? But we will have to write 10 if statements, each for each number. That's tedious but doable.
• What's about 100 numbers?
• There must be a way for us to tell Python to repeat this statement for all the numbers provided!
• That is the idea of loops in programming!

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2. for Loops

• for loops are for iterating over a list of objects (or a collection of objects or an iterater).
• The standard syntax for a for loop is:

for value in list:
    # do something with value

• Example:

nums = [5, 2, 9, 14, 3]
for i in nums:
    print(i)

5
2
9
14
3

• Let's do something more interesting than printing out the number:

nums = [5, 2, 9, 14, 3]
sum = 0
for i in nums:
    if i % 2 == 0:
        sum = sum + i
sum

16

• See video for the step by step walkthrough of the code segment above.

2.1 Use for loop for the smallest even problem

• Let's say 10 numbers are given to us in a list named nums.

even = []
for i in nums:
    if i % 2 == 0:
        even.append(i)

if even == []:
    result = -1
else:
    result = min(even)

• Similarly, we can use the same logic above for the other variations of this problem: smallest odd, largest even, largest odd.

2.2 for loop with strings

• for loop works very well with string!

s = 'hello'
for c in s:
    print(c)

h
e
l
l
o

• Example:
  • You're a given a string s containing a mathematical expression with 2 operands and an addition. For example, s = '3+2.5'. There is no space between the number (operand) and the operator. Write a Python program to evaluate the expression and store the result in a variable named result.

# method 1
temp = ''
for c in s:
    if c == '+':
        num1 = float(temp)
        temp = ''
    else:
        temp = temp + c
num2 = float(temp)
result = num1 + num2

• The split() function in Python splits a string by a specified delimiter.

'hello'.split('e')

['h', 'llo']

• We can split s by +!

# method 2
nums = s.split('+')
result = float(nums[0]) + float(nums[1])

2.3 range() function

• The range() function in Python returns an iterator that yields a sequence of evenly spaced integers.
• What does this mean?

range(10)

range(0, 10)

list(range(10))

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

• The syntax of the range() function is: range(start, end, step)

list(range(0, 20, 2))

[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

list(range(5, 0, -1))

[5, 4, 3, 2, 1]

• Sometimes, we don't want to iterate over a list of objects in the "default" way, instead we want to iterate the index of the list and access the elements by the index:

nums = [5, 2, 9, 14, 3]
total = 0
for i in range(len(nums)):
    if nums[i] % 2 == 0:
        total = total + nums[i]
total

16

total2 = 0
for i in nums:
    if i % 2 == 0:
        total2 = total2 + i
total2

16

---

3. while Loops

• A while loop specifies a condition and a block of code that is to be executed until the condition evaluates to False or the loop is explicitly ended with break.

nums = [1, 14, 3, 7, 32, 19, 4]
i = 0
total = 0
while total < 20:
    total = total + nums[i]
    i = i + 1
total

25

• The use of the break keyword is extremely useful in cases like this:

nums = [1, 1, 3, 2, 2, 1, 4]
i = 0
total = 0
while total < 20:
    total = total + nums[i]
    i = i + 1

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-12-6acf41fe9194> in <module>
      3 total = 0
      4 while total < 20:
----> 5     total = total + nums[i]
      6     i = i + 1

IndexError: list index out of range

• To fix this error:

nums = [1, 1, 3, 2, 2, 1, 4]
i = 0
total = 0
while total < 20:
    if i == len(nums):
        break
    total = total + nums[i]
    i = i + 1
total

14