5 More Strings and Loops

5_Loops

Notebook 5 - Loops¶

You can make a copy of this notebook by selecting File->Save a copy in Drive from the menu bar above.

Things you'll learn in this lesson:

more about strings
while loops and for loops
break and continue statements

Loops¶

Stop repeating yourself¶

Let's write a program that prints the first three integers along with the square of each (the number multiplied by itself).

i = 1
print(i, i*i)
i = 2
print(i, i*i)
i = 3
print(i, i*i)

In [ ]:

i = 1
print(i, i*i)
i = 2
print(i, i*i)
i = 3
print(i, i*i)

Two problems with this program¶

scalability - what happens to this program if I change the specification from the first three integers to the first 1,000 integers?

You need to type 2,000 more lines of code.
maintainability - what happens to this program if I change the specification to require the cube of each number?

You need to edit every single print statement. After expanding this program to cover the first 1,000 integers, you'd have to change 1,000 lines!

`while` Loops¶

The while statement is one way we can express repeatable logic in Python.
while statements define a condition and a sequence of statements in this format...

while condition:    
    block of indented statements
    ...
print('done with loop')

The condition is checked on entry to the loop and after each execution of the loop block.
Similar to if statement except the block is repeatedly executed as long as the condition is true.
When the condition becomes false, the loop is exited, i.e. it stops repeating and control moves to the statement after the loop.
If the condition is initially false, the loop is never entered, i.e. the loop block is never executed, not even once.

`while` Loop Structure¶

In Python, the extent of a while block is defined by indentation (just like if statement blocks)

i = 0
while i < 3:    
    print(i)    
    i = i + 1
print(i)

What does the output look like...

when i is initialized to 0?
when i is initialized to 1?
when i is initialized to 2?
when i is initialized to 3?
when i is initialized to 4?

In [ ]:

i = 0
while i < 3:
    print(i)
    i = i + 1
print(i)

Loop Counter¶

variable that controls entry into the loop body
often keeps track of the number of times the loop is executed
must be initialized
must be changed at some point in the loop block to ensure the loop is executed the intended number of times

Assignment Operators¶

i = i + i can be written like this: i += 1
More generally, i = i + x can be written as i += x, which adds the value of x to i and assigns the result to i.
You'll often see this syntax inside loops when we want, for example, to increment a loop counter every time through the loop body.
We have similar assignment operators available corresponding to the other arithmetic operators:

operator	equivalent
x += y	x = x + y
x -= y	x = x - y
x *= y	x = x * y
x /= y	x = x / y
x **= y	x = x ** y
x %= y	x = x % y

Generating Squares Using a `while` Loop¶

i = 1
while i <= 3:    
    print(i, i*i)    
    i += 1

Does this work?
Is this version an improvement? Why?

In [ ]:

i = 1
while i <= 3:
    print(i, i*i)
    i += 1

Scalability¶

Can you make this work for the first 1,000 integers?

i = 1
while i <= 1000:  # changed 3 to 1000    
    print(i, i*i)    
    i += 1

We scaled this version to 1,000 integers by changing only one line of code!

In [ ]:

i = 1
while i <= 1000:  # changed 3 to 1000
    print(i, i*i)
    i += 1

Maintainability¶

Can you print both the square and the cube (3rd power) for the first 1,000 integers?

i = 1
while i <= 1000:    
    print(i, i*i, i**3) # added cube to print    
    i += 1

The previous version would have required us to edit 1,000 print statements.
We added support for printing cubes in this version by changing only one line of code!

In [ ]:

i = 1
while i <= 1000:
    print(i, i**2, i**3) # added cube to print
    i += 1

Another Problem¶

Notice that the loop limit is embedded in the while condition.
It would be nice if this "configuration data" were separate from the program logic.
It would also be nice if we could change the limit dynamically without having to change the code, for example, as a function of user input.

A Better Version¶

limit = 10
i = 1
while i <= limit:
    print(i, i*i, i**3) # added cube to print
    i += 1

By using a variable, we can set the limit separately from the loop condition.

In [ ]:

limit = 10
i = 1
while i <= limit:
    print(i, i*i, i**3) # added cube to print
    i += 1

An Even Better Version¶

limit = int(input('desired number of integers: '))
i = 1
while i <= limit:
    print(i, i*i, i**3) # added cube to print
    i += 1

By using a variable, we can set the limit separately from the loop condition.
We can also set the limit dynamically, by gathering input from the end user.

In [ ]:

limit = int(input('desired number of integers: '))
i = 1
while i <= limit:
    print(i, i*i, i**3) # added cube to print
    i += 1

Challenge - Find the Bugs¶

There are (at least) four bugs in this program. Can you find them?

start = input('desired starting integer: ")
input('desired ending integer: ')
while i < end:
  print(i, i*i, i**3)

In [ ]:

start = input('desired starting integer: ")
input('desired ending integer: ')
while i < end:
  print(i, i*i, i**3)

Nested `while` Loops¶

Just as we had if statements of if statements, we can have while loops of while loops (and so on...)

i = 1
while i <= 3:
    j = 1
    while j <= 3:
        print(i, '/', j, end='\t')
        j += 1
    print()
    i += 1

Result:

1 / 1    1 / 2    1 / 3
2 / 1    2 / 2    2 / 3
3 / 1    3 / 2    3 / 3

In [ ]:

i = 1
while i <= 3:
    j = 1
    while j <= 3:
        print(i, '/', j, end='\t')
        j += 1
    print()
    i += 1

Infinite Loops¶

An infinite loop is a loop that never ends.
This is usually not by design (!).

Does this loop end?

i = 1
while i <= 3:
    print(i, i*i, i**3)

Challenge: how about this one...

i = 5
while i != 0:
    print(i, i*i, i**3)
    i -= 2 # same as i = i - 2

In [ ]:

i = 5
while i != 0:
    print(i, i*i, i**3)
    i -= 2 # same as i = i - 2

Debugging Infinite Loops¶

inserting print statements to see what's going on
we call this "tracing code"
when problem is fixed, you can comment out or delete your tracing code

Example:

# calculate sum of first n integers
i = 1
n = 10
sum = 0
while i <= n:
    i += 1
    #print(f"adding i={i} to sum={sum}")
    sum += i
print('sum of 1 to', n, '=', sum)

In [ ]:

i = 1
while i < 100:
  print("I'm running forever")

In [ ]:

# calculate sum of first n integers
i = 1
n = 10
sum = 0
while i <= n:
    i += 1
    #print(f"adding i={i} to sum={sum}")
    sum += i
print('sum of 1 to', n, '=', sum)

This progam is fail-safe, right?¶

start = int(input('starting integer: '))
end   = int(input('ending integer: '))
incr  = int(input('increment: '))
i = start
while i <= end:
    print(i, i*i, i**3, i**4)
    i += incr

Is there any way this program can go into an infinite loop?

In [ ]:

start = int(input('starting integer: '))
end   = int(input('ending integer: '))
incr  = int(input('increment: '))
i = start
while i <= end:
    print(i, i*i, i**3, i**4)
    i += incr

Trusting User Input¶

Getting input from external sources adds flexibility because program behavior can be changed at runtime (without requiring you to change your code).
The price of that flexibility is that you have to worry about the consequences of receiving bad input.

A More Robust Version¶

import sys # needed for exit() function
start = int(input('starting integer: '))
end   = int(input('ending integer: '))
incr  = int(input('increment (>0): '))
if (incr <= 0):
    sys.exit('incr must be positive')
i = start
while i <= end:
    print(i, i*i, i**3, i**4)
    i += incr

you should also verify start and end are numeric
you can do that with the string method isnumeric()

`break` Statement¶

The break statement exits a loop immediately.
It's often used to exit a loop under certain circumstances.
It's Python's way of saying "get me out of here!"

`break` Example¶

Our last version catches bad input but is very unforgiving - it exits as soon as it gets bad input.
This version gives the user more chances to get it right...

while True:
    incr = int(input('increment (>0): '))    
    if incr > 0:        
        break
    print('bad input, increment must be positive')
print('off we go...')

In [ ]:

while True:
    value = int(input('positive value (>0): '))
    if value > 0:
        break
    print('bad input, value must be positive')
print('off we go...')

`continue` Statement¶

The continue statement jumps to the top of a loop immediately.
The loop condition is retested and the next iteration begun (if true) or the loop is exited (if false).
It's Python's way of saying "let's take it from the top".

`continue` Example¶

Assume we want start and end to be positive.
If the start value is bad, don't bother prompting for end.

start = 0
end = 0
while (start <= 0) or (end <= 0):    
    if start <= 0:
        start = int(input('starting integer (>0): '))        
        if (start <= 0):            
            continue
    if end <= 0:
        end = int(input('ending integer (>0): '))
        if end <= 0:            
            continue
    print(‘using start’, start, ‘and end’, end)

In [ ]:

start = 0
end = 0
while (start <= 0) or (end <= 0):
  if start <= 0:
      start = int(input('starting integer (>0): '))
      if (start <= 0):
          continue
  if end <= 0:
      end = int(input('ending integer (>0): '))
      if end <= 0:
          continue
  print(f"using start {start} and end {end}")

`for` Loops¶

while loops repeat program logic based on a condition.
for loops repeat program logic based on the contents of a sequence .
We say they "iterate over a sequence".

For example, this program prints the characters in a string, one character per line:

mystr = 'this is a test'
for i in mystr:
    print(i)

Result:

t
h
i
s
...

In [ ]:

for i in 'Python':
    print(i)

Numerical `for` Loop¶

For loops can also be used with a sequence of numbers, similar to while loops.
For example, this program prints the first three integers and their squares...

for i in (1, 2, 3):
    print(i, i*i)

Result:

1 1
2 4
3 9

Notice a big advantage over while loops: there's no need to separately initialize and increment a loop counter.
But enumerating the range of numbers is cumbersome.

In [ ]:

for i in (1,2,3, 4, 5, 6, 7, 8, 9, 10):
  print(i, i*i)

The `range()` Function¶

the Python range() function generates a sequence of numbers
prototype: range([start], end, [increment])
start is optional and defaults to zero
end is required and terminates the sequence at end-1
increment dictates the interval between elements
increment is optional and the defaults to one
range() returns a special Python type called a "range"

In [ ]:

for i in range(0, 10):
  print(i, i*i)

`range` works great in `for` loops¶

Print the print first 20 integers and their squares

for i in range(1, 21):
    print(i, i*i)

In [ ]:

for i in range(1, 21):
    print(i, i*i)

Nested `for` Loop¶

Just as we have seen if statements of if statements, and while loops of while loops, we can have for loops of for loops (and so on...)

Example: print a 9-by-9 multiplication table

print('X', end='\t')
for i in range(1, 10):
    print(i, end='\t') # column headings
print()
for i in range(1, 10):
    print(i, end='\t') # row headings
    for j in range(1, 10):
        print(i * j, end='\t') # products
    # print newline at end of each row
    print()

In [ ]:

print('X', end='\t')
for i in range(2, 10):
    print(i, end='\t') # column headings
print()
for i in range(2, 10):
    print(i, end='\t') # row headings
    for j in range(2, 10):
        print(i * j, end='\t') # products
    # print newline at end of each row
    print()

Example Program - Image Manipulation¶

In [ ]:

from IPython.display import display, Javascript
from google.colab.output import eval_js
from base64 import b64decode
def take_photo(filename=‘photo.jpg’, quality=0.8):
js = Javascript(’’’
    async function takePhoto(quality) {
      const div = document.createElement(‘div’);
      const capture = document.createElement(‘button’);
      capture.textContent = ‘Capture’;
      div.appendChild(capture);
      const video = document.createElement(‘video’);
      video.style.display = ‘block’;
      const stream = await navigator.mediaDevices.getUserMedia({video: true});
      document.body.appendChild(div);
      div.appendChild(video);
      video.srcObject = stream;
      await video.play();
      // Resize the output to fit the video element.
      google.colab.output.setIframeHeight(document.documentElement.scrollHeight, true);
      // Wait for Capture to be clicked.
      await new Promise((resolve) => capture.onclick = resolve);
      const canvas = document.createElement(‘canvas’);
      canvas.width = video.videoWidth;
      canvas.height = video.videoHeight;
      canvas.getContext(‘2d’).drawImage(video, 0, 0);
      stream.getVideoTracks()[0].stop();
      div.remove();
      return canvas.toDataURL(‘image/jpeg’, quality);
    }
    ‘’’)
display(js)
data = eval_js(‘takePhoto({})’.format(quality))
binary = b64decode(data.split(’,’)[1])
with open(filename, ‘wb’) as f:
f.write(binary)
return filename

In [ ]:

from IPython.display import Image
try:
  file = take_photo()
  print(f"Image saved to {file}")
  image = Image(file)
# Show the image which was just taken.
display(image)
except Exception as err:
# Errors will be thrown if the user does not have a webcam or if they do not
# grant the page permission to access it.
print(str(err))

In [ ]:

import matplotlib.pyplot as plt
I = plt.imread('photo.jpg')
I = I.sum(axis=-1)
plt.figure()
plt.imshow(I, cmap="gray")
iterations = 10
for x in range(iterations):
  for i in range(1,I.shape[0]-1):
    for j in range(1,I.shape[1]-1):
      I[i,j] = (
                 I[i-1,j-1] + I[i-1,j] + I[i-1,j+1] +
                 I[i,j-1]   + I[i,j]   + I[i,j+1] +
                 I[i+1,j-1] + I[i+1,j] + I[i+1,j+1]
                ) / 9.0
plt.figure()
plt.imshow(I, cmap="gray")

Challenges¶

Question 1¶

Using a while loop, repeatedly prompt the user for an input string, and print the input string.
Exit the loop when the user enters only the character ‘q’ or an empty string.

Enter a string (‘q’ to quit): this is a test
You entered: this is a test
Enter a string (‘q’ to quit): another test
You entered: another test
Enter a string (‘q’ to quit): q
Exiting...

In [ ]:

# Add your code here

In [ ]:

#@title Double click here to reveal solution
while True:
value = input(“Enter a string (‘q’ to quit): “)
if not value or value == ‘q’:
break
print(f“You entered: {value}")

Question 2¶

Building on the previous program, for each string entered, print the string as entered, followed by an all upper case version and an all lower case version, like this:

Enter a string (‘q’ to quit):  I Am The Walrus
Original string: I Am The Walrus
Upper case:      I AM THE WALRUS
Lower case:      i am the walrus

In [ ]:

# Add your code here

In [ ]:

#@title Double click here to reveal solution
while True:
value = input(“Enter a string (‘q’ to quit): “)
if not value or value == ‘q’:
break
print(f“Original string: {value}")
print(f“Upper case:      {value.upper()}")
print(f“Lower case:      {value.lower()}")

Question 3¶

Building on the previous program, for each string entered, print whether the entered string is all upper, all lower or mixed case, like this:

Enter a string (‘q’ to quit): this is a test
You entered: this is a test, which is lower case
Enter a string (‘q’ to quit): ANOTHER TEST
You entered: ANOTHER TEST, which is upper case
Enter a string (‘q’ to quit): mIxEd CaSe
You entered: mIxEd CaSe, which is mixed case

Hints:

Use a for loop to examine the characters in the string.
Use isupper() and islower() string methods to examine the case of each character in the loop.
What can you infer by counting the upper and lower case characters in a given string?

In [ ]:

# Add your code here

In [ ]:

#@title Double click here to reveal solution
while True:
value = input(“Enter a string (‘q’ to quit): “)
if not value or value == ‘q’:
break
upper_cnt = 0
lower_cnt = 0
length = 0
case = None
for c in value:
if c.isspace():
continue
length += 1
if c.isupper():
upper_cnt += 1
elif c.islower():
lower_cnt += 1
if upper_cnt == length:
case = “upper”
elif lower_cnt == length:
case = “lower”
else:
case = “mixed”
print(f“You entered: {value}, which is {case} case”)

Question 4¶

The factorial of a number (written x!) is the product of 1, 2, ..., x.
In other words, it's the result of multiplying 1, 2, ..., all the way up to (and including) the number x itself.
Prompt the user for a number and print a table of factorials from 1 up to the entered number.
For example, if the user inputs 5, the output should look something like this (with table headings):

Don't worry if your columns don't line up perfectly.

In [ ]:

# Add your code here

In [ ]:

#@title Double click here to reveal solution
while True:
value = input(“Enter a number (‘q’ to quit): “)
if not value or value == ‘q’:
break
value = int(value)
fact = 1
print()
for i in range(1, value + 1):
fact *= i
print(f"{i}\t{fact}")
print()

Question 5¶

Repeatedly prompt the user for a string (until receiving ‘q’ or an empty string).
For each string received, use a for loop to create a reversed version of the string.
As you step through the characters in the string, successively build a reversed version by prepending each char to the reversed version of the string (i.e. revstr = char + revstr)
Print the reversed string along with an indication as to whether or not it's a palindrome, written the same forwards and backwards, like 'racecar'. * Don't worry about dealing with upper/lower case issues. You can assume all input is lower case.
Also don't worry about doing anything special with whitespace characters (assume "race car" is not a palindrome).

Enter a string: hello
hello/olleh - is NOT a palindrome
Enter a string: redivider
redivider/redivider - is a palindrome!

In [ ]:

# Add your code here

In [ ]:

#@title Double click here to reveal solution
while True:
value = input(“Enter a number (‘q’ to quit): “)
if not value or value == ‘q’:
break
revstr = ”"
for c in value:
revstr = c + revstr
if value == revstr:
print(f"{value}/{revstr} - is a palindrome!")
else:
print(f"{value}/{revstr} - is NOT a palindrome”)
print()

Just For Fun - Some Palindromes¶

Character-wise

A man, a plan, a canal, Panama!
I prefer pi
Dammit, I'm mad!
Ma is as selfless as I am
Never odd or even
Madam, I'm Adam

Word-wise

King, are you glad you are king?
Says Mom, "What do you do?" – You do what Mom says.
You know, I did little for you, for little did I know you.
Please me by standing by me please.
Blessed are they that believe they are blessed.
Escher, drawing hands, drew hands drawing Escher.

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Last modified April 11, 2025: adjust notebook 9 (18d1654)

5 More Strings and Loops

Notebook 5 - Loops¶

Loops¶

Stop repeating yourself¶

Two problems with this program¶

while Loops¶

while Loop Structure¶

Loop Counter¶

Assignment Operators¶

Generating Squares Using a while Loop¶

Scalability¶

Maintainability¶

Another Problem¶

A Better Version¶

An Even Better Version¶

Challenge - Find the Bugs¶

Nested while Loops¶

Infinite Loops¶

Debugging Infinite Loops¶

This progam is fail-safe, right?¶

Trusting User Input¶

A More Robust Version¶

break Statement¶

break Example¶

continue Statement¶

continue Example¶

for Loops¶

Numerical for Loop¶

The range() Function¶

range works great in for loops¶

Nested for Loop¶

Example Program - Image Manipulation¶

Challenges¶

Question 1¶

Question 2¶

Question 3¶

Question 4¶

Question 5¶

Just For Fun - Some Palindromes¶

`while` Loops¶

`while` Loop Structure¶

Generating Squares Using a `while` Loop¶

Nested `while` Loops¶

`break` Statement¶

`break` Example¶

`continue` Statement¶

`continue` Example¶

`for` Loops¶

Numerical `for` Loop¶

The `range()` Function¶

`range` works great in `for` loops¶

Nested `for` Loop¶