guess.py redux

Consider our class exercise guess.py from last week. How can we

  • keep track of whether the user guessed the number or not (outside the loop)?

  • stop asking the user for more numbers after they have guessed correctly?

To keep track of whether the user was successful of not, we need a boolean variable. We have two choices to guard against asking the user for numbers after they guess the right one:

  • if statement

  • break statement

guess.py if statement

wk04 guess if

guess.py break

wk04 guess break

While loop

for loops are an example of a definite loop. They always loop for a fixed number of times.

while loops can loop an undetermined amount of time.

while <condition>:
    <body>

where <body> executes for as long as <condition> evaluates to True.

wk04 while ex

iteration

i

i < 3

i + 1

1

0

T

1

2

1

T

2

3

2

T

3

4

3

F

exits

Infinite loops

A while loop can repeat forever if its <condition> never becomes False! This is called an infinite loop. If your program infinite loops, it will become stuck at the console.

  • Type Ctrl-C to forcibly exit your program when it infinite loops!

Exercise: guess.py while

The code below replicates the program with the for loop. Neither the if-statement or a break is needed.

wk04 guess 5

This example lets the user guess as many times as they wish!

wk04 guess inf

String formatting

Python3 offers several methods for string formatting.

Method 1:

Documentation

"string containing <token1> <token2> ... <token N>"%(<value1>, <value2>, ..., <valueN>)

where tokens can be

  • %s for strings

  • %f for floats

  • %d for integers

Tokens can take additional arguments.

  • %<NUM>s pads NUM spaces before the string

  • %<NUM>d pads NUM spaces before the integer

  • %<NUM1>.<NUM2>f pads NUM1 spaces before the float and NUM2 digits after the decimal

Demo

rosemary[~]$ python3
Python 3.6.9 (default, Nov  7 2019, 10:44:02)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> text = "apples"
>>> val = 3.14567
>>> quantity = 10
>>> "The string format: %s %f %f"%(text, val, quantity)
'The string format: apples 3.145670 10.000000'
>>> "The string format: %20s %f %f"%(text, val, quantity)
'The string format:               apples 3.145670 10.000000'
>>> "The string format: %2s %2.2f %2d"%(text, val, quantity)
'The string format: apples 3.15 10'
>>> "The string format: %5s %5.2f %5d"%(text, val, quantity)
'The string format: apples  3.15    10'
>>> "The string format: %10s %10.2f %10d"%(text, val, quantity)
'The string format:     apples       3.15         10'
>>> "The string format: %s %.3f %d"%(text, val, quantity)
'The string format: apples 3.146 10'

Method 2:

Documentation

"string containing {<token1>} {<token2>} ... {<token N>}".format(<value1>, <value2>, ..., <valueN>)
f"string containing {<value1>:<token1>} {<value2>:<token2>} ... {<valueN>:<token N>}"

where tokens can be

  • :s for strings

  • :f for floats

  • :d for integers

Tokens can take additional arguments similarly to the old method. However, you can also include the values to print along with their format if you pre-pend the string with f.

>>> "The string format: {:s} {:.3f} {:d}".format(text, val, quantity)
'The string format: apples 3.146 10'
>>> "The string format: {:s} {:f} {:d}".format(text, val, quantity)
'The string format: apples 3.145670 10'
>>> "The string format: {:10s} {:10.2f} {:10d}".format(text, val, quantity)
'The string format: apples           3.15         10'
>>> f"The string format: {text} {val} {quantity}"
'The string format: apples 3.14567 10'
>>> "The string format: {text} {val} {quantity}"
'The string format: {text} {val} {quantity}'
>>> f"The string format: {text} {val:0.2f} {quantity}"
'The string format: apples 3.15 10'

in keyword

wk04 in exs

Exercise: Boolean types practice

Handout 4

Handout 4: Answers

Functions

Function definition syntax:

def <name>(<param1>, <param2>, ...., <paramN>):
    <body>
    [return <value>]

  • You can pass as many arguments (aka parameters, aka inputs) as you like to a function

  • the return value is optional. If there is no return value, the function doesn’t return anything.

  • Above is a function definition. To actually call the function, you use the name and pass in parameters

validate.py

wk04 validate

area.py

wk04 area
wk04 area stack

Function execution summary

  • Pause when you see a function call

  • Create a stack frame for the function and push it onto the stack

    • allocate parameters

  • Copy argument values to parameters

  • Execute the function, line by line, until return

  • Send back the return value (if there is one)

  • Pop the function off the stack. Destroy the frame

  • Resume the calling function

Practice: Function stacks

Handout5.pdf

img/Handout5-Ans.jpg