An Introduction To Python - Strings & I/O

An Introduction To Software
Development Using Python
Spring Semester, 2015
Class #4:
Strings & I/O

First, Do It By Hand!
• Upon being handed a problem, you may first
want to start coding – don’t!
• First solve the problem by hand.
• If you can’t do that, then you can’t code it.
Image Credit: blogs.msdn.com

The Floor Tile Problem
• A row of black and white tiles needs to be placed along a wall. For aesthetic
reasons, the architect has specified that the first and last tile shall be black.
• Your task is to compute the number of tiles needed and the gap at each end, given
the space available and the width of each tile.
• To make the problem more concrete, let’s assume the following dimensions:
• Total width: 100 inches
• Tile width: 5 inches
100
5”

Solving The Tile Problem
Total Width / Tile Size = 100 / 5 = 20 tiles
Problem:
Wrong Color!
5” 95”
10”
95” / 10” = 9.5 sets of tiles
Answer: 1 black tile + 9 sets of 2 tiles (18 tiles) = 19 tiles
Side Gap = 100” – (19 x 5”) = 100” – 95” = 5”, 5” / 2 = 2.5” on each side

Tile Problem Algorithm
number of pairs of tiles =
integer part of [(total width – one tile width) / (2 x one tile width)]
number of tiles = 1 + (2 x number of pairs)
gap at each end = (total width – (number of tiles x tile width)) / 2
Note: We can now solve this problem for any total width and tile width!

Describe The Steps Necessary For
Finding A Solution To A Problem
Problem:
You put $10,000 into a bank account that earns 5 percent interest per year.
How many years does it take for the account balance to be double the original?

It’s All In The Description
Create a description of the steps for finding the
solution.
Each step must be clear and unambiguous, requiring
no guesswork.
Your informal description is called pseudocode.
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What Is An Algorithm?
An algorithm for
solving a problem is
a sequence of steps
that is unambiguous,
executable, and
terminating.
The existence of an algorithm
is an essential prerequisite for
programming a task.
You need to first discover and
describe an algorithm for the
task that you want to solve
before you start programming
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Say Hello To Strings!
• Your computer programs will have to process
text in addition to numbers.
• Text consists of characters: letters, numbers,
punctuation, spaces, and so on.
• A string is a sequence of characters.
• Example: “John Smith”
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How Do We Deal With Strings?
• Strings can be stored in variables:
answer = “a”
• A string literal denotes a particular string:
“Mississippi”
• In Python, string literals are specified by enclosing a
sequence of characters within a matching pair of
either single or double quotes.
“fire truck” or ‘fire truck’
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Concatenation and Repetition
• Concatenation
– Join two strings into one long string
• Vacation = “under”+”water” results in “underwater”
• How to insert a space:
holiday = “Christmas”+ “ ”+”Tree” results in
“Christmas Tree”
• Repetition
– Dashes = “-” * 25 creates “-------------------------”
Must be an integer
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Converting Between
Numbers and Strings
• Convert a number to a string
– str(): str(17) = “17”
– maxFallRate = str(9.8)+” meters / sec”
• “9.8 meters / sec”
• Convert a string to a number
– Integer: int(“17”) = 17
– Float: float(“17.0 ”) = 17.0
– houseNumber = int(“15715”)
Blank spaces are ignored
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Strings and Characters
• Strings are sequences of Unicode characters.
• You can access the individual characters of a
string based on their position within the
string.
• This position is called the index of the
character.
Image Credit: www.fontspace.com
0 1 2 3 4
First Last

String Indexes
• name = “TILEZ”
• len(name) = 5
• name[0] = “T”, name[4] = “Z”
0 1 2 3 4

String Methods
• A method, like a function, is a collection of
programming instructions that carry out a
particular task.

All Characters Have Numbers
• A character is stored internally as an integer value. The
specific value used for a given character
is based on its ASCII code (or Unicode code).
• Python provides two functions related to character encodings.
– The ord function returns the number used to represent a given
character. ord(“A”) = 65
– The chr function returns the character associated
with a given code. chr(65) = “A”
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What The Cool Kids Know
• Escape Sequences
– To include a quotation mark in a literal string, precede it with a backslash (),
like this: "He said "Hello""
– The backslash is not included in the string. It indicates that the quotation mark
that follows should be a part of the string and not mark the end of the string.
The sequence " is called an escape sequence.
– To include a backslash in a string, use the escape sequence , like this:
"C: TempSecret.txt"
– Another common escape sequence is n, which denotes a newline character.
Printing a newline character causes the start of a new line on the display. For
example, the statement print("*n**n***") prints the characters
• *
• **
• ***
on three separate lines
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Input and Output
• When a program asks for user input, it should first print a message that
tells the user which input is expected. Such a message is called a prompt.
In Python, displaying a prompt and reading the keyboard input is
combined in one operation.
– first = input("Enter your first name: ")
• The input function displays the string argument in the console window
and places the cursor on the same line, immediately following the string.
– Enter your first name: █
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Numerical Input
• The input function can only obtain a string of text from the
user.
• To read an integer value, first use the input function to obtain
the data as a string, then convert it to an integer using the int
function.
numBottles = input("Please enter the number of bottles: ")
bottles = int(numBottles)
bottlePrice = input("Enter price per bottle: ")
price = float(bottlePrice)
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Sample Program
• Write a program that reads a number between 1,000
and 999,999 from the user, where the user enters a
comma in the input. Then print the number without
a comma.
• Here is a sample dialog; the user input is in color:
Please enter an integer between 10,000 and 99,999:
23,456
23456
Note: there are 2 different ways to solve this

Formatted Output
• To control how your output looks, you print a formatted string and then
provide the values that are to be “plugged in”.
• If the value on the right of the “%” is a string, then the % symbol becomes
the string format operator.
• The construct %10.2f is called a format specifier: it describes how a value
should be formatted.
– The 10 specifies the size of the field, the 2 specified the number of digits after the “.”.
– The letter f at the end of the format specifier indicates that we are formatting a floating-
point value. Use d for an integer value and s for a string;

Formatted Output
• To specify left justification for a string, add a
minus sign before the string field width:
title1 = "Quantity:“
title2 = "Price:"
print("%-10s %10d" % (title1, 24))
print("%-10s %10.2f" % (title2, 17.29))
• The result is:
Quantity: 24
Price: 17.29
Image Credit: www.theindianrepublic.com

Python Format Specifier Examples

What’s In Your Python Toolbox?
print() math strings I/O

What We Covered Today
1. Algorithms
2. String
3. Input / Output
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What We’ll Be Covering Next Time
1. IF Statement
2. Relational Operators
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An Introduction To Python - Strings & I/O

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