Cs190 295 programming in python for life sciences lecture 6
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CS190/295 Programming in Python for Life Sciences: Lecture 6 PowerPoint PPT Presentation

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CS190/295 Programming in Python for Life Sciences: Lecture 6. Instructor: Xiaohui Xie University of California, Irvine. Announcement. Homework #4 will be out this week (will send out emails). Due on Feb 16 (next Thur) Lab session next Tuesday. Control Structures.

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CS190/295 Programming in Python for Life Sciences: Lecture 6

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CS190/295 Programming in Python for Life Sciences: Lecture 6

Instructor: Xiaohui Xie

University of California, Irvine


  • Homework #4 will be out this week (will send out emails). Due on Feb 16 (next Thur)

  • Lab session next Tuesday.

Control Structures

Part I: Decision structures

Multi-way decisions: if-elif-else

  • Python will evaluate each condition in turn looking for the first one that is true. If a true condition is found, the statements indented under that condition are executed, and control passes to the next statement after the entire if-elif-else.

  • If none of the conditions are true, the statements under the else are performed.

  • The else clause is optional; if omitted, it is possible that no indented statement block will be executed.

Control Structures

Part 2: Loop Structures

For Loops

  • It allows us to iterate through a sequence of values

  • The loop index variable var takes on each successive value in the sequence, and the statements in the body of the loop are executed once for each value.

Indefinite Loops: while-statement

  • An indefinite loop keeps iterating until certain conditions are met.

  • Here condition is a Boolean expression, just like in if statements. The body is, as usual, a sequence of one or more statements.

  • The body of the loop executes repeatedly as long as the condition remains true. When the condition is false, the loop terminates

Nested Loops

  • Again in the number averaging example, suppose instead of one-per-line we allow any number of values on a line, separated by comma.

Post-Test Loop

  • Suppose you are writing an input algorithm that is supposed to get a nonnegative number from the user. If the user types an incorrect input, the program asks for another value. It continues to reprompt until the user enters

  • a valid value. This process is called input validation.

  • Here is a simple algorithm:

    • repeat

    • get a number from the user

    • until number is >= 0

Post-Test Loop

  • Python does not have a statement that directly implements a post-test loop. However you can implemented with a while statement using the trick of “seeding”:

  • Or use the break statement:

Data Collections

Most real-world programs deal with large collections of data

  • A few examples:

  • Words in a document.

  • Students in a course.

  • Data from an experiment.

  • Customers of a business.

  • Graphics objects drawn on the screen.

  • Cards in a deck.

Example problem: simple statistics

Extend this program so that it computes not only the mean, but also the median and standard deviation of the data

Basic summary statistics


  • Lists are ordered sequences of items, a collection of values denoted by the enclosing square brackets

  • Lists can be created by listing items inside square brackets

Lists vs. Strings

  • In Python strings and lists are both sequences that can be indexed. In fact, all of the built-in string operations that we discussed previously are sequence operations and can also be applied to lists:

Lists vs. Strings: differences

  • The items in a list can be any data type, including instances of programmer-defined classes. Strings, obviously, are always sequences of characters.

  • Second, lists are mutable. That means that the contents of a list can be modified. Strings cannot be changed “in place.”


  • A tuple is a sequence of immutable Python objects. Just like lists. The only difference is that types cannot be changed.

  • Tuples are defined using parentheses while lists use square brackets.

  • Examples:

    Tup1 = (1,2,3,4)

    Tup2 = (‘UCI’, ‘UCLA,’UCSD’)

List operations

  • Python lists are dynamic. They can grow and shrink on demand. They are also heterogeneous. You can mix arbitrary data types in a single list. In a nutshell, Python lists are mutable sequences of arbitrary objects. This is very different from arrays in other programming languages.

  • A list of identical items can be created using the repetition operator.

  • Typically, lists are built up one piece at a time using the append method.

List operations: remove elements

List operations

Using lists is easy if you keep these basic principles in mind

  • A list is a sequence of items stored as a single object.

  • Items in a list can be accessed by indexing, and sublists can be accessed by slicing.

  • Lists are mutable; individual items or entire slices can be replaced through assignment statements.

  • Lists will grow and shrink as needed.

Statistics with Lists

# get a list of numbers and store in a list

Statistics with Lists: Mean

Statistics with Lists: Standard Deviation

Statistics with Lists: Median

Simple Statistics: put together

Non-sequential collections

  • Dictionary is a build-in data type for non-sequential collections in Python.

  • Python dictionaries are mappings: Keys -> values

  • A dictionary can be created by listing key-value pairs inside a curly braces.

  • We can access the value associated with a particular key using:

  • Dictionaries are mutable:

Dictionary Operations

  • You can also extend a dictionary by adding new entries

  • In fact, a common method for building dictionaries is to start with an empty collection and add the key-value pairs one at a time

Dictionary Operations

Dictionary methods: examples

References and Objects

  • Consider the following example:

    x = [1,2,3,4]

    y = x

    y[1] = ‘hello’

    >>> y


    >>> x


    Note that items in x are also changed after y assignment.

Call by reference in functions

  • Consider the following example:

    >>> def my_func(x):


    >>> x = [1,2,3,4]

    >>> my_func(x)

    >>> x


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