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Interlude: Why learn to program?
(which is subtly distinct from, although a core part of, computer science itself) 3 From the Economist: “Teach computing, not Word” http://www.economist.com/blogs/babbage/2010/08/computing_schools Like philosophy, computing qua computing is worth teaching less for the subject matter itself and more for the habits of mind that studying it encourages. The best way to encourage interest in computing in school is to ditch the vocational stuff that strangles the subject currently, give the kids a simple programming language, and then get out of the way and let them experiment. For some, at least, it could be the start of a life-long love affair.
Interlude, continued
4 That, for me, sums up the seductive intellectual core of computers and computer programming: here is a magic black box. You can tell it to do whatever you want, within a certain set of rules, and it will do it; within the confines of the box you are more or less God, your powers limited only by your imagination. But the price of that power is strict discipline: you have to really know what you want, and you have to be able to express it clearly in a formal, structured way that leaves no room for the fuzzy thinking and ambiguity found everywhere else in life… The sense of freedom on offer - the ability to make the machine dance to any tune you care to play - is thrilling.
Intro Programming Classes Compared
CS 1110: Python
- No prior programming experience necessary
- No calculus
- Non-numerical problems
- More about software design
- Focus is on training future computer scientists
CS 1112: Matlab
- No prior programming experience necessary
- One semester of calculus
- Engineering-type problems
- Less about software design
- Focus is on training future engineers that compute 1/21/13 Overview; Types & Expressions 8 But either course serves as a pre-requisite to CS 2110
Academic Integrity
• Do not cheat , in any way, shape, or form
• Will be very explicit about this throughout course
• Pay attention to all assignment instructions
• In return, we try to be fair about amount of work,
grading the work, and giving you a course grade
• See website for more information
1/21/13 Overview; Types & Expressions 12
Getting Started with Python
- Designed to be used from the “command line” § OS X/Linux: Terminal § Windows: Command Prompt § Purpose of the first lab
- Once installed type “python” § Starts an interactive shell § Type commands at >>> § Shell responds to commands
- Can use it like a calculator § Use to evaluate expressions Overview; Types & Expressions 14 This class uses Python 2.7. - Python 3 is too cutting edge - Minimal software support 1/21/
The Basics
1/21/13 Overview; Types & Expressions 15
42 “Hello!” int eger
Values
Types
Expressions
float (real number) str ing (of characters) 34 * (23 + 14) "Hel" + "lo!" 1.0 / 3.
Expressions vs. Statements
Expression
• Represents something
§ Python evaluates it § End result is a value
• Examples:
Statement
• Does something
§ Python executes it § Need not result in a value
• Examples:
§ print "Hello" § import sys 1/21/13 Overview; Types & Expressions 17 Literal An expression with four literals and some operators
Type: int
- Type int (integer): § values: …, –3, –2, –1, 0, 1, 2, 3, 4, 5, … - Integer literals look like this: 1 , 45 , 43028030 (no commas or periods) § operations: +, – , *, /, **, unary –
- Principle : operations on int values must yield an int § Example: 1 / 2 rounds result down to 0 - Companion operation: % (remainder) - 7 % 3 evaluates to 1, remainder when dividing 7 by 3 § Operator / is not an int operation in Python 3 (use // instead) 1/21/13 Overview; Types & Expressions 18 multiply to power of
Floats Have Finite Precision
• Python stores floats as binary fractions
§ Integer mantissa times a power of 2 § Example: 1.25 is 10 * 2
• Impossible to write most real numbers this way exactly
§ Similar to problem of writing 1/3 with decimals § Python chooses the closest binary fraction it can
• This approximation results in representation error
§ When combined in expressions, the error can get worse § Example : type 0.1 + 0.2 at the prompt >>> 1/21/13 Overview; Types & Expressions 20 mantissa exponent
Type: str
- Type str (string of characters): § values: any sequence of characters § operation(s): + (catenation, or concatenation)
- String literal : sequence of characters in quotes § Double quotes: " abcex3$g<&" or "Hello World!" § Single quotes: 'Hello World!'
- Concatenation can only apply to Strings. § "ab" + "cd" evaluates to "abcd"
§ "ab" + 2 produces an error
1/21/13 Overview; Types & Expressions 21
Converting Values Between Types
• Basic form: type ( value )
§ float(2) converts value 2 to type float (value now 2.0)
§ int(2.6) converts value 2.6 to type int (value now 2)
§ Explicit conversion is also called “casting”
• Narrow to wide: bool ⇒ int ⇒ float
• Widening. Python does automatically if needed
§ Example : 1/2.0 evaluates to 0.5 (casts 1 to float )
• Narrowing. Python never does this automatically
§ Narrowing conversions cause information to be lost § Example : float(int(2.6)) evaluates to 2. 1/21/13 Overview; Types & Expressions 23
