Physics 2660: Fundamentals of Scientific Computing Lecture 3 News and Announcements
•  I am not Professor Neu!
•  He is away this week at CERN
•  Working on preparing results for an important upcoming series of conferences
–  Studies of the Higgs boson interaction with the top quark
–  Studies of b quarks produced with top-­‐‑quark pairs
–  Search for production of black holes
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Reminders
•  Weekly readings and homeworks will be posted on the class wiki site in general w/o any official notification to you – please check the links frequently
•  Prepare for labs!
–  Make sure the readings are complete
–  Review the class notes from lecture
•  Do your homework!
–  Electronic submissions by noon every Thursday unless otherwise scheduled
–  HW02 due this Thursday by noon
•  Do not fall behind – this is a crucial time the next few weeks.
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Reminder
•  Prof. Neu’s office hours this week are canceled
–  He is available via email, chat, twiSer all week
–  Do not hesitate to contact him if you have questions or concerns
•  TA office hours, also in Room 022
–  Mondays 5:00-­‐‑8pm
–  Tuesdays 5:00-­‐‑8pm
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Announcement: Repeat from Last Time
•  From now on, compile your code using the –Wall (“minus W all”) flag:
•  The –Wall flag tells the compiler to report to you ALL warnings the compiler wants to issue (by default these are suppressed.
•  Types of warnings:
–  use of un-­‐‑initialized variables
–  definition of unused variables
–  syntax problems
•  Warnings are not good!!
•  We will start penalizing in homework grading for compiler warnings
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Outline
•  From last time:
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C program structure
Intro to a basic C program
Defining simple variables and doing arithmetic
FormaSed input/output via printf() and scanf()
•  Today:
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How variables’ values are stored on a computer
More on formaSed I/O
Operators Intro to functions
Conditional executions in C (if/else, switch/case)
Loops ( for() , while(), do )
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Storing Variables
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Bits and Bytes
•  How is data stored on a computer?
–  Data is stored in bits – think of them as values that can have either the value 0 or 1, or switches
–  One value is called a bit
–  bits are grouped together in groups of 8, called a byte
–  8 bits are convenient for binary arithmetic – using the binary number system (1s and 0s only)
–  Examples: subscripts reflect base system
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1d = 1b = 1*2d0
5d = 101b = 1*2d2 + 0*2d1 + 1*2d0
12d = 1100b
21d = 10101b
•  182d = 10110110b à
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Familiar Storage Sizes
100
103 106
109
1015
1012
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Storing Variables
•  Recall last time we introduced the most-­‐‑common C variable types:
–  int
–  float and double
–  char
int main(){
double velocity = 0.0;
float x = 0.0;
int number = 0;
char a = ‘a’;
float y = 0.0;
•  When you run your program, an area in your computer’s memory is }
set up to store the value for each of the variables you define.
…
•  Different variable types require a different amount of space.
•  Hence, one must be careful to match variable types with the actual use one envisions …cannot put a decimal number in an int, for instance
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Storage of an integer: Example
•  For an integer we have up to 4 bytes = 32 bits of storage
•  So the variable “num” defined as this:
int num = 909337759;
is stored this way:
•  Comments:
–  One bit is reserved for the sign +/-­‐‑ (leading bit): •  leading bit = 1 •  leading bit = 0
–  So max value we can store is 2d31 -­‐‑ 1d = 2,147,483,647d
Implication:
If your OS uses only 32-­‐‑bit words
for memory addresses, the max memory address is 4,294,967,296…
meaning up to 4.3 GB of memory can be handled…
hence 32-­‐‑bit systems cannot have much more than 4 GB of memory.
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Variable Storage: Examples
4 bytes
4 bytes
1 byte
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Variables: How to know their size
Note the last two lines: compound types. 5/2 is rightfully evaluated as a decimal number but has two int’s as args – hence it is assigned the size of an integer.
So don’t try to divide integers and expect the right answer – unless you define things properly!
And the last one – the size of the result is given by the highest precision argument.
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Using sizeof()
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FormaWed Input / Output
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I/O Format Specifications: Review from Last Time
•  Recall from lab00, you used printf() to print values to screen
•  Similar function scanf() is used to read in values from the user:
•  % is the control key indicating some I/O formaSed data
•  followed by
an I/O specifier:
“i” and “d” are similar but very different. Use “d” for integers until we cover in more detail.
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Format Mismatches
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Format Mismatches
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Format Mismatches
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Format Mismatches
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Format Mismatches
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Format Mismatches
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Format Mismatches
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Controlling the Appearance of Output: Display
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Controlling the Appearance of Output: Escape Characters
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Files and Input/Output
•  Essentially all of the lessons about controlling output can be applied to controlling / designing input to your program
•  printf() and scanf() are not very different
•  What about non-­‐‑interactive I/O?
–  Use of files
•  fopen(), fclose(), etc.
•  Some experience in last week’s lab02 and hw02
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Operators
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Operator Types
•  What kind of operations do we need to be able to do in a computer program?
•  Here are some:
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Arithmetic (eg., +, -­‐‑, *, / )
Assignment (eg., = )
Increment / decrement (eg., ++, -­‐‑-­‐‑) used in counters
Logical / Conditional (eg., && , || )
Bitwise operations…
Pointer operations…
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Arithmetic Operators
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Assignment Operators
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Increment/Decrement Operators
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Logical / Conditional Operators
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Operator Precedence: Which Comes First?
or
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Bad Coding Example: I
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Bad Coding Example: II
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Functions
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Introduction to Functions
•  C is a simple language
•  Its utility is extended through the use of re-­‐‑usable functions
•  Some functions are found in standard libraries like stdio.h and math.h
•  Users can write functions too – for many good reasons!
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C Standard Library Functions
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C Standard Library Functions
Common C Standard Libraries we will use:
•  stdio.h
•  math.h
Best to get in the habit of including these in
every program you write!
You don’t even know WHERE these files are
on galileo in order to use them.
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Functions from the C math library
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User-­‐‑Defined Functions
•  Why would one want to write their own functions?
–  Avoid duplicating the same code many times within a program: streamlined simplicity
–  Re-­‐‑usable functions are easier to maintain and modify
–  Functions are portable – can be used in other programs
–  Simplicity: code something intricate once and call it via a simple single line rather than multi-­‐‑line complications
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No functions – repetitive!
User-­‐‑Defined Functions: Example Use Case
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Three main parts:
Prototype, Definition, and Calls
User-­‐‑Defined Functions: Use Case Example
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User-­‐‑Defined Functions: Use Case Example
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User-­‐‑Defined Functions: Reusable?
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Conditional Executions in C
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Linear Code Execution in C
•  Simple pieces of code have lines of instructions that are executed linearly, calling external functions like printf:
#include <stdio.h>
int main(void)
{
int num = 5;
printf(Hello, world. num = %d\n”,num);
return 0;
}
•  This is not typical however
•  Most programs need to do more intricate things in their execution
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Conditional Executions in C: if statements
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Conditional Checks
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Conditional Execution: if and else
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Conditional Execution: if and else
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We’ll pick up from here next time. Don’t forget to do submit homework by Thursday @ noon. And of course do not forget lab on Thursday. See you then!
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