Physics 2660: Fundamentals of Scientific Computing Lecture 6 News and Announcements
•  Voting Day!
•  Piazza
•  HW05 is due Friday 4 March at noon
•  HW06 will be assigned this week and will be due Thursday 17 March at noon
•  Office hours reminder:
–  My office hours are in Room 022-­‐‑C (our computer lab) from 3:30-­‐‑5pm on Tuesdays or by appointment
–  TA office hours, also in Room 022-­‐‑C
•  Mondays 5-­‐‑8pm
•  Tuesdays 5-­‐‑8pm
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News and Announcements
•  Mid-­‐‑term exam will be Tuesday 15 March at 7pm in this room
–  Two weeks from now!
–  Will cover everything we have covered so far, including today!
–  Format: Mix of multiple choice, matching, short answer – no in-­‐‑class coding, just need a Number 2 pencil!
–  More later in today’s lecture
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Review and Today’s Outline
•  Last time:
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Random numbers
MC integration
Pointers
Recursion
•  Today:
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Pointers to functions
Arrays in C
Passing arguments to main(….)
Making re-­‐‑usable code
Intro to statistics
Review of C so far and exam examples
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Pointers to Functions and Trapezoidal Rule for Integration
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Pointers to Variables: A Review
•  Pointers allow direct access to memory locations where variables are stored in 1’s and 0’s
–  address via &
–  content via *
•  Passing pointers into functions:
–  In function prototype
#include <stdio.h>
void circle_stuff(double radius, double
*a_ptr, double *c_ptr);
void main(){
double radius = 3.0;
double area = 0.0;
double circumference = 0.0;
•  use pointer-­‐‑to-­‐‑values as argument(s)
circle_stuff(radius, &area,
&circumference);
printf(“area=%8.3f
circumference=
%8.3f\n”, area, circumference);
–  In call to function
•  call with pointer-­‐‑to-­‐‑address as argument(s)
–  In function definition, •  use pointer-­‐‑to-­‐‑values as argument(s) as done in prototype
•  use pointer-­‐‑to-­‐‑values in body of function
}
void circle_stuff(double radius,
double *a_ptr, double *c_ptr){
*a_ptr = 3.1416*radius*radius;
*c_ptr = 2.0*3.1416*circumference;
}
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How are Functions Stored in Memory?
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Pointers to Functions: Example
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Pointers to Functions: Pieces
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Pointers to Functions: Example – Numerical Integration
Monte Carlo integration is one method one can use to integrate some complicated function, especially useful in multiplie dimensions.
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Pointers to Functions: Example – Numerical Integration
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Pointers to Functions: Example – Numerical Integration
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Pointers to Functions: Example – Numerical Integration
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Arrays in C
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What is an Array?
•  Very often we need to store and access many instances or sets of related data
•  Example: coordinates in 3-­‐‑space:
–  x=5, y=23, z=1
–  Can be represented by a vector: (5,23,1)
–  The points in space are of the same type – would like a “container” that holds them in a convenient format
•  Arrays do this for us!
•  Not just god for vectors in 3-­‐‑space
–  Good for matrices
–  Good for collections of related items
–  Good generally for similar pieces of data
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An example: A vector in 3-­‐‑space
*
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An example: A vector in 3-­‐‑space using an array
*
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Defining Arrays
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Using Arrays
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Storage of Arrays in Memory
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Storage of Arrays in Memory: Danger
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Using Arrays: Danger
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Passing Arrays to Functions
*
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Multidimensional Arrays
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Multidimensional Arrays
Do remember though: When you define an array of any size, the footprint of that array’s size is dedicated in the computer’s memory.
It is very easy to exhaust a computer’s memory by defining arrays that are too large.
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Arrays of Characters
*
Single characters are nice but …many in succession can be more meaningful to us.
Array of characters are called a string.
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Arrays and Pointers
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Passing arguments to main(…)
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Passing arguments to main(…)
•  We have done this many times already….
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Special parameters in C: argc and argv
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Special parameters in C: argc and argv
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An Example
*
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The Keyboard Enters Strings Not Numbers!
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Making Pieces of Code that Are Reusable
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Reusable Code
•  Many tasks in computer programming for scientific purposes are confronted over and over again
–  imagine calculating the sine of some angle or the distance between two points in some linear algebra project
–  imagine every time you needed that value, having to write a piece of new code to do the calculation
•  There is point in reinventing the wheel every time
•  Utilities such as these – and many others we can think of – are best coded once and reused over and over again
–  reduction of chance for bugs
–  easier debugging
–  portability of code to other users
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Reusable Code: Option I
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Reusable Code: Option I
Drawback of this Option:
You have to compile the extra code every time!
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Reusable Code: Option II
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Reusable Code: Option II
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Mid-­‐‑term Exam
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Information on Mid Term Exam
•  Your mid-­‐‑term is coming!
•  Tuesday 15 March in this room at 7pm
–  You will have 60 min to finish
–  Bring a number 2 pencil
–  Regular lecture and labs will of course meet that week
•  Format:
–  multiple choice
–  matching
–  short answer
•  Let’s review some of the things we have learned so far
•  Some examples follow
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Review of C: What We Have Learned So Far
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Variables
Variables…
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Input and Output
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Files
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Loops
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Conditionals
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Functions
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Pointers
•  Pointers allow direct access to memory locations where variables are stored in 1’s and 0’s
–  address via &
–  content via *
•  Passing pointers to functions:
–  In function prototype
#include <stdio.h>
void circle_stuff(double radius, double
*a_ptr, double *c_ptr);
void main(){
double radius = 3.0;
double area = 0.0;
double circumference = 0.0;
•  use pointer-­‐‑to-­‐‑values as argument(s)
circle_stuff(radius, &area,
&circumference);
printf(“area=%8.3f
circumference=
%8.3f\n”, area, circumference);
–  In call to function
•  call with pointer-­‐‑to-­‐‑address as argument(s)
–  In function definition, •  use pointer-­‐‑to-­‐‑values as argument(s) as done in prototype
•  use pointer-­‐‑to-­‐‑values in body of function
}
void circle_stuff(double radius,
double *a_ptr, double *c_ptr){
*a_ptr = 3.1416*radius*radius;
*c_ptr = 2.0*3.1416*circumference;
}
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Questions About the Basics
•  What is an operating system?
•  What are the main hardware parts of a computer?
•  What is a compiler?
•  Etc.
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Questions About the Linux Shell
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Questions About the C Language
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Questions About the C Language
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Questions About the C Language
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Questions About the C Language
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We’ll pick up from here next time. Thursday labs -­‐‑-­‐‑ see you then.
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