Physics 2660: Fundamentals of Scientific Computing. Lecture 5 Instructor: Prof. Chris Neu

Transcription

1 Physics 2660: Fundamentals of Scientific Computing Lecture 5 Instructor: Prof. Chris Neu (chris.neu@virginia.edu)

2 Reminder I am back! HW04 due Thursday 22 Feb electronically by noon HW grades are coming. Please be patient. Office hours: all held in our computer lab, room 022-C of this bldg Me: After lecture 3:30-4:30 every Tuesday TAs: Mondays: 3-5pm and 6-8pm Wednesdays: 5-9pm 2

3 Last time: C program structure Intro to a basic C program Review and Outline Defining simple variables and doing arithmetic FormaWed input/output via printf() and scanf() How variables values are stored on a computer More on formawed I/O Today: Comments Functions Conditional structures: Loops if, if/else, if/elseif/else switch/case Count-controlled loops: for()more on loops Conditioned controlled loops: while(), do Random numbers 3

4 Comments Inside Your Code 4

5 Internally Documenting / Comments 5

6 Functions 6

7 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! 7

8 C Standard Library Functions 8

9 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. 9

10 Functions from the C math library 10

11 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 11

12 User-Defined Functions: Example Use Case No functions repetitive! 12

13 User-Defined Functions: Use Case Example Three main parts: Prototype, Definition, and Calls 13

14 User-Defined Functions: Use Case Example 14

15 User-Defined Functions: Reusable? 15

16 Conditional Executions in C 16

17 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 17

18 Conditional Executions in C: if statements 18

19 Conditional Checks 19

20 Conditional Execution: if and else 20

21 Conditional Execution: if and else 21

22 Conditional Execution: switch and case 22

23 Conditional Execution: switch and case 23

24 if/else versus switch/case 24

25 The? Operator 25

26 DANGER: = versus == 26

27 DANGER: = versus == Take-away message: Be careful to use == in conditionals and not =. 27

28 Return Values and Tests Values are returned for all conditional checks: if false, the value is zero.. the value of the expression (5 < 3) is 0 if true, the value is not zero.. the value of the expression (5 > 3) is 1 We can use these characteristics in our code: 28

29 Return Values and Tests: From Functions One can call a function that performs some test Did it fail? 29

30 DANGER: Take Care with Floating-Point Numbers 30

31 DANGER: Take Care with Floating-Point Numbers Quiz time: 31

32 DANGER: Take Care with Floating-Point Numbers Quiz time: Lets say A = ( / 3.0) *

33 DANGER: Take Care with Floating-Point Numbers Quiz time: Lets say A = ( / 3.0) * 3.0 What is A? 33

34 DANGER: Take Care with Floating-Point Numbers Quiz time: Lets say A = ( / 3.0) * 3.0 What is A? Well, we all know the calculation should yield

35 DANGER: Take Care with Floating-Point Numbers Quiz time: Lets say A = ( / 3.0) * 3.0 What is A? Well, we all know the calculation should yield The computer would evaluate things in this manner: float A = ( /3.0)*3.0; if (A== ){ printf( A=%lf\n,A); } else printf( A is not \n ); 35

36 DANGER: Take Care with Floating-Point Numbers Quiz time: Lets say A = ( / 3.0) * 3.0 What is A? Well, we all know the calculation should yield The computer would evaluate things in this manner: What will be printed? float A = ( /3.0)*3.0; if (A== ){ printf( A=%lf\n,A); } else printf( A is not \n ); 36

37 DANGER: Take Care with Floating-Point Numbers Quiz time: Lets say A = ( / 3.0) * 3.0 What is A? Well, we all know the calculation should yield * The computer would evaluate things in this manner: What will be printed? float A = ( /3.0)*3.0; if (A== ){ printf( A=%lf\n,A); } else printf( A is not \n ); A is not What? 37

38 Floats and Comparisons 38

39 Floats and Comparisons 39

40 Loops! 40

41 Loops Loops allow computers to do what computers do best: Execute repetive boring tasks efficiently and accruately over and over again Two types: 41

42 Count-Controlled Loops! 42

43 Count-controlled loops: for() 43

44 How a for() loop works 44

45 for() loop: Important Parts Every for() loop has four important parts: counter initialization test condition counter update body of execution 45

46 Example: Good for() loop usage 46

47 Example: BAD for() loop usage 47

48 for() loops and break statements 48

49 for() loops and continue statements 49

50 Nested Loops 50

51 Condition-Controlled Loops! 51

52 Count-controlled Loops vs. Condition-controlled Loops 52

53 Pre-test Conditional Loops: while() 53

54 Post-test Conditional Loops: do/while() 54

55 We ll pick up from here next time. See you Thursday in labs! 55