1. CSC 107 – Programming For Science

2. The Week’s Goal

3. Variables  Variable names location to store data  Memory location's initial value is unknown  Assignments update memory location with new value  Memory location updated by assignment ONLY  When variable is used in program…  …uses current value at that memory location  Just about everything (interesting) uses variables

4. Variable Declarations  Variables must be declared before can be used  Way of getting computer to make space for variable  States how to interpret memory in future uses  Allows the compiler to check if uses are legal

5. Variable Names  Begin with letter or underscore ( _ )  Then use any letters, numbers, or underscore  Unique name * needed for each variable  Computer wouldn't know which of 1,000 bob s to use  Reserved words are… reserved and can't be used  Includes all type names on p. 83 of book  void, unsigned, class also reserved words

6. Data Types  Each variable also has data type  How program treats variable’s value defined by this  Single true or false value held by bool  C/C++ defines 7 numeric data types  Integer types: short, int, long, long long  Decimal types: float, double, long double not really standardized  Ranges for each type is not really standardized  Non-negative versions using unsigned ______  char data type can hold a character

7. Making Programs Interesting  Declaring & printing boring after 1 st 6 years  Want to do something with variable to use it  C++ has large variety of statements to do this  First statement covered in today’s lecture

8. Making Programs Interesting

9. Assignments  Variable declaration creates “box” to store data  Box can get values placed in it using assignments  General form of assignment is variable = expression;  Computer works by first evaluating expression  Single value must result from this expression  Value of variable set to this result

10. What Is The Expression?  Simplest expressions are literal values  Examples: double d; int i; char doe; i = 6; i = 7; d = -7; d = 34.5691; doe = ‘a’; doe = ‘0’; 125612.345-56‘a’

11. What Is The Expression?  Examples of other simple expressions double d; int i; char doe; i = 6; i = 7; d = -i; i = d; d = 34.5691; i = d; doe = ‘0’; // 0 == ASCII 48 i = doe; doe = i;

12. Data Types  Assignments are legal only if always safe  C++ defines ordering of legal assignments long double double float long int short char Legal to assign to higher type

13. What Is The Expression ?  Can also include basic arithmetic operators  Addition +i = 4 + 6;  Subtraction -d = i – 2.3;  Multiplication *i = 120 * 8;  Division /d = 4.0 / i;  Modulus %i = 120 % 8;  Modulus computes remainder between two integers: 4 % 5 equals 4 5 % 4 equals 1 9 % 3 equals 0 12823 % 812 equals 643

14. Tracing A Program  Important for understanding & debugging code  Step-by-step execution of program shown  To see what is happening, done via pencil-and-paper  Execute each line of program like computer does  Within trace, add row whenever variable declared  Update variable’s value each time it is assigned  Off to side, show any output from cout statements

15. Program Trace 1 int x = 4 + 2; 2 int y = 8 * 1; 3 double z = y – 3; 4 x = x + 1; 5 y = 7 % x; 6 z = y + 1.0 / 2.0; 7 z = 8.0 / 4 + x * x; 8 y = (x – 3) * (y + 2); 9 y = x / 4; 10 cout << x << “ ” << y << “ ” << z << endl;

16. Integer Division  Dividing two integers computes an integer  Literals or variables does not matter, only their type  Important to remember, 12 is integer & 12.0 is not  C++ ignores result after decimal to get integer 2 / 5 equals 0 (the.4 was thrown away) 5 / 2 equals 2 (the.5 was thrown away) 16 / 4 equals 4 -5 / 2 equals -2 (the.5 was thrown away) 2.0 / 5 equals 0.4 ( 2.0 is not an integer!)

17. Floating Point Arithmetic  Operations using decimal has decimal result  Even if whole number is result of the operation  For example, all the assignments to i are illegal: int i; double d = i; i = 6.0 / 3.0; i = 2.0 * d; i = d + 1; i = 4 * 2.0; i = (d * 1) + 5; i = 8 + (9 * 3) – (2 / 1.0) * 4 + 2;

18. Priority of Operations  Equations can become very complex  4 + 5 * 6 * 9 - 2 + 1 = …?  Very  Very strict order of operations used by computer  ( ) Solve from inner- to outermost  + (positive) & - (negative) Solve from right to left  * & % & / (division) Solve from left to right  + (addition) & - (subtraction) Solve from left to right use lots of parentheses  My suggestion: use lots of parentheses

19. Compound Assignment Operators  Short simple operators that allow us to be lazy  Save some typing for several common actions  Lowest priority operation; expression evaluated first OperatorEquivalent C++ Expression a += 2;a = a + 2;a = a + 2; b -= d;b = b – d;b = b – d; c *= 4 + 5.6;c = c * (4 + 5.6); d /= 0.3 * e;d = d / (0.3 * e);

20. How To Shoot Yourself in Foot  Also increment ( ++ ) & decrement ( -- ) operators  Use with variables only; no exceptions possible  Used anywhere to save typing an additional line  Two different ways these operators applied v = ++b % c;  b = b + 1; v = b % c; c = f * --h;  h = h – 1; C = f * h; a = b++ * c;  a = b * c; b = b + 1;

21. Your Turn  Get in groups of 3 & work on following activity

22. For Next Lecture  Read sections 6.8 – 6.9, 9.8 for Friday  How do we call these built-in functions?  What types of data do they work with and return?  What does “typecasting” mean?  Week #2 weekly assignment due Tuesday  Problems available on Angel  If problem takes more than 10 minutes, TALK TO ME!