Understanding Native Types and Memory in C++

1. CS351 – Week 1

2. Topics • Native Types • Memory representation • Variable initialization declaration and allocation

3. Previous knowledge • How to select variable type (string, integer, float, or boolean) • Concept of capturing external files for use in programming (import or include) • Reading strings from standard input and parsing for numeric types if needed • Converting and concatenating strings for output to screen • Compute and save numerical results • Understanding of how to use a randomize function

4. Objectives • Identify native types in C++ that are generally available • Understand how to declare, allocate and initialize a variable • Declare variables in global or local scope • Create a basic C++ program that takes input from standard in, computes a value and prints the value to the screen • Using constant values • Appropriately including needed files • Correctly coding the main signature • Using object for inputting and outputting values

5. Including files • Reusing code from a library • Python import math • C #include <math.h> • Differences between C and Python? • How do you find what file has the function you want?

6. Computer memory Conceptually the memory in a computer is very much like building lots laid out by lot number on a map of the streets of a city before any houses have been built.

7. Computer memory Lots can be used individually or as groups Although memory has a basic unit, units can be used together to represent more complex data

8. Data for variables • “Jane Doe” • $475.24 • 123-45-6789 • 26 years of age • January

9. Native types in C++ • char – typically holds data encoded to represent written character sets • integer types – whole numbers; can be signed or unsigned; variations represent integers that use from 8 bits to 64 bits • float types – Stores numbers as fraction, exponent and sign; not precise • boolean – true or false • Reference (pointer) – location that refers to another location • strings – usually as array of characters; have special handling in the language; variable name refers to first location Size of types is implementation dependent Resources: http://en.wikipedia.org/wiki/Primitive_data_type, http://www.cplusplus.com/doc/tutorial/variables/

10. wget http:/bama.ua.edu/~anderson/size.cc Sample program //size.cc #include <iosteam> using namespace std; int main() { cout<<“sizeof(int) is ”<<sizeof(int)<<“ bytes”<<endl; return 0; } Needed for cout and endl Like java packages Will discuss later sizeof give the size in bytes of a type or variable Needed since sizes vary String literal ** What are the non-reserved words?

11. Sample program //size.cc #include <iosteam> using namespace std; intmain() { cout<<“sizeof(int) is ”<<sizeof(int)<<“ bytes”<<endl; return 0; }

12. Other general rules • Everything is case sensitive • These identifiers are reserved (can’t be used as variable names) What does it mean to be a reserved word? http://en.cppreference.com/w/cpp/keyword

13. Other syntax • semicolons are used to denote the end of a statement except • after the #include statement • before a block • Whitespace is not part of syntax //size.cc #include <iosteam> using namespace std; int main() { cout<<“sizeof(int) is ”<<sizeof(int)<<“ bytes”<<endl; return 0; }

14. Programmed Example - 10 minWork in pairs (only pairs) • Create a C++ program that prints out the size of the following variables • char • int • unsigned int • short • long • long long • float • double • Fix w1ex1.cc so that it compiles and prints Hello World Turning in programs Put both names at top Comments start with // cat <program name> |mailx –s “<lastname 1> <lastname2> Week # Date” cs351@cs.ua.edu Example: cat w1ex1.cc |mailx –s “honganderson Week 1 011414” cs351@cs.ua.edu

15. User input form keyboard //cin.cc #include <iosteam> using namespace std; int main() { int value; cin >>value; cout<<“The user entered” <<value<<endl; cout<<“value * 3 ==<<” <<value*3<<endl; return 0; }

16. 1st assignment – 5 minWork in pairs (only pairs) • Fix w1ex2.cc so that it calculates the user’s mileage reimbursement Turning in programs Put both names at top Comments start with // cat w1ex2.cc|mailx –s “Week 1: 011414” cs351@cs.ua.edu

17. C++ variables • Declaration – Give variable a name and a type • Allocation – Identify location where variable will be stored • Initialization – Set variable to appropriate initial value When this happens depends on scope…

18. Variable scope • A scope is a region of the program and broadly speaking there are three places, where variables can be declared: • Inside a function or a block which is called local variables, • In the definition of function parameters which is called formal parameters. • Outside of all functions which is called global variables.

19. C++ native type rules • For local variables • <type> <identifier>; will only declare and allocate • <type> <identifier>=<value>; will declare, allocate and initialize

20. Local variable declaration #include <iostream> using namespace std; int main () { // Local variable declaration: int a, b; int c; // actual initialization a = 10; b = 20; c = a + b; cout << c; return 0; }

21. C++ native type rules • For global declarations • <type> <identifier>; will declare, allocate and initialize • <type> <identifier>=<value>; will declare, allocate and initialize

22. Mixed declaration //mixed.cc #include <iostream> using namespace std; int g; int main () { // Local variable declaration/allocation: int a, b; // actual initialization a = 10; b = 20; g = a + b; cout << g; return 0; } cout <<a; //not guaranteed to be coherent

23. Same variable in two scopes //twoscopes.cc #include <iostream> using namespace std; // Global variable declaration: int g = 20; int main () { // Local variable declaration: int g = 10; cout << g; return 0; } Is this an error? If not, what prints here? The enclosing scope has precedence

24. Variable scope matters • If the scope of a variable is global (defined outside of braces), • Can be used throughout a program (any function or code; even in other source files • Memory is know to be needed at compile time • Compiler can designate a place for this memory as part of the program setup • If a variable has scope local to some block • it can only be used by statements in that block • compiler knows about it *but* we don’t know when functions will be run; • To conserve memory, we reused the memory when it is not needed (out of scope) Memory in these two different cases are allocated in different places in the process space (program)

25. C++ constants • Uses const modifier in front of variable declaration • Value cannot be changed (compiler code will not be generated) • Allows compiler to optimize access • Acts as a semantic check • Better than #define because the type is specified (allows type checking by compiler) • #define is a macro; replaces text before compiling

26. Constants //const.cc #include <iostream> using namespace std; // Global variable declaration: constintmonthsInYear = 12; int main () { cout<<monthsInYear<<endl; int age; cin>>age; //what happens if you put in a floating poitn number? cout<<“Entered age “<<age<<endl; monthsInYear=0; //error line inttotalMonths = monthsInYear*age; count<<totalMonths<<endl; return 0; }

27. C++ reference types • Strings, objects, arrays, and structures are not native types but are reference types • Composed of primitive types • Different rules for declaration, allocation and initialization • For all types, if you initialize, you automatically allocate

28. 2nd Assignment • Fix w1ex3.cc so that it: • prints prompts for user input • prints out the number of weeks in the current month • Calculates the current weekly salary based on monthly salary input and weeks in the current month • Write a program that takes an angle in degrees from the user and converts it to radians. Use a constant for π • Consult instructions for submission • Quiz online on this material posted Monday; complete before class on Tuesday