Declaration of variables

Declaration of variables

• 1.
  C++ Basics Variables, Identifiers, Assignments,Input/Output
• 2.
  Variables variable can holda number or a data of other types, it always holds something. A variable has a name the data held in variable is called value variables are implemented as memory locations and assigned certain memory address. The exact address depends on computer and compiler. we think as though the memory locations are actually labeled with variable names 12.5 32 'c' y Temperature Letter 1001 1002 1003 1004 1005 1006 1007 -Number 1008 1009 2
• 3.
  Identifiers identifier – nameof a variable or any other named construct identifier must start with a letter or underscore symbol (_), the rest of the characters should be letters, digits or underscores the following are legal identifiers: x x1 x_1 _abc sum RateAveragE the following are not legal identifiers. Why? 13 3X %change data-1 my.identifier a(3) C++ is case sensitive: MyVar and myvar are different identifiers 3
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  Identifier Style careful selectionof identifiers makes your program clearer identifiers should be short enough to be reasonable to type (single word is norm) – standard abbreviations are acceptable long enough to be understandable two styles of identifiers C-style - terse, use abbreviations and underscores to separate the words, never use capital letters for variables Camel Case - if multiple words: capitalize, do not use underscores – variant: first letter lowercased pick style and use consistently ex: Camel Case 1 C-style Camel Case 2 Min min min Temperature temperature temperature CameraAngle camera_angle cameraAngle CurrentNumberPoints cur_point_nmbr currentNumberPoints 4
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  Keywords keywords are identifiersreserved as part of the language int, return, float, double cannot be used by the programmer to name things consist of lowercase letters only have special meaning to the compiler 5
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  Keywords (cont.) asm doif return typedef auto double inline short typeid bool dynamic_cast int signed typename break delete long sizeof union case else mutable static unsigned catch enum namespace static_cast using char explicit new struct virtual class extern operator switch void const false private template volatile const_cast float protected this wchar_t continue for public throw while default friend register true union delete goto reinterpret_cast try unsigned 6
• 7.
  Variable Declarations before use,every variable in C++ program needs to be declared type – the kind of data stored in a variable declaration – introduces a variable to a compiler a variable declaration specifies type name declaration syntax: two commonly used numeric types are: int - whole positive or negative numbers: 1,2, -1,0,-288, etc. double - positive or negative numbers with fractional part: 1.75, -0.55 example declarations: int numberOfBars; double weight, totalWeight; type id, id, ..., id; list of one or more identifiers 7
• 8.
  Declaration Location, Initial Value avariable should be declared as close to its use as possible variable contains a value after it is declared until assigned, this value is arbitrary 8
• 9.
  Assignment assignment statement isan order to the computer to set the value of the variable on the left hand side of equal sign to what is written on the right hand side it looks like a math equation, but it is not example: numberOfBars = 37; totalWeight = oneWeight; totalWeight = oneWeight * numberOfBars; numberOfBars = numberOfBars + 3; variable = value; 9
• 10.
  Output to do input/output,at the beginning of your program insert #include <iostream> using std::cout; using std::endl; C++ uses streams for input an output stream – a sequence of data to be read (input stream) or a sequence of data generated by the program to be output (output stream) variable values as well as strings of text can be output to the screen using cout (console output): cout << numberOfBars; cout << ”candy bars”; cout << endl; << is called insertion operator, it inserts data into the output stream, anything within double quotes will be output literally (without changes) - ”candy bars taste good” note the space before letter “ c” - the computer does not insert space on its own keyword endl tells the computer to start the output from the next line 10
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  More Output several insertionoperators can be stacked together in a single statement: cout << numberOfBars << ”candy barsn”; symbol n at the end of the string serves the same purpose as endl arithmetic expressions can be used with the output statement: cout << ”The total cost is $” << (price + tax); 11
• 12.
  Input cin - (standsfor Console INput) – stream used to give variables user-input values need to add the following to the beginning of your program using std::cin; when the program reaches the input statement it just pauses until the user types something and presses <Enter> key therefore, it is beneficial to precede the input statement with some explanatory output called prompt: cout << ”Enter the number of candy bars”; cout << ”and weight in ounces.n”; cout << ”then press returnn”; cin >> numberOfBars >> oneWeight; >> is extraction operator dialog – collection of program prompts and user responses input operator (similar to output operator) can be stacked input token – sequence of characters separated by white space (spaces, tabs, newlines) the values typed are inserted into variables when <Enter> is pressed, if more values needed - program waits, if extra typed - they are used in next input statements if needed 12