IT150/IT152 Concepts Summary Sheet

Transcription

1 (Examples within this study guide/summary sheet are given in C#.) Variables All data in a computer program, whether calculated during runtime or entered by the user, must be stored somewhere in the memory (RAM) of the computer. To accomplish this, computer programs use variables, which can be defined as a named space in memory. To capture and name a piece of memory for use as storage, a programmer will declare a variable like this: Int32 imyinteger; String smystring; Double dmydouble; Boolean bmyboolean; Any variable declared in a program must also be initialized prior to use by the program. This is to prevent the program from inadvertently using a value that the program itself did not generate or obtain from a user. The initialization step assigns a known value into the variable at the outset of the program. Usually this value is 0 (in the case of numerical variables) or an empty string (in the case of text or string variables). A programmer will initialize a variable like this: imyinteger = 0; smystring = String.Empty; dmydouble = 0.0; bmyboolean = false; The declaration and initialization can occur on the same line like this: Int32 imyinteger = 0; String smystring = String.Empty; Double dmydouble = 0.0; Boolean bmyboolean = false; Other data types include Int64, Float, and Decimal. These are used when needed, depending on the values involved. H. W. Gould, Winter Page 1

2 Input/Output Input is handled differently in a console application from how it is handled in a Windows application. (All input in either type of application is in string format.) In a console application, a programmer will use the ReadLine( ) method of the Console class to get input from the user. Any input gathered from a user with the ReadLine( ) method must be assigned into a string variable like this: smystring = Console.ReadLine( ); When a user enters information into the computer and presses the <enter> key, the Console.ReadLine( ) function takes the input from the keyboard (console) and assigns it into the variable smystring. A single equal sign ( = ) is an assignment, meaning whatever value is produced by the code on the right side of the equal sign is stored into the variable on the left side of the equal sign. (For comparisons between values, the equals is represented by two equals signs ( == ).) If the user needs to enter numerical data (for calculations of some kind), the string value (being held in a string variable) must be converted to a number. The method to do this is the Convert.ToDouble(sMyString) or Convert.ToInt32(sMyString) method (or the appropriate corresponding method for other data types like Int64 or Float). Note that smystring would be replaced by whatever variable name the programmer used for his input string. Input in a Windows application (Windows Forms) would be accomplished by using the.text property of a textbox. When using the value out of a textbox, the programmer must refer to it with the textbox name and the Text property, Ex. txtinputvalue.text. Conversion of text in textboxes to numerical values is accomplished in the same fashion, using the Convert method. Output in a console application involves the use of the Console.Write( ) or Console.WriteLine( ) methods. The difference between the two is simple: Console.Write( ) writes a string value (represented here by ) to the screen and leaves the cursor on the same line as the output, while Console.WriteLine( ) writes a string value to the screen and moves the cursor to the left edge of the screen on the next line. In other words, WriteLine( ) automatically appends a carriage return (new line) to the text; Write( ) does not. The format for these output statements is: Console.Write( This will appear on the screen. ); Console.WriteLine( This will appear on the screen and the cursor will drop to the next line. ); Console.WriteLine(sMyString); (Writes the value stored in smystring to the screen.) To write numerical output to the screen involves invoking the.tostring( ) method that is a part of each of the numerical data types, for example, imyinteger.tostring( ) converts the integer value in imyinteger to a string representation of that value (such as 4 converting to 4 ). Output in a Windows application involves the use of labels and sometimes textboxes. Textboxes are usually reserved for input, though, and you can set the properties of a label to make it look like a textbox, if you desire to do so. The handling of numerical data being output to a label or textbox are the same as for console applications the ToString( ) method. H. W. Gould, Winter Page 2

3 Decision Statements IT150/IT152 Concepts Summary Sheet There are times when a computer program needs to do different things, depending on certain values that have been calculated or entered by the user. The example I gave in classes is a bank ATM responding to a user s request for twenty dollars. Quite simply, if the bank account in question has sufficient funds, one course of action will occur (the money will be dispensed), whereas if there are not sufficient funds, a different course of action will occur (a message telling the user that the request is declined.) The primary decision statements are the if, the if else, and the switch statements. The switch statement has several options (cases) which are distinguished from each other based on discrete values that can be distinctly determined. Here is an example of the syntax of the if else statement (using the ATM example): if(dacctbalance > damountrequested) else damounttodispense = damountrequested; damounttodispense = 0.0; Note: if there is a need to do one thing if a condition exists, but do nothing if the condition doesn t exist, the if statement can be used without the else portion (Assume braining is a Boolean variable and TakeUmbrella( ) is a method or function.): if(braining) TakeUmbrella( ); Switch statements operate on the basis of any number of possible values, each distinct from the others and specifically identified, such as integers. Values of type double are not used in switch statements due to their widely varied possible values. (Continued next page) H. W. Gould, Winter Page 3

4 A switch statement gets its name from electric switches that have several distinct settings, such as the temperature settings on a clothes dryer. The choice of which case to execute is determined by the value in the parameter of the switch statement. An example would be a code block that determines the name of the day of the week, based on the numerical calculation of the day s position in the week (1 st day of the week = Sunday, 2 nd day of the week = Monday, etc.): switch (idaynumber) Case 1: sdayname = Sunday ; Case 2: sdayname = Monday ; Case 3: sdayname = Tuesday ; Case 4: sdayname = Wednesday ; Case 5: sdayname = Thursday ; Case 6: sdayname = Friday ; Case 7: sdayname = Saturday ; default: sdayname = String.Empty; Remember that each Case must have a statement in it. H. W. Gould, Winter Page 4

5 Loops There are times when a block of code must be executed repeatedly, sometimes for a specified number of times and other times, for as long as a specified condition exists. The first kind, with the specified number of repeats, is accomplished with the use of a for loop. Here is the syntax of a for loop: for(int32 i = 0; i < count; i++) // do some junk // next statement after the loop completes. When the program execution reaches this code, the value in count will already be determined (as an integer). 1. The code executes Int32 i = 0; one time when the for loop is reached, creating the temporary variable i as a counter for the loop. This variable will disappear after the for loop is completed. 2. The code then tests the value of i in the second parameter (i < count;) of the for loop. If this statement returns a value of true, the loop will execute. 3. After the code inside the loop executes and the closing curly brace is encountered, the third parameter (i++) will execute, incrementing the value of i by one. 4. After that, the second parameter is tested once again, executing the code in the loop if the test condition returns true again. This continues until the test condition (i < count) returns false, at which time the code execution goes to the first statement after the for loop. An example of how this might be used is a case in which the user needs to add up a list of numbers. The app would inquire of the user how many values need to be summed up. After the user enters a number, the for loop would execute that many times, getting a number from the user and adding it to a running total. After the loop completes its execution, the code after it would do something with the sum, most likely printing it out for the user. The code might then ask if the user wishes to sum up another list of values, to which the user might reply yes. This brings up the other kinds of loop the while and dowhile loops. Either of these loops the while and the do-while will execute as long as the test condition returns a value of true. For example, a loop in a program that continues to increment a value as long as the value remains less than another value. An example of this is when we did our roll the dice application. As long as the user responded in the affirmative (said yes ) to the question Roll again? the loop would continue to repeat. This test condition can be any kind of comparison that returns a Boolean value. It is imperative, however, that code inside the loop affects that test condition such that the test condition at some point will return false. Otherwise, it becomes an infinite loop. In the case of the dice-roll application, it asks the user the Roll again? question inside the loop so that the test condition (suserresponse == Y ) can be affected to return false, such as if the suserresponse gets a value other than Y. H. W. Gould, Winter Page 5

6 A while or do-while loop will continue to repeat until the test condition returns false, giving the user control of how many times the loop executes. As long as the user wants more dice rolls, the dice will continue to roll. What is the difference, you ask, between a while and a do-while? (Why, I thought you d never ask!) A while loop: while(<test condition>) // do some garbage maybe A do-while loop: do // do some garbage at least once while(<test condition>); // note the semicolon here Note that the test condition on the while loop is at the beginning of the loop, but the test condition on the do-while loop is at the end. The primary difference between the two is that the do-while loop will always execute at least one time and then will check the test condition, whereas a while loop will test the condition prior to the first execution of the loop, meaning the code in the loop might never execute, in the case where the test condition is false prior to getting to the loop in code. The best example I can think of would be a program that controls a valve to a tank of fluid, based upon readings from an instrument. The programmer would use a while loop that tests to see if the fluid level in the tank is below a certain level, indicating the need to add more fluid. If the instrument indicates that the fluid in the tank is high enough, then the code to open the valve and fill the tank more will never execute. Only when the fluid level in the tank drops to a low level will the test condition return true, thus allowing the loop to add measured amounts of fluid into the tank. Another difference between the do-while and while loops is in syntax: a while loop has no semicolon after the loop. A do-while loop does have a semicolon. This is so the compiler can distinguish between the two. There are many instances in which either of the two will work, but for those occasions when one or the other is better suited, you ll have to make the determination. H. W. Gould, Winter Page 6

7 Methods, Functions, and Subroutines A method is a block of code that is given a name and that performs a specific purpose that may be needed from various places in the overall application. Executing this block of code is as simple as calling the name of the method from the point in code at which you wish to run the code. In some cases, the method will have a parameter list for which values must be sent (example to follow). These parameters are to hold values that are passed into the method so the method can do its work. There are two kinds of methods (functions and subroutines), although the three terms listed above tend to be used interchangeably, albeit incorrectly. A method that returns a value back to the calling program is called a function. A method that does not return a value, but simply does a chore or task for the calling code (the code that calls for the method to execute), is called a subroutine. Either can take parameters. Here is an example of each: A Function static Int32 AddThem(Int32 ifirstvalue, Int32 isecondvalue) Int32 iresult = ifirstvalue + isecondvalue; Return iresult; Note several things: the return type (in the example, Int32) is the data type of the value you are returning to the main program. The names of the variables in your parameter list (in this case, ifirstvalue and isecondvalue) are irrelevant. They exist only while the function is running. You could just as easily call the variables Abbott and Costello, or Laurel and Hardy. Ideally, though, you d use intuitive names as is the convention for professional software engineers. When the calling code passes information to the function, it passes the value by default. It does not pass the variable, even if the variable in the function has the same name as the variable in the main program. This is important to remember. This is due to the scope of the variables. When you are in the code in the function, everything in the main program is hidden, or in cyber-limbo. The above function example takes two integer values passed into it, which it stores in its two variables ifirstvalue and isecondvalue. It creates a third variable named iresult, which holds the sum of the two that results from the addition in the first statement. The second statement returns the value that is stored in iresult back to the main program, where the call to this function is in an assignment statement. That assignment statement might look like this: isum = AddThem(iAlpha, ibeta); The above statement involves isum, ialpha, and ibeta all being Int32 variables. H. W. Gould, Winter Page 7

8 A Subroutine static void Roll(Int32 ifv, Int32 isv) Console.WriteLine( Your sum is + (ifv+isv).tostring() +. ; This subroutine takes the two values sent to it and use them to complete the addition and output it to the screen, using the WriteLine( ) and an accompanying string literal. Notice that any value or mathematical equation that results in a number has the built-in ToString( ) method with it. So, ifv + isv results in an Int32 value, which means it has the ToString( ) in it. We do the math, then we convert to string, then we include it into a text output for the reader s sake. No value is returned in this case, which is why we use the void return type. This method simply does the task of writing output to the screen. Just as the function Roll in the dice program we did at midterm did not return a value, but simply handled the task of printing the correct dice to the screen. H. W. Gould, Winter Page 8

9 Classes and Objects (IT152) IT150/IT152 Concepts Summary Sheet Object-oriented programming established the concept of writing computer programs that more accurately reflect the real world. Objects can be designed to match real world objects such as cars or buildings or even abstract things like hotel reservations or client accounts. The relationship between a class and an object can easily be explained with the words automobile and your specific automobile. The word automobile has with it certain understood properties. Automobiles have tires, doors, motors, windshields, etc., regardless of make and model. However, the specifications on these various properties varies with each individual automobile. A class (and any object or instance of that class) will have certain data properties and functions in it that affect its own data. The automobile class, for example, has the data value of how much fuel is onboard. A member function might be one that adds gas in the amount specified when the method is called. An objects methods will only affect that particular object, not all objects of the same class. If I add gas to my car, it doesn t affect the fuel level of all the other cars in the parking lot. If my car is blue and I paint it red, the color data property of my car will change, but your car will remain the same color as always, unless you run the same function in your instance of automobile. To create an object of a particular class is to instantiate the class. It means, simply put, that you created a specific instance of that class, just as your car is a specific instance of the automobile class. To run a method or function in a particular object, you run it using the object name and the method/function name, like this: omycar.paintme( Red ); which would run the method inside the object omycar. Thus, the object omycar s data value for color would now be red. The data value for color on oyourcar would remain untouched. By designing well-planned objects, such as a Person class, you can use and re-use your classes. A Person class might be used to track employees of a company, passengers on a plane, or clients of a law firm among other possibilities. Anywhere in the world of software development where dealing with people, this class would apply. A class has two methods in it which must always be included the Constructors. This is a method that creates the object of the class. One of them will take parameters such as (for the car class) make, model, year, engine size, number of doors, etc. The other, the default Constructor, takes no parameters whatsoever, and creates an object of the class that fits default values. Even if you do not believe you will ever need it, you always (by convention) include a default constructor. Everything that applies in a main program can also be done in a class. Classes can contain functions, subroutines, and data values of all data types. Generally, the data members are listed as private, which means they cannot be viewed nor modified from outside the object. A well-written class will have methods that will display the values to the calling program, and will have methods that modify the values, if it is appropriate to do so. H. W. Gould, Winter Page 9

10 Summary Writing software it not difficult once the concept of think like a computer is achieved, which simply means that the software developer can think through a process step-by-step without worrying about what will happen several steps down the road, so to speak. We all do this; we just don t think about this. When you go to Facebook, you follow the individual steps like going to your computer, logging into it, opening your browser, entering the Facebook URL into it, and entering your login and password. You don t think about this process; you simply do it. It comes naturally to you. H. W. Gould, Winter Page 10