Select the ONE best answer to the question from the choices provided.

Transcription

1 FINAL EXAM Introduction to Computer Science UAlbany, Coll. Comp. Info ICSI 201 Spring 2013 Questions explained for post-exam review and future session studying. Closed book/notes with 1 paper sheet of notes. Multiple choice (17 questions, 4 points each) Select the ONE best answer to the question from the choices provided. 1. A Java comment consists of everything from // to the end of the line, or everything between a matching pair of /* and */ sequences. 1 When you put Java code inside a comment: a. The compiler checks the comment for spelling but otherwise ignores it. b. That code is compiled and run just like any other code because the comment simply emphasizes to people that it is important. c. That code, like anything else inside the comment, can be seen by people who look at the program, but is otherwise completely ignored when the program is compiled or run. d. That code is printed when the program is run. The item starts with a reminder of the (correct) syntax for comments. The question is about (1) purposes of comments and (2) about what processing or printing is done when the program is compiled or run: Briefly, NONE!

2 Here is some Java code. What's printed when it runs? (In the first response below, 52 was printed as a 5 and a 2 in a row, not number fiftytwo. Similarly for all the others.) int Adam; int Billi; Adam = 1; Billi = 2; Adam = Billi + 3; //line 5 Billi = Adam + 4; //line 6 System.out.print(Adam); System.out.print(Billi); Tests your skill to carefully simulate code: First, Adam is set to 1. Second, Billi is set to 2. Third, the 2 in Billi is retrieved, added with literal 3 to give 5, and the 5 is stored in Adam. Adam is not changed again before being printed, so what is printed first is 5. Fourth, the 5 just stored in Adam is retrieved, added with literal 4 to get 9, and the 9 is stored in Billi (overwriting the 2 that Billi was set to first). Billi is not changed again before being printed, so what is printed second is What if lines 5 and 6 were interchanged? Billi = Adam + 4; Adam = Billi + 3; a. 37 b. 35 c. 57 d. 58 e. 59 a. 85 b. 58 c. 59 d. 21 e. 55 It's wise to completely repeat your figurings. Perhaps cross out and rewrite to code to avoid confusion. Here is a way to write the computation trace in a diagram. int Adam; int Billi; Adam = 1; Billi = 2; Billi = Adam + 4; //line 6 Adam = Billi + 3; //line 5 System.out.print(Adam); System.out.print(Billi); After Adam = 1; Billi = 2; Adam Billi 2 1 After Billi = Adam + 4; Adam Billi 5 1 After Adam = Billi + 3; Adam Billi 5 8 So what's printed is 85

3 4 4. According to the Mad Ph.D. video, what does the code House reftohouse = new House(); make the computer do? a. Make a new House object and throw away its address or location. b. Make a new House class and save its address or location. c. Make a new House object, make a local variable named reftohouse, and copy that object's address or location into that local variable as its value. d. Same answer as c. except what is made is a class (or blueprint), not an actual House object. new House( ) makes a new House object (whose design or blueprint had been coded in the House class defined in House.java) and returns its address or location. As usual in an assignment, the computation on the RIGHT is done first. The House reftohouse code means make a local variable named reftohouse So, after that, the address or location returned is copied into that local variable as its value. Here is the head of a method definition: public double weightedavg( double n1, double n2, double wgt1 ) 5. One of the method bodies computes and returns the weighted average of the first two double parameter values, with the first value weighted with weight w1, the third parameter value. Pick the one. a. return ( wgt1*n1 + (1.0 wgt1)*n2 ); 5 6 b. return ( (wgt1*n1 + (1.0 wgt1)*n2)/2.0 ); c. return ( 0.5*n *n2 ); d return ( (n1*wgt1) + ((1.0 n1)*wgt2) ); e. return ( wgt1*n wgt1*n2 ); Choice c. is the ordinary average; it is when the weight wgt1 is 0.5 The weights multiplying the numbers n1 and n2 to be averaged must add up to 1.0 So when the weight for n1 is wgt1, the other weight must be (1.0 wgt1)

4 6. Please reconsider the above question. 4 of the 5 choices are in good Java syntax and will compile, but one of the 5 will fail to compile. Which one will fail to compile? (Even though the other 4 are legal and will compile, only one is the correct answer to question 5.) The only variables declared in this question are n1, n2, and wgt1 All the choices have good Java syntax, but just one of the choices has the variable wgt2 which was NOT declared. Hence that one will fail to compile. The other 3 (wrong for question 5) choices recall that code that is useless or wrong from the application's point of view will still be compiled if it has no Java language errors. Such errors are called logical errors as opposed to syntax errors. 7. The full-credit version of Project 5 required that a row of images be copied so they are centered horizontally when they are viewed. Here are 4 computing operations that are involved (among others): C: Actually copy a Picture. I: Make the Picture objects from input files. L: Calculate where to copy the first Picture. W: Calculate (by adding up) the total width. In what order must the computer do these operations? a. C I L W b. I C W L 7 c. I W L C d. L I W C e. other order not shown The final step is copying the picture (C). Preceeding (C), calculating WHERE to copy it must be done (L). The geometric location depends on the total width so (W) must preceed that. The Picture must be made (with new Picture) before getwidth will return its width, so that must be done first a. public int next( int x ) return (x + 1); defines a method and next( 7 ); calls a method. b. public int next( int x ) return (x + 1); calls a method and next( 7 ); defines a method. 9. In the code above: a. x is a parameter value and 7 is a parameter variable. b. x is a parameter variable and 7 is a parameter value.

5 A Java method definition always specifies the type(s) of the parameter varibles (such as int in int x). Only method definitions specify a return value type (such as int in int next) type. Only method definitions can specify properties of the method (such as public). Only method definitions can specify the body of code to be run (The code in... ) at some future time when the method is called. A parameter variable, like any other variable, is named by its name and must be declared (in Java) with its type. A variable can never be a literal number like 7. A situation NOT covered in these two question is a variable whose value is used for the parameter value in a method call. For example, in the code sequence below, invar is a local variable whose value comes from the user and whose value is used as a parameter value. toprint is another local variable whose value is used as a parameter value in another method call. int invar; invar = sc.nextint(); int toprint; toprint = next( invar ); System.out.println(toPrint); For green chromakey substitution, select the correct way to program processing of all the Pixels of the Picture with the green background: a. fors or whiles loop through all those Pixels. A for or while decide whether a color is green enough to substitute. b. fors or whiles loop through all those Pixels. An if decides whether a color is green enough to substitute. c. if statements loop through all those Pixels. A for or while decides whether a color is green enough to substitute. d. if statements loop through all those Pixels. An if statement decides whether a color is green enough to substitute. Just remember: for and while are for loops, and if is for decisions. Here is a mnemonic: "if" is a shorter word than the others.

6 11.Here is a variation of familiar code. It takes the Color of each Pixel from one Picture and copies a brighter version of that Color into the corresponding Pixel of a different Picture. Assume both Pictures have the same width and the same height dimensions. The question asks which is the direction of the copying...put simply, from (a.) A to B, or (b.) from B to A? Picture A = Good code that makes or provides a Picture object ; Picture B = Good code that makes or provides another Picture object ; int y = 0; while (y < A.getHeight()) int x = 0; while (x < A.getWidth()) (B.getPixel(x, y)).setcolor( (A.getPixel(x, y)).getcolor().brighter( ) ); x++; y++; 11 a. The Picture referred to by A is the source of the colors. They are brightened and then copied into the Picture referred to by B. b. The Picture referred to by B is the source of the colors. They are brightened and then copied into the Picture referred to by A. The Pixel whose Color is gotten (by getcolor) is returned when getpixel( ) is called on Picture (referred to by) A. Therefore that Pixel (from Picture A) is the source of the Colors.

7 12 12.When a computer finds the minimum of numbers within an array by processing the elements one at at time, what should it do to decide whether or not to replace the number stored as the minimum found so far? a. In the (... ) of a while or for statement, compare the previous element to the current element. b. In the (... ) of a while or for statement, compare the minimum found so far to the current element. c. In the (... ) of an if statement, compare the previous element to the current element. d. In the (... ) of an if statement, compare the minimum found so far to the current element. First, the decision to replace the minimum found so far with the current element is made, as decisions are typically made, by an if statement, NOT a for or while statement. Second, it is NOT sufficient to decide that the current element is a new minimum based on it being smaller than the preceeding element! (Quite a few beginners think this is so, BUT IT IS NOT!) That is because the current element, even if it is smaller than the preceeding element, might actually be bigger than an EVEN SMALLER element that was somewhere even further preceeding. For example, when the numbers are 1 15, 14; 1 is the smallest although the 14 is smaller than the 15 which immediately preceeds it.

8 13. void plot( int xlocation, int height ) Lab 8 concluded with an assignment to extend G&E's Picture class to a Plotter class by adding the above method to paint a vertical black line at the given x-location with the given height (in pixel units.) Which body is correct? 13 a. for(int y = 0; y < height; y = y + 1) this.getpixel( xlocation, y).setcolor(java.awt.color.black); b. for(int y = 0; y < height; y = y + 1) this.getpixel( y, xlocation).setcolor(java.awt.color.black); c. for(int y = 0; y < height; y = y + 1) this.getpixel( y, y).setcolor(java.awt.color.black); d. for(int y = 0; y < height; y = y + 1) this.getpixel( xlocation, xlocation).setcolor(java.awt.color.black); x x drawn by (a). drawn by (b). y x y x drawn by (c). drawn by (d). y y

9 14. In Lab 8 and many other labs and projects, you extended G&E book classes by coding class definitions like public Plotter extends Picture /* In here, we define a constructor calling a superconstructor, plus we add (public) methods to add our own potential behaviors to Picture objects.*/ Suppose (like in the problem above) you added a public method, like plot. Suppose your application starts with this code: public class App public static void main(string[ ] a) Picture pictr = new Picture( good parameter(s) for constructing a Picture ); Plotter plotr = new Plotter( good parameter(s) for constructing a Plotter ); [Note: (0, 0) are valid parameter values for any call to plot. ] 14 a. On pictr, you can call both plot(0,0) and show( ), and on plotr you can also call both plot(0,0) and show( ). b. On pictr, you cannot call plot(0,0) but you can call show( ), and on plotr you can call both plot(0,0) and show( ). c. On pictr, you can call both plot(0,0) and show( ), and on plotr you cannot call show( ) but you can call plot(0,0). d. On pictr, you cannot call plot(0,0)but you can call show( ), and on plotr, you cannot call show( ) but you can call plot(0,0). 15.When is a constructor method called automatically? (It is special because its name is the same as the name of the class in which the constructor is defined.) a. Before the new operator builds the object. b. Immediately after the new operator builds the object. c. Some time after the rest of the program starts using the object. d. Never automatically: Methods only run when they are called! 15

10 16.Tracing with an array. Suppose the code referring to an array of integers through arr is started with the contents of the array, and k, given on the right. Trace the code using the boxes underneath to carefully determine what a computer would leave in the array when the code finishes. int k; k = 0; while( k < arr.length 1 ) arr[k] = arr[k + 1]; k = k + 1; Now what is stored in the array when the loop finishes? 0 k [0] [1] [2] [3] [4] a. b c d. e

11 17. This question is to analyze what nested loops do; the code is from a lecture and Project 5's Sudoku class definition. public String tostring() String result = ""; /*A*/for( int rowofblocks = 0; rowofblocks < 3; rowofblocks = rowofblocks + 1 ) /*B*/for( int rowinablock = 0; rowinablock < 3; rowinablock = rowinablock + 1 ) /*C*/for( int colofblocks = 0; colofblocks < 3; colofblocks = colofblocks + 1 ) /*D*/for( int colinablock = 0; colinablock < 3; colinablock = colinablock + 1 ) result=result+ board[colofblocks*3+colinablock][rowofblocks*3+rowinablock] + " "; result = result + " "; result = result + "\n"; result = result + "\n"; return result; When the above method is called on a Sudoku (or CheckableSudoko) object, it returns a string that looks like what is on the right: Please consider the Sudoku board as 9 rows consisting of 9 numbers each. One of those 9 rows is circled. One of the 4 for loops, labeled /*A*/, /*B*/, /*C*/, /*D*/, provides the 9 numbers of each row. They are 9 numbers added to the result string each (single) time that (whole) loop is run Which of the 4 loops has this purpose of making a row of 9 numbers? a. /*A*/ b. /*B*/ c. /*C*/ d. /*D*/ (End of the multiple choice questions. The remaining 2 questions are for writing Java code.)

12 (20 points) Coding Complete to code below so that the method finds and then prints each of the following (some steps require one or more loops): 1. The number of ints in the array located by the parameter value. (That is the length of the array, the number of elements.) 2. The smallest int in that array. 3. The location (index or subscript value) where the smallest int is stored*(see footnote). 4. The average of all the ints in that array. You MUST program the printing by filling in the FOUR BLANK SPACES in the last four lines of the method, which we wrote for you. (Correct answers don't refer to this and don't depend on whether the method is static or not.) public void static printstats( int[] array ) System.out.println( Length is + ); System.out.println( Smallest element is + ); System.out.println( Smallest is at index + ); System.out.println( The average is + ); *(Don't worry about two or more array elements storing the same int value. If the minimum is repeated, any index at which a copy is stored is acceptable.)

13 (15 points) More coding. Here's the NumCollector class that plans objects that have an array for collecting numbers (like the House of Project 5 which collected Picture refs.) Your job is to complete the showoffcollection method so it does all 3 things: 1. Prints all the numbers in the collection (must use a loop). 2. Prints how many odd numbers are in the collection, after using a loop to figure out what to print. (Cheat sheet info: ((X % 2) == 0) evaluates to true exactly when X is even.) 3. If ALL 10 numbers in the collection are odd, prints the literal line ALL YOUR NUMBERS ARE ODD. But, if there is one or more even number, it should print instead YOU HAVE AN EVEN NUMBER. public class NumCollector int array; public NumCollector( ) this.array = new int[10]; public void addnumberwhere( int num, int where ) this.array[ where ] = num; public void showoffcollection( )