CS Exam 2 Study Guide and Practice Exam

Transcription

1 CS Exam 2 Study Guide and Practice Exam March 28, 2016 Summary 1 Disclaimer 2 Loops 2.1 For loops While loops Do-While Loops Hints and Warnings Methods 3.1 Static vs. Non-Static Return Values Parameters Pass-by-Reference vs Pass-by-Value Examples Arrays 4.1 Basics Warnings Discrete Math 5.1 Propositional Logic Practice Written Exam 6.1 Short Answer Tracing Code Programming Quiz Practice Exam 8 Suggestions for Studying and Test Taking 8.1 Written Programming Quiz Challenges

2 1 Disclaimer This is a review of the material so far, but there may be material on the exam not covered in this study guide. 2 Loops 2.1 For loops For loops are used when you know how many iterations the loop needs to complete. General syntax: for ( int i = 0; i < somelength; i++){ //some code You can also think of for loops as such: for (initialization; range; update) { //some code Real life example : A loop used print every second letter backwards (z, x, v, t...). for ( char c = z ; c >= a ; c-=2){ System.out.print(c); 2.2 While loops While loops are generally used when you don t know how many iterations you need the loop to complete (i.e. if you are reading entered words from the keyboard until the user types stop ). General syntax: int i = 0; while (i < somelength){ //some code i++; You can also think of for loops as such: initialization; //outside of loop while (range){ //some code update Same Real life example as above. char c = z ; while (c >= a ){ System.out.print(c); c-=2; 2.3 Do-While Loops Do-While loops are used when you want the loop to run at least once, regardless of whether the value is in the range. General syntax:

3 int i = 0; do { //code i++; while (range); Same Real life example as from before. char c = z ; do { System.out.print(c); c-=2; while (c >= a ); 2.4 Hints and Warnings Warning: Watch the ranges! For an array of size 8, i < 9 is the same as i apple 8 (which would iterate over all of the values in the array). However, i apple 9 will give you an Index Out Of Bounds Exception. Hint: You can always change your update by using the += or -= method, especially if you are updating by more than 1. Hint: You can always change your starting point, you don t have to start at 0, it is just the most common. If you were asked to reverse a string, you can always make the starting point at stringname.length-1! (Remember the -1 though, lengths are 1 based while loops are 0 based usually). Summary: Basically you can solve a problem with loops in many di erent ways whether it s using a di erent loop, starting at a di erent place, changing your ending value, or changing how you update. 3 Methods 3.1 Static vs. Non-Static Static methods are used when you used when you aren t going to use instance variables. To call a static method that you ve made: public static void main (String [] args){ int mycoolandcleverintegername = 0; printmyinteger(mycoolandcleverintegername); //prints 0 public static void printmyinteger(int i){ System.out.println(i); //it s a cool method, I know :) To call a static method from a di erent class: public static void main (String [] args){ int mynumber = 10; // Math is the class, pow is the static method int mynumbersquared = Math.pow(myNumber,2); System.out.printf("my number: %d, my number squared: %d", mynumber, mynumbersquared);

4 Non-static methods use instance variables. To call a non-static method: public class Exam2 { private int mycoolandcleverintegername = 0; public static void main (String [] args){ Exam2 exam = new Exam2 (); /* because mycoolandcleverintegername is * an instance variable it can be used in * ANY method in this class. */ exam.printmyinteger(); //prints 0 public void printmyinteger(){ System.out.println(myCoolandCleverIntegerName); 3.2 Return Values When a method has a return value of void it returns no value. You can print or manipulate instance variables and objects, without using the return keyword. When a method has a return value of double, int, String, String [ ], etc you must return a value of that type. For example, public int multiply(int a, int b) must return an integer or public String [] toarray(string s) must return a String array. 3.3 Parameters Parameters are values that are passed in through the method when it is called. These are variables that you can use and manipulate (BEWARE of pass-by-reference and pass-by-value!). Given the following code, notice that we passed the multiply method myint and mydouble but we used the variables i and d. This is where the warning comes in, this does not change any value. In order for the values to be changed in the main method we would need to assign. Also, when you call multiply(myint,mydouble);, you need to use a println or store the value in a variable and then print it for anything to be shown in the console. public static void main (String [] args0){ int myint = 3; double mydouble = 4.0; // will this do anything? multiply(myint, mydouble); public static double multiply(int i, double d){ double result = i * d; return result; Pass-by-Reference vs Pass-by-Value Pass-by-Reference are usually objects. This is because they have their own specified memory (aka it has memory allocated for the variable), so when a method calls that variable it accesses that place in memory and manipulates that. Therefore, Pass-by-Reference variables ARE CHANGED! For example:

5 public static void main (String [] args0){ int [] intarray = {1, 2, 3; System.out.println(Arrays.toString(multiplyIndex0(intArray,9))); //prints [9,2,3] System.out.println(Arrays.toString(intArray)); //prints [9,2,3] public static int [] multiplyindex0(int [] i, int p){ i[0] = p; return i; Pass-by-Value are primitive types that are passed into a method s parameter. THESE VALUES ARE NOT CHANGED OUTSIDE THE METHOD THAT INITIALIZES THEM. The calling method creates a copy of the values so the original values are never changed. For example: //method that is calling ( aka caller method) public static void main (String [] args0) { int number = 100; increment(number); System.out.println("Number: " + number); //method being called ( aka calling method) public static void increment(int n){ n++; //NUMBER IS NEVER CHANGED 3.4 Examples public class PsuedoString{ char [] greeting = { H, e, l, l, o ; //the charat String method public char charat(int index){ //must return a char return greeting[index]; //calling the non- static method public static void main(string [] args){ Psuedo p = new Pseudo(); p.charat(someinteger); public class Practice{ //a static method that makes a string from 3 characters public static String chartostring(char a, char b, char c){ String string1 = Character.toString(a) + Character.toString(b) + Character.toString(c); return string1;

6 //a method that prints a String array s elements public static void printarray (String [] sarray) { for ( int i = 0; i < sarray.length; i++) System.out.print(sArray[i] + " "); public static void main (String [] args){ String [] arr = {" Twas", "the", "night", "before", "my", "CS 160", "Exam.", " Twas", "a", "dark", "stormy", "night", "without", "a", "soul", "in sight", "..."; 4 Arrays 4.1 Basics printarray(arr); String newstring = chartostring( h, i,! ); System.out.println(newString); //two ways to initialize an array //you know only the size typeofarray [] nameofarray = new typeofarray [sizeofarray]; //you know what the values are typeofarray [] nameofarray1 = {values, you, want, in, the, array; Some Basics: To print you either need to make a loop or usearrays.tostring(). for ( int i = 0; i < arrayname.length; i++){ //Notice the lack of parenthesis after length! System.out.print(arrayName[i]); //Read directions carefully, you may need to add a space, an underscore, hyphen, semicolon, etc to the string. System.out.println(Arrays.toString(arrayName)); To manipulate the array String [] sarray = new String [3]; // to change on value in the array sarray[0] = "Hola"; // to find the length of a string int [] iarray = {1, 1, 1, 1, 1, 0; //remember there are no parenthesis after length like there is with Strings. System.out.println(iArray.length);

7 Multi-Dimensional Arrays // Initialization int [][] array = {{1,2,3,{1,2,3; // Printing a 2D Array for ( int row = 0; row < array.length; row++){ for ( int col = 0; col < array[row].length; col++){ // prints the value at array[ row][col] followed // by a comma then a space. System.out.print(array[row][col] + ", "); 2D Arrays are like tables, the first [ ] represent the rows and the second [ ] represent the columns. 4.2 Warnings Warning: Remember the length method starts at 1 and index starts at 0. If you had an array of length 3 and you try to change the value at arrayname[3] you will get an IndexOutOfBoundsException. Warning: Remember that you don t add ( ) to the length method. It is just arrayname.length. Warning: Arrays are objects, regardless if the type of array is a primitive type. So you can t use == on an array YOU MUST USE THE.EQUALS METHOD! 5 Discrete Math 5.1 Propositional Logic This is not comprehensive. Tautology: a proposition that is always true Contradiction: a proposition that is always false Contingency: a proposition that is neither always true and neither false (aka neither a tautology nor a contradiction) Given p! q Converse: q! p - Reverse the order Inverse: p! q - Negate the original order Contrapositive: q! p - Negate and reverse the order.

8 6 Practice Written Exam If you would like practice in Discrete Math: go to the CS 160 Homepage and under the Tutorials tab there are practice quizzes. 6.1 Short Answer 1. Declare an integer array name myintarray that contains 3 elements. 2. Use a for loop to assign the value 1 to every element in myintarray. 3. Change the last element in myintarray to 99, don t hard code. 4. Create a double 2D array called darray with 3x3 elements. Challenge: Create a double 2D array called darray initialized to the values {1.0, 2.0, 3.0 and {4.0, 5.0, 6.0, 7.0. (Note: 2D arrays can have di erent sizes for each row or column(in this example row 0 is length of 3 and row 1 is length of 4.)) 5. Declare the header to a public, static method that s called printer, it takes an integer as a parameter, returns nothing. 6. Declare a public method called multiplier that has two integers as parameters and it returns a double of the two integers. Call this method (which is in a class called Multiplier in your main with the integers 3 and Write a for loop, while loop, and do-while loop that prints odd numbers from 1 to 101 all on one line. 8. Declare a public, static method called find that has a String and character parameter. This method s purpose is to count the number of times a character is in the given string. find returns the count. Call this method (in your main method), with a stereos are as good as oreos and a character of o. 9. Write a method that is public, non-static, called toascii which has a String parameter, and returns an integer array. This method takes each letter in the String and converts the characters into Ascii values and places each character into a di erent element of an integer array (with the same order as the string). 6.2 Tracing Code /* Instructions: * By each commented number write down what the console would print. * There are some questions that return errors. Either write " error" or you can write the exception name. * If you find an infinite loop. Continue through the questions( even though the rest of the program would never print. * Write " infinite loop" for those cases for the questions. * */ import java.util.arrays; public class Trace { private int[] iflip; private String concat = ""; public int[] flip(int [] iarray){ iflip = new int [iarray.length]; for ( int i = iarray.length-1; i >= 0; i--) iflip[i] = iarray[i]; return iflip; public static double[] update(double[] d){ for ( int i = 0; i < d.length; i++){ d[i] = 1; return d;

9 public static void printshift (char c){ System.out.print("Shifted right 2 characters from " + c + " to "); c+=2; System.out.println(c); public int increment (int i){ return i++; public void makestring(string [] stringarray){ for ( int i = 0; i < stringarray.length; i++) concat += stringarray[i]; return concat; public void arrayprinter(){ for ( int i = 0; i < iflip.length; i++) System.out.print(iFlip[i]); public static void main (String [] args){ Trace trace = new Trace(); int [] integerarray = {1, 2, 3; int [] secondintarray = {4, 5, 6, 7, 8, 9, 10; double [] darray = {1.0, 22.0, 333.0, ; String [] badnamemashups = {"Bobbylene", "Jimmeroo", "Samandra", "Georgina", "Henrietta"; String [] boringstringarray = {"I am", "a", "boring", "string"; int counter = 0; char c = a ; //Question 1: What does the console print EXACTLY for ( int i = 0; i < 30; i++){ if (i % 3 == 0){ System.out.print(i + ", "); System.out.println(); int usernum = 3; //Question 2: Does this print anything? If not write " none", if it does write what s printed do { System.out.println("The usernum value is: " + usernum); while (usernum > 5); //Question 3: for ( int i = 0; i < integerarray.length; i++) System.out.print(integerArray[i]); System.out.println();

10 //Question 4: System.out.println( Arrays.toString(trace.flip(integerArray))); update(darray); //Question 5: System.out.println(Arrays.toString(dArray)); darray[4] = -2.0; //Question 6: System.out.println(dArray[4]); //Question 7: while (counter >= 0){ System.out.print(counter); trace.increment(counter); //Question 8: printshift(c); //Question 9: System.out.println(c); //Question 10: for ( int i = 0; i <= badnamemashups.length; i++){ System.out.print(badNameMashUps[i] + " & "); //Question 11: trace.makestring(boringstringarray); //Question 12: arrayprinter(secondintarray); 7 Programming Quiz Practice Exam Instructions: Your program must have only 11 lines of output. If you don t know how to solve one of the question print an empty line so you get credit for the rest of the quiz. 1. Make a public, non-static method that takes a parameter of one String array, returns a String of every other letter in each string in the string array (include spaces), call the method everyother. 2. Make a public, static method called info that when given a String (as a parameter) the method prints the length of the the String, the first and last character, and it prints the index of a, if it does not have an a in the string it returns -1. Do not make another String variable to copy/edit, use the parameter. (returns nothing, everything is on di erent lines) 3. Create a private instance variable of type String, called personalitystring. Write a public, non-static method, called personality, this method will use the personalitystring (hint: don t use a parameter) and returns another String. Use a conditional statement of your choice to return your String. If the parameter String has a length of less than or equal to 3, return silent, if it has a length between 4 and inclusively 9, return smarty, if the length is more than 9 then return talker. Inside your main method: 4. Make a String array called sarray and assign Hola, Hi there, Hello, and Sup. Use this to test

11 question Make a String variable called s1 and assign supercalifragilisticexpialidocious to it (fun fact: this is 5th longest English word, exciting I know). Use this to test question Make four String variables called s2, s3, s4, 25 and use these variables to test the method from question Make a 2D int array called iarray2d, that is a 4x4 box. 8. Initialize all values in the array to 42 (because it s the answer to everything). 9. Print out the 2D array s values with commas separating the values, all on one line. (Expect a trailing comma) 10. Make a double array called doublearray and initialize the array to the values 1.0, 2.0, 3.0, 4.0, 5.0, and Print the 3rd value in doublearray. 8 Suggestions for Studying and Test Taking 8.1 Written Write down for, while, do-while loops/templates. Know what each piece does, that way when you come across a problem that needs a loop then you can quickly change a piece (initialization/starting point, range, or ending place) of the loop to solve your problem. Practice writing code in Eclipse and before you run your program guess what the output would be. is good practice for testing your own programs and also for the code tracing part of the exam. This For your discrete math there are interactive quizzes under the Tutorials tab on the CS 160 Homepage. 8.2 Programming Quiz Go back to past recitations and assignments and redo them without using the internet, friends, or past recitations/assignments. Practice writing code in Eclipse. Make up projects and problems or ask a TA and they can give you some challenges. Look at code, the more exposure you get to code (whether it s your own code or not) the easier it is to understand. Under the Progress Page many of the units have programs that you can look at. 8.3 Challenges Create a method that reverses a string array. There are some cool rhymes/poems that make sense forward and backwards. You can also make this into an array challenge by separating the poem/rhyme by words and putting each word into a di erent element of a String array. Then reverse that. Under the Resources tab on the CS 160 Homepage there are some coding practice sites, like CodingBat.