Glossary. (Very) Simple Java Reference (draft, v0.2)

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1 (Very) Simple Java Reference (draft, v0.2) (Very) Simple Java Reference (draft, v0.2) Explanations and examples for if and for (other things to-be-done). This is supposed to be a reference manual, so we expect that you re somewhat familiar with stuff already. (Please send requests for improvements, corrections and expansions to anya@ii.uib.no.) Glossary Statement (utsagn eller setning) code that does something; either calling a method (e.g., System.out.println ("hei!");, or deciding which statement to execute next (e.g., if, for,...) Expression (uttrykk) code that yields a value when the program is running. For example, ( 7), a < b ( true or false), i ( the value of variable i), i++ ( the value of variable i, and increase i after), a[i] ( the value stored at index i in the array stored in the variable a). Value (verdi) the result of a computing an expression. In Java, it will be a number or character (7, 'a', 2.0,...), or a reference to an array or object. All values have types. Variable (variabel) variables store values (either primitive ones like numbers, or references to objects). All variables have types only a value with a compatible type can be put into a variable. Type (type) classification of values, expression and variable. In Java, all expressions will have a type this tells us roughly what sort of value it can have when the program is running. For variable, the type tells you what values you are allowed to store in it. For example, if our program says int a, we know that a will be an integer, even if we don t know which exact integer it will be, and we know that in an assignment like a = e;, the expression e must also compute an integer, otherwise we wouldn t be allowed to put it into a. Types are used to determine what expressions you are allowed to write 1. For example, the things you can do with a string would probably be different from what you can do with a number. So, with String s; int a;, it makes sense to say s.length() (length of a string), but not a.length() (?); and you can say s+"b" (string concatenation) and a+2 (addition), even though you re not allowed to say s-"b". The primitive types are boolean, byte, short, int, long, float, double, char. All other types are reference types, such as String (a reference to a string), int[] (a reference to an array of integers), String[] (a reference to an array of reference to strings). Parameter (parameter) (also argument) a variable containing input to a method. The value used in the method call is placed in the parameter variable before the method starts. Apart from this, they behave like normal variables. Field variable (feltvariabel) (also field, member variable) a variable which is part of an object. Inside methods, you can refer to them directly, or by this.x. 1 This is typically called static typing the compiler checks that all your expressions are well-formed, with compatible types, before the program is run. 1

2 If Statement If Statement Syntax if(condition1) { then-branch1 } else if(condition2) { then-branch2 }... else { else-branch } Condition is tested first and decides which branch should be executed. Must be a Boolean expression (e.g., a logic expression or a comparison) Then-branch is executed if the condition was true. Else-branch (optional) is executed if the condition was false. Else if the else branch may contain another if, for an arbitrarily long sequence of alternatives. The next condition is tested only if the previous one was false. Curly braces around the then and else branches are recommended, but not necessary if the branch has only one statement. Definitely recommended for multi-line statements. Typical Examples 1 if(a > b) 2 return a; 3 else 4 return b; 1 if(s.equals("help")) 2 givehelp(); 3 else if(s.equals("quit")) 4 System.exit(0); 5 else if(s.equals("start")) 6 start(); 7 else 8 error("unknown command: " + s); More If examples Test even/odd: 15 int a = 3; 16 if (a % 2 == 0) { 17 System.out.println("a is even!"); 18 } else { 19 System.out.println("a is odd!"); 20 } (Example 1) a is odd! if vs else if: 24 // what does this print? 25 a = 1; (Example 2) 2

3 26 if (a < 1) 27 System.out.println("less than one!"); 28 else if (a < 2) 29 System.out.println("less than two!"); 30 if (a < 3) 31 System.out.println("less than three!"); less than two! less than three! Empty then-branch (sometimes useful): (Example 3) 35 // here we avoid having to say "a!= 0 &&..." in every condition 36 if (a == 0) 37 ; // do nothing for zero 38 else if (a < 10) 39 System.out.println("between one and ten!"); 40 else if (a < 20) 41 System.out.println("ten and twenty!"); between one and ten! For Loops Syntax label: for(initialiser; condition; update) { body } Initialiser is executed before the first loop iteration starts. (Does nothing if empty.) Loop condition is tested before each iteration (loop exits if false). (Always true if empty.) Loop update is executed after each loop iteration, before the condition is tested. (Does nothing if empty.) Loop body is executed once per iteration. (Does nothing if empty.) Loop label (hardly ever used) used with continue/break if you have nested loops Typical Example 1 for(int i = 0; i < n; i++) { 2 doonething(i); 3 doanotherthing(i); 4 } Strange Examples 3

4 1 // infinite loop: 2 for(;;) { 3 // (same as while(true) {...}) 4 } 5 6 // multiple variables 7 for(int i = 0, j = 100; j > 0; i++, j--) {... } Scope of variables in a for-loop Scope when loop variable is declared in loop initialiser (Example 4) 1 for(int i = 0; i < 10; i++) { // i=0,...,9 2 i for(int j = 0; j < 10; j++) { // j=0,..,9 for each i=0,...,9 3 j System.out.println(" (" + i + "," + j + ")"); 4 } 5 } 6 // new loop, different i: 7 for(int i = 0; i < 10; i++) { // i=0,...,9 8 i System.out.println(" " + i); 9 } Scope when loop variable is declared outside (Example 5) 1 { 2 int i = 0; // loop variable can be declared before: 3 ifor (i = 2; i < 10; i++) { 4 // i=2,...,9 5 System.out.print(" " + i); 6 } 7 // i is now 10: 8 System.out.println(" Value of i after loop: " + i); } A bunch of loop examples Counting : 11 for (int i = 0; i < 10; i++) { // repeat 10 times 12 System.out.print(" " + i); // i=0,1,...,9 13 } (Example 6) Counting : 17 for (int i = 10; i > 0; i--) { // repeat 10 times 18 System.out.print(" " + i); // i=9,8,...,1 19 } (Example 7) Counting again: 23 for (int i = 0; i < 10; i++) { // this is a different i 24 System.out.print(" " + i); 25 } 26 // Note: i is no longer available after the loop: 27 // System.out.println(i); => "i cannot be resolved to a variable" (Example 8) 4

5 Printing with commas between: (Example 9) 31 // many ways to solve this: 32 // if(!first) {print(", "); first=false;}, to avoid printing comma before first number 33 // if(i!=0) print (", ");, to avoid printing comma before first number 34 // if(i!=n 1) print (", ");, to avoid printing comma after last number 35 // or store text of comma, as we have done 36 String comma = ""; 37 for (int i = 0; i < 10; i++) { // repeat 10 times 38 System.out.print(comma + i); // i=0,1,...,9 39 comma = ", "; // next i will get a comma in front of it 40 } 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 Do-nothing loop: 44 // this loop stops before it does anything (0 > 0 == false) 45 for (int i = 0; i > 0; i++) { 46 System.out.print(" " + i); 47 } (Example 10) Nested loops: 51 for (int i = 0; i < 10; i++) { // i=0,...,9 52 for (int j = 0; j < 10; j++) { // j=0,..,9 for each i=0,...,9 53 System.out.print(" (" + i + "," + j + ")"); 54 } 55 System.out.println(); // new line for each iteration of outer loop 56 } (Example 11) (0,0) (0,1) (0,2) (0,3) (0,4) (0,5) (0,6) (0,7) (0,8) (0,9) (1,0) (1,1) (1,2) (1,3) (1,4) (1,5) (1,6) (1,7) (1,8) (1,9) (2,0) (2,1) (2,2) (2,3) (2,4) (2,5) (2,6) (2,7) (2,8) (2,9) (3,0) (3,1) (3,2) (3,3) (3,4) (3,5) (3,6) (3,7) (3,8) (3,9) (4,0) (4,1) (4,2) (4,3) (4,4) (4,5) (4,6) (4,7) (4,8) (4,9) (5,0) (5,1) (5,2) (5,3) (5,4) (5,5) (5,6) (5,7) (5,8) (5,9) (6,0) (6,1) (6,2) (6,3) (6,4) (6,5) (6,6) (6,7) (6,8) (6,9) (7,0) (7,1) (7,2) (7,3) (7,4) (7,5) (7,6) (7,7) (7,8) (7,9) (8,0) (8,1) (8,2) (8,3) (8,4) (8,5) (8,6) (8,7) (8,8) (8,9) (9,0) (9,1) (9,2) (9,3) (9,4) (9,5) (9,6) (9,7) (9,8) (9,9) Two loops after each other: (Example 12) 60 for (int i = 0; i < 10; i++) { // i=0,...,9 61 System.out.print(" " + i); 62 } 63 for (int j = 0; j < 10; j++) { // j=0,...,9 64 System.out.print(" " + j); 65 }

6 Variable declared outside loop: (Example 13) 69 // loop variable can be declared before: 70 int i = 0; 71 for (i = 2; i < 10; i++) { 72 // i=2,...,9 73 System.out.print(" " + i); 74 } 75 // i is now 10: 76 System.out.println(" Value of i after loop: " + i); 77 // i continues to exist ( be in scope ) for the rest of the block 78 // until we reach } 79 // 80 // we can't declare a new i in a loop now, because there's already 81 // an i in scope: 82 // for(int i = 0; i < 10; i++) { } => "i already declared" 83 // but we can reuse the same i: 84 // for(i = 0; i < 10; i++) { } => ok Value of i after loop: 10 Very silly loop: 88 for (int j = 0; j < 10; j++) { 89 for (j = 0; j < 10; j++) { // WTF happens now? 90 System.out.print(" " + j); 91 } 92 } (Example 14) Two equivalent loops: (Example 15) 96 System.out.print(" A for-loop: "); 97 for (int counter = 0; counter < 10; counter++) { 98 System.out.print(" " + counter); 99 } 100 System.out.println(); System.out.print(" A while-loop: "); 103 int counter = 0; 104 while (counter < 10) { 105 System.out.print(" " + counter); 106 counter++; 107 } A for-loop: A while-loop: A loop that operates on strings: (Example 16) 111 for (String s = ""; s.length() < 10; s = s + "*") { 112 System.out.println("'" + s + "'"); 113 } 6

7 '' '*' '**' '***' '****' '*****' '******' '*******' '********' '*********' A loop that computes min/max of a list: (Example 17) 117 int[] myarray = { 3, 2, -1, 9, 5, -10, 0, 8 }; 118 int maxseen = Integer.MIN_VALUE; // largest number seen so far 119 int minseen = Integer.MAX_VALUE; // smallest number seen so far 120 // what should we set maxseen and minseen to before we've seen 121 // any numbers? 122 // we can set them to the first number, myarray[0] 123 // => we must check that we have at least one element! 124 // or the smallest/largest number we can think of 125 for (int k = 0; k < myarray.length; k++) { 126 System.out.printf(" k=%4d, myarray[k]=%3d, minseen=%5d, maxseen=%5d\n", 127 k, myarray[k], minseen, maxseen); 128 if (myarray[k] > maxseen) { 129 maxseen = myarray[k]; 130 } 131 if (myarray[k] < minseen) { 132 minseen = myarray[k]; 133 } 134 } 135 System.out.println(" min=" + minseen + ", max=" + maxseen); k= 0, myarray[k]= 3, minseen= , maxseen= k= 1, myarray[k]= 2, minseen= 3, maxseen= 3 k= 2, myarray[k]= -1, minseen= 2, maxseen= 3 k= 3, myarray[k]= 9, minseen= -1, maxseen= 3 k= 4, myarray[k]= 5, minseen= -1, maxseen= 9 k= 5, myarray[k]=-10, minseen= -1, maxseen= 9 k= 6, myarray[k]= 0, minseen= -10, maxseen= 9 k= 7, myarray[k]= 8, minseen= -10, maxseen= 9 min=-10, max=9 A loop that computes min/max without index (a.k.a. enhanced for-loop"): (Example 18) 139 // same setup as above 140 // int[] myarray = {3,2, 1,9,5, 10,0,8}; 141 // int maxseen = 0; // largest number seen so far 142 // int minseen = 0; // smallest number seen so far 143 if (myarray.length == 0) { 144 throw new IllegalArgumentException("Can't compute min/max of empty array"); 145 } 146 maxseen = myarray[0]; // pick first value as min/max so far 147 minseen = myarray[0]; 148 System.out.println(" myarray=" + Arrays.toString(myArray)); 149 System.out.println(" Loop:"); 150 System.out.printf(" %4s %7s %7s\n", "x", "minseen", "maxseen"); 7

8 151 System.out.printf(" \n"); 152 for (int x : myarray) { // x = myarray[0],myarray[1],..myarray[n 1] 153 System.out.printf(" %4d %7d %7d\n", x, minseen, maxseen); 154 // Math.max/min does the job easier than with "if": 155 maxseen = Math.max(maxSeen, x); 156 minseen = Math.min(minSeen, x); 157 } 158 System.out.println(" result: min=" + minseen + ", max=" + maxseen); myarray=[3, 2, -1, 9, 5, -10, 0, 8] Loop: x minseen maxseen result: min=-10, max=9 Advanced examples A loop with break and continue: (Example 19) 164 for (int k = 0; k < 10; k++) { 165 if (k % 2 == 1) 166 { 167 continue; // jump to next iteration 168 } 169 if (k == 8) 170 { 171 break; // stop if we reach } 173 System.out.print(" " + k); 174 } A loop with continue, finds prime numbers: (Example 20) 178 // warning: this is somewhat bad style 179 // warning:...and it's inefficient. A guy called 180 // Eratosthenes found a better algorithm over 2200 years ago 181 // outer: for (int k = 2; k < 100; k++) { 183 // check if k is dividable by a smaller number 184 // if so, it's not a prime, move on to next 185 for (int divisor = 2; divisor < k; divisor++) { 186 if (k % divisor == 0) 187 { 188 continue outer; // go to next k (label "outer") 189 } 190 } 191 // if we get here, we didn't find any divisor 192 System.out.print(" " + k); 193 } 8

9 A Better Prime Number Generator: (Example 21) 202 // We need an ArrayList, since it can grow in size 203 List<Integer> primes = new ArrayList<>(); 204 outer: for (int k = 2; k < 100; k++) { 205 // we only need to try dividing by prime number; 206 // for any other number, there'll a smaller number / 207 // prime factor that k would be divisible with. 208 // The only possible divisors would be a smaller prime number, 209 // and they'll be in our list of primes! 210 for (int divisor : primes) { 211 if(divisor * divisor > k) { 212 break; // our other primes will be too large too 213 } else if (k % divisor == 0) 214 { 215 continue outer; // go to next k (label "outer") 216 } 217 } 218 // if we get here, we didn't find any divisor 219 primes.add(k); 220 } 221 System.out.println(primes); [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97] Find longest consecutive series of positive or negative numbers: (Example 22) 227 int[] numbers = {0, 1, 3, 0, 4, -1, -3, -1, -2, -5, 1, -7, -8, -2}; 228 int last = 0; // last number seen 229 int current = 0; // length of current sequence 230 int longest = current; // longest seen so far 231 System.out.printf(" %4s %7s%7s %7s %7s %-9s %-9s\n", "k", "numbers[k]", "last", "current", "longest", "samesign?", "nonzero?"); 232 System.out.printf(" \n"); 233 for(int k = 0; k < numbers.length; k++) { 234 System.out.printf(" %4d %7d %7d %7d %7d %-9b %-9b\n", k, numbers[k], last, current, longest, last * numbers[k] > 0, numbers[k]!= 0); 235 // slightly better: 236 // Math.signum(numbers[k 1]) Math.signum(numbers[k]) > // (multiplies signs ( 1,0,1) rather than numbers themselves) 238 if(last * numbers[k] > 0) { // same sign 239 current++; 240 } 241 else if(numbers[k]!= 0) { // starting a new series 242 current = 1; 243 } 244 else { // zero is not part of a series 245 current = 0; 246 } 247 last = numbers[k]; 248 longest = Math.max(longest, current); 249 } 250 System.out.println(" Longest series: " + longest); 9

10 k numbers[k] last current longest samesign? nonzero? false false false true true true false false false true false true true true true true true true true true false true false true true true true true Longest series: 5 10