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Introduction to Programming - Concepts and Tools

Introduction to Programming - Concepts and Tools. Lecture 2, 13 February, 2004 Jens Chr. Godskesen http://www.it/courses/ITBR/F2004/. Overview Lecture 2. Control structures Sequencing, conditionals, and loops Statements if and if-else for -loops while-loops do-while loops

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Introduction to Programming - Concepts and Tools

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  1. Introduction to Programming - Concepts and Tools Lecture 2, 13 February, 2004 Jens Chr. Godskesen http://www.it/courses/ITBR/F2004/

  2. Overview Lecture 2 • Control structures • Sequencing, conditionals, and loops • Statements • if and if-else • for-loops • while-loops • do-while loops • Termination of loops • break and continue • Mandatory assignment

  3. Sequences, Conditionals, and Iterations conditional b = 0 yes no c = a % b iteration sequence a = b b = c return a

  4. Sequences, Conditionals, and Iterations In Java we would write while (b != 0) { c = a % b; a = b; b = c; } System.out.println("The gcd is: ” + a); using the fundamentals: sequencing, conditioning and iterating.

  5. Statements The things that can be put together in sequence are statements i.e.: • variable declaration statements • (assignment and method call) expression statements • blocks • if and if-else statements • while statements • for statements • break and continue statements • switch statements

  6. Variable Declaration Statements The simplest variable declaration statement is a comma separated list of variables (of the same type) int i; int a, b, c; Variables may be initialized when declared int a, b = 0, c;

  7. Expression Statements We have seen assignment expression statements e.g. c = a % b; and method call expression statements e.g. System.out.println("The gcd is: ” + a); In general an expression statement is on the form expr; for some expression expr (notice the semicolon is part of the statement). E.g. i++; is an expression statement whereas a % b; is not.

  8. Blocks A seqence of statements may be grouped inside a block (which is a statement) , e.g. { c = a % b; a = b; b = c; } A block is a sequnce of statements (of any kind) started by a left ({) and terminated by a right brace (}).

  9. Blocks while (b != 0) { c = a % b; a = b; b = c; } while (b != 0) c = a % b; a = b; b = c; The while loops above are inequivalent

  10. Blocks while (b != 0) { c = a % b; a = b; b = c; } while (b != 0) c = a % b; a = b; b = c; The while loops above are inequivalent, but the two below are have the same behaviour while (b != 0) c = a % b; a = b; b = c; while (b != 0) c = a % b; a = b; b = c;

  11. Nested Blocks Blocks may be nested in blocks as in int i = 0; { int j = 0; while (j < 10) { System.out.println("i+j is " + (i+j)); j++; i++; } } The localvariablej is created and initialized when entering the inner block but it doesn’t exists in memory when the block is exited.

  12. Empty Statements A semicolon all by itself is an empty statement doing nothing int i = 0; { int j = 0; while (j < 10) { System.out.println("i+j is " + (i+j)); ;;;;;; // before we had j++ } } What is the outcome of running the code above? And what about: int j = 0; while (j < 10);

  13. Boolean Expressions A boolean expression is any expression evaluating to either true or false. E.g. • Literals: true andfalse • Equality operators:a == b and b != 0 • Relational operators: • a < b, 1 > a, 0 <= b, b >= 10 • Logical operators: • negation: !(b == 0) • or: (a < b) || (b > a) • and: (a <= b) && (b > a) The equality and relational operators can be used between any pairs of numeric values.

  14. Precedence and Associativity The precedence between the operators are || < && < equational < relational < ! B || C && D equals B || (C && D) ! B == a < b means (!B)==(a < b) All the operators associates from left to right; except negation (!). a != b == B means (a != b) == B ! ! B means !(!B)

  15. Short-Curciut Evaluation && and || uses short-curciut evaluation, i.e. expressions are evaluated from left to right in so far further evaluation is needed. In boolean B =(i != 0) && (j/i > 1); (j/i > 1) is evaluated only in case (i != 0) is evaluated to true, and in B = B || !B !B is only evaluated if B is false.

  16. Conditional Statements We shall consider three conditional statements • if statements • if-else statements • switch statements

  17. if statements The simplest conditional statement is the if-statement if (BooleanExpr) Statement BooleanExpr true Statement false

  18. if statement examples if (b > a) a = b; System.out.println("The maximum of a and b is: ” + a); if (a > b) { int tmp = a; a = b; b = tmp; } System.out.print(”The sorted order of a and b is: ”); System.out.println(a + ”, ” + b);

  19. if-else statements The if-else-statement has the form if (BooleanExpr) Statement1 else Statement2 BooleanExpr true false Statement1 Statement2

  20. if-else statement example int max; if (a > b) max = a; else max = b; // if !(a > b), i.e. b >= a System.out.println("The maximum of a and b is:" + max);

  21. if-else statements Either branch of an if-else statement may contain a block System.out.print(”The sorted order of a, b, and c is: ”); if (a >= b) { if (b >= c) System.out.println(a + ", " + b + ", " + c); } else { if (a >= c) { if (c >= b) System.out.println(a + ", " + c + ", " + b); } } …..

  22. if-else statements Since if-else statements are genuine statements in this case we may skip the block structure getting: System.out.print(”The sorted order of a, b, and c is: ”); if (a >= b) if (b >= c) System.out.println(a + ", " + b + ", " + c); else if (a >= c) if (c >= b) System.out.println(a + ", " + c + ", " + b); …..

  23. if-else statements Since the true branch of the if statements contains only an if statement (and no else statement) it’s more elegant to combine the expressions with &&. System.out.print(”The sorted order of a, b, and c is: ”); if (a >= b && b >= c) System.out.println(a + ", " + b + ", " + c); else if (a >= c && c >= b) System.out.println(a + ", " + c + ", " + b); else if (b >= a && a >= c) System.out.println(b + ", " + a + ", " + c); else if (b >= c && c >= a) System.out.println(b + ", " + c + ", " + a); else if (c >= a && a >= b) System.out.println(c + ", " + a + ", " + b); else System.out.println(c + ", " + b + ", " + a);

  24. if-else statements But this code is more efficient System.out.print(”The sorted order of a, b, and c is: ”); if (a > b) if (a > c) if (b > c) // a > b > c System.out.println(a + ", " + b + ", " + c); else // a > c >= b System.out.println(a + ", " + c + ", " + b); else // c >= a > b System.out.println(c + ", " + a + ", " + b); else // b >= a if (a > c) // b >= a > c System.out.println(b + ", " + a + ", " + c); else // b >= a && c >= a if (b > c) // b > c >= a System.out.println(b + ", " + c + ", " + a); else // c >= b >= a System.out.println(c + ", " + b + ", " + a);

  25. The Dangling else • An else is always matched with the nears preceeding if that doesn’t have an else, hence the indentation below is misleading • if (BooleanExpr1) • if (BooleanExpr2) • Statement1 • else Statement2 • it should have been • if (BooleanExpr1) • if (BooleanExpr2) • Statement1 • else Statement2

  26. switch Statements Suppose we want to print the day of the week assuming dayOfWeek holds values from 1 to 7. if (dayOfWeek == 1) nameOfDay = "Sunday"; if (dayOfWeek == 2) nameOfDay = "Monday"; if (dayOfWeek == 3) nameOfDay = "Tuesday"; if (dayOfWeek == 4) nameOfDay = "Weednsday"; if (dayOfWeek == 5) nameOfDay = "Thursday"; if (dayOfWeek == 6) nameOfDay = "Friday"; if (dayOfWeek == 7) nameOfDay = "Saturday"; System.out.println(”It is ” + nameOfDay); Too many comparisons!

  27. switch Statements The following code is better if (dayOfWeek == 1) nameOfDay = "Sunday"; else if (dayOfWeek == 2) nameOfDay = "Monday"; else if (dayOfWeek == 3) nameOfDay = "Tuesday"; else if (dayOfWeek == 4) nameOfDay = "Weednsday"; else if (dayOfWeek == 5) nameOfDay = "Thursday"; else if (dayOfWeek == 6) nameOfDay = "Friday"; else if (dayOfWeek == 7) nameOfDay = "Saturday"; System.out.println(”It is ” + nameOfDay); But even better (more structured) to use a switch

  28. switch Statements switch (dayOfWeek) { case 1: nameOfDay = "Sunday"; break; case 2: nameOfDay = "Monday"; break; case 3: nameOfDay = "Tuesday"; break; case 4: nameOfDay = "Weednsday"; break; case 5: nameOfDay = "Thursday"; break; case 6: nameOfDay = "Friday"; break; case 7: nameOfDay = "Saturday"; } System.out.println("It is " + nameOfDay); A switch requires that the condition evaluates to an integer type and that the constants after the case labels are unique. A break causes execution to continue immediately after the switch statement Notice that the braces are part of the construct.

  29. switch Statements switch (dayOfWeek) { case 7: case 1: System.out.println("It is weekend"); break; case 2: case 3: case 4: case 5: case 6: System.out.println("It is a week day"); break; default: System.out.println("Not a day number " + dayOfWeek); } If there is no break statement the execution “falls through”. There can only be one default label, it can occur anywhere.

  30. Iterations Iteration is to execute certain parts of the code several times as e.g. in a = Console.in.readInt(); b = Console.in.readInt(); while (b != 0) { c = a % b; a = b; b = c; } How would you ever manage to implement the algorithm above without iterations? We considerthree types of iterations • for statements • while statements • do-while statements

  31. for statements The for statement (for loop) is most often used to perform a definite number of iterations, say int i; double square_root; for (i = 1; i <= 10; i++) { square_root = Math.sqrt(i); System.out.println("the square root of " + i + " is " + square_root); } Using the initialization i = 1 (only once) and once for each interation the conditioni <= 10 and the incrementioni++. Notice the use of semicolons to separate the three.

  32. for statements A for statement may contain a local variable in its initialization for (char c = 1; c <= 100; c++) System.out.println("the code for " + c + " is " + (int)c); that is not visible outside the for loop, it is said to have local scope.

  33. for statements The general form of a for statement is for(forInit; BooleanExpr; UpdateExpr) statement forInit UpdateExpr BooleanExpr true Statement false

  34. while statements i = 1; // initialization of the loop while (i <= 10) { square_root = Math.sqrt(i); System.out.println("the square root of " + i + " is " + square_root); i++; // prepare for next iteration } is equivalent to for (i = 1; i <= 10; i++) { square_root = Math.sqrt(i); System.out.println("the square root of " + i + " is " + square_root); }

  35. while statements The while statement has the following general form while (BooleanExpr) Statement BooleanExpr true false Statement

  36. while statements while (B) S is equivalent to if (B) { S while (B) S } Note that the body of a while loop may not be executed at all.

  37. do-while statements The do-while statement has the following form do Statement while (BooleanExpr); Statement true Statementis always executed at least once. Notice the semicolon BooleanExpr false

  38. Example In [JbD] you may find code similar to this (p.74) double number, runningTotal = 0; int count = 0; System.out.println("Type some numbers, ” + "the last one being 0"); number = Console.in.readDouble(); while (number != 0) { runningTotal = runningTotal + number; count = count + 1; number = Console.in.readDouble(); } System.out.print("The average of the ” + count); System.out.print(" numbers is ” + runningTotal/count); for calculating the average of a seqence of numbers. Let’s redo it using a do-while loop.

  39. Example Pseudo-code (with variables number, runningTotal, and count) • do • Get a number number • Add number to the running total runningTotal • Increment the count of numbers count read so far • while number is not zero • return runningTotal divided by count-1

  40. Example double number, runningTotal = 0; int count = 0; System.out.println("Type some numbers, “ + "the last one being 0"); do { number = Console.in.readDouble(); runningTotal = runningTotal + number; count++; } while (number != 0); System.out.print("The average of the " + (count-1)); System.out.print(" numbers is " + runningTotal/(count-1)); Notice that count is subtracted by one!

  41. Example double number, runningTotal = 0; int count = -1; System.out.println("Type some numbers, “ + "the last one being 0"); do { number = Console.in.readDouble(); runningTotal = runningTotal + number; count++; } while (number != 0); System.out.print("The average of the " + count); System.out.print(" numbers is " + runningTotal/count); What happens if only a 0 is typed?

  42. Termination, sentinels On the previous slide we used a sentinel to terminate a loop System.out.println("Type some numbers, “ + "the last one being 0"); do { number = Console.in.readDouble(); … } while (number != 0); If the user don’t input 0 the loop will not terminate.

  43. Termination, no counter incrementation i = 1; while (i <= 10) { square_root = Math.sqrt(i); System.out.println("the square root of " + i + " is " + square_root); //i++; // prepare for next iteration } If i is not incremented the loop will not terminate. What will make the equivalent code on slide 31 not terminate? It’s the programmers responsability to make sure the program terminates

  44. Termination, missing check // print multiples of 13 between 1 and 100 i = 13; while (i != 100) { System.out.println(i); i += 13; } Testing for <=or >=is usually better than testing for equality or inequality.

  45. Termination cannot be guaranteed System.out.println("Type a natural number: "); int n = Console.in.readInt(); while (n > 1) { Console.in.readLine(); // allows to step through if ( n%2 == 0) n = n/2; else n = 3*n + 1; System.out.println("The current value of n is: " + n ); } System.out.println("The final value of n is: " + n ); It’s unknown if the code terminates for any value of n! [Collatz]

  46. break and continue do { System.out.print("Enter a positive integer or 0 to exit: "); n = Console.in.readInt(); if (n == 0) break; // exit loop if (n < 0) { System.out.println("Wrong value " + n); continue; // stop current iteration } System.out.println("squareroot of " + n + " = " + Math.sqrt(n)); //continue lands here } while (true); //seemingly an infinite loop //break lands here Many believe that break and continue should be avoided, it’s not good programming style.

  47. Without break and continue do { System.out.print("Enter a positive integer or 0 to exit: "); n = Console.in.readInt(); if (n < 0) // replaces continue System.out.println("Wrong value " + n); else if (n > 0) System.out.println("squareroot of " + n + " = " + Math.sqrt(n)); } while (n != 0);// replaces break Many uses of break and continue can be eliminated using other control structs

  48. Mandatory Assignment Today the first mandatory assignment is posed. Before doing the assignment • Make groups! • Make sure you can run Java programs at ITU Before handing in the solution on February 20 • Make code public at your homepage and write link on the cover sheet of the pile of answer handed in • Only hand in code you’ve tried to compile

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