Control structures and logic

Transcription

1 Programming with Java Module 2 Control structures and logic Theoretical part Contents 1 Module overview Commands and blocks Operators (Part II) Relational Operators Logical operators Branches One-sided branch: conditional program execution Two-sided branch Multilevel branches Switch-command Loops for-loops while-loop do-while loop Nested loops

2 Keywords Command block Command head Command body Logical operators Truth value Relational operators Branch for-loop while-loops do-while-schleife Nested loops Authors: Lukas Fässler, Barbara Scheuner, David Sichau Date: 07 August 2017 Version: 1.1 Hash: a9ee87a The authors try the best to provide an error free work, however there still might be some errors. The Authors are thankful for your feedback and suggestions. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. To view a copy of this license, visit 2

3 1 Module overview An algorithm formulated as a program usually consists of several commands. These commands are executed in an order determined by the programmer. This program sequence (sequence of commands) seldom proceeds linearly. Often, a program sequence branches into two or more program sequences whereby each individual sequence is only executed under certain conditions (branching). To simplify an algorithm, certain program sequences are often executed repeatedly (loops). The program process can be influenced by means of control structures which occur in most programing languages. The decision how to control the process has to be formulated in the condition. 1.1 Commands and blocks As already mentioned in module 0, individual commands end with a semicolon. Several commands can be combined into one command block. Java uses curly brackets { to combine commands into code blocks: { \\opens the block command1; command2;... \\closes the block Control structures (e.g. branches or loops) can control the execution of the code blocks. These control structures consist of a head and a body. { \\head \\body In the following program, the command block 1 is interrupted by a command block 2: 3

4 \\Begin command block 1 Head 1 { \\Body 1 \\Begin command block 2 Head 2 { \\Body 2 \\End command block 2 \\Continuing block Body 1 \\End command block 1 2 Operators (Part II) The arithmetic operators have already been described in module 1. In connection with control structures, logical and relational operators are employed. 2.1 Relational Operators Relational operators are used to compare values (operands). They provide a logical result True or False. Values that are connected by relational operators are called atomic propositions in the propositional calculus. Relational operators are summarized in table 1. Operator Expression Description provides True if... > a > b greater than a is greater than b. < a < b smaller than a is smaller than b. == a == b equal a and b have the same value.!= a!= b unequal a and b have unequal values. >= a >= b greater or equal a is greater than or equal to b. <= a <= b smaller or equal a is smaller than or equal to b. Table 1: Relational Operators in Java. 4

5 2.2 Logical operators Logical operators concatenate atomic propositions. Thereby, truth values are compared. The result is also a truth value, i.e. True or False. Since these are the operands of the Boolean propositional logic, their data type is Boolean. The logical operators used in Java are presented in table (table 2). Operator Expression provides True if...!!a a is false (NOT). && a && b a and b are true (AND). If a is false, b is not evaluated. a b either a or b is true (OR). If a is true, b is not evaluated. ˆ aˆb a and b have different logical values (EXLUSIVE OR) Table 2: logical operators in Java. 3 Branches Branches verify a state of the program. Depending on whether a condition is applied or not, the program continues with different blocks of commands. In Java, as in many other programming languages, branches are initiated by the keyword if. The meaning of if is the same as in the natural language. If it is raining, then I will take the bus, otherwise I will walk. This could be written in Java as follows: if (rain) {bus else {walk; If the condition rain is True, the block containing the command bus is executed, otherwise the block with the command walk is executed. Generally, it can be decided at runtime whether a command or command block should be executed or not by means of an if-command. To formulate conditions, Boolean variables, relations such as equality, greater or smaller as well as logical operators can be used. Depending on the number of cases to be distinguished, a one-sided (3.1), twosided (3.2) or multilevel branch (3.3) needs to be chosen. 5

6 3.1 One-sided branch: conditional program execution A one-sided branch consists of a condition check and a command block which is executed or not Notation: if (Condition) { Command block; Example: if (rain == true) { System.out.println("It is raining."); The sentence "It is raining." is only output if the variable rain has the value True. 3.2 Two-sided branch In a two-sided branch, we can also specify what should be executed in the other case (else), i.e. if the condition does not apply. Notation: if (Condition) { Command block1; else { Command block2; Beispiel: if (rain == true) { System.out.println("It is raining."); else { System.out.println("It is not raining."); 6

7 The sentence "It is raining." is output if the variable rain has the value True and the sentence "It is not raining." is output in the other case (False). 3.3 Multilevel branches By using a multilevel branch, several comparisons can be performed. This can be necessary for checking different possibilities in a certain order. Notation: if (Condition1) { Command block1; else if (Condition2) { Command block2; else if (Condition3) { Command block3;... Example: if (rain == true) { System.out.println("It is raining."); else if (snow == true) { System.out.println("It is snowing."); else if (sun == true) { System.out.println("The sun is shining."); else { System.out.println("The weather conditions are not clear."); The sentence "It is raining." is output if the variable rain has the value True. If it has the value False, the variable snow is checked in a next step. If the variable snow has the value True, the sentence "It is snowing." is output. If it has the value False, the variable sun is checked in a next step. If the variable sun has the value True, the sentence "The sun is shining." is output. If it has the value False, the sentence "The weather conditions are not clear." is output. 7

8 3.4 Switch-command Another way to differentiate between different possibilities is the switch-command. Thereby the value of a variable is compared with different values. Notation: switch (expression){ case constant: Command block; default: Command block; In contrast to the if-condition, the switch-command can only check for equality. Comparisons with greater of smaller operators are not possible. All whole number data types and since Java 7 also string types are feasible as expression in the switch-command. Additional the switch statement executes all commands from the entry point. If that behavior is not desired the statement block needs to be closed with a break. Example: switch (test) { case 1: System.out.println("I were chosen."); break; case 2: System.out.println("You were chosen."); break; case 3: System.out.println("We were chosen."); break; default: System.out.println("Nobody was chosen."); 4 Loops Commands can be executed repeatedly by means of loops. As in other programming languages, there are different kinds of loops in Java. A loop consists of a 8

9 loop head and a loop body. The loop body consists of the command block to be repeated. The loop head controls the loop. It indicates how often or under which conditions the commands of the loop body should be repeated. 4.1 for-loops In a counter-controlled for-loop, a counter variable is counted in the loop head in a range from a start value to the end value. The counter changes in each loop iteration. Schreibweise: for (init; test; update){ Command block Initialization (init): Declaration of the counter and initialization to the start value. Logic expression (test): after each loop iteration it is checked if the loop should continue or the final value is already reached. Update (update): The counter is changed after each loop iteration. Example: The following statement prints the values from 0 to 4 to the screen. for (int i=0; i<5; i++){ System.out.println(i); In the first step the counter variable i is declared and set to 0. At the begining of the loop it is checked if i is smaller than 5. If that is the case the loop body is executed, otherwise the loop is finished. The variable i is increased by one inside the loop body (i++ is short for i=i+1). 4.2 while-loop It is not always predictable how often a command has to be repeated since the number of repetitions depends on what happens in the loop body. Here, the counter-controlled loops reach their limits. In condition-related loops, the number of repetitions does not depend on a counter but on a condition. This condition is checked in each loop iteration. While- and do-while-loops are distinguished from each other by whether the condition is checked before or after executing the command block. 9

10 Notation: initializing the variables while (Condition){ Command block Update Initialization: declaration of one or several variables and initialization of them. Condition: The condition is checked as soon as the while-loop is reached. If the condition is True, the loop body is executed. If the condition is False, the loop is terminated and the commands are not executed anymore. The condition is rechecked after each iteration. Update: The values within the loop body has to change in such a way that the condition will eventually be satisfied, otherwise we will get an endless loop which contradicts the definition of an algorithm (an algorithm must always terminate its work). Example: The following commands output the values 0 to 4 on the screen: int i=0; while (i<5){ System.out.println(i); i++; First, the variable i is initialized and set to 0. At the beginning of the loop, it is checked whether i is smaller than 5. If this is the case (true), the loop body is executed. If this is not the case (false), the loop is terminated. The variable i is increased by 1 within the loop body each time the loop is run. 4.3 do-while loop The loop body of a do-while loop is executed at least once unlike the while-loop since the condition check that decide whether the loop body is executed again is at the end of the loop head. 10

11 Notation: initializing the variables do { Command block Update while (Condition); Initialization: declaration of one or several variables and initialization of them. Update: Inside the loop body the values must change such that the condition can be fulfilled, otherwise an endless loop would be the result. Condition: The condition is checked after the loop body is executed. It contains the condition for the repetion of the loop. If the condition evaluation returns true the loop body is executed again, otherwise the loop is terminated. Example: The following commands output the values 0 to 4 on the screen: int i=0; do { System.out.println(i); i++; while (i<5); In the first step the variable i is initialized and set to 0. Inside the loop body the variable i is increased by 1. Only after this step it is checked if i is smaller than 5. As soon as the value 5 is reached the loop is terminated. 4.4 Nested loops During programming it happens quiet frequently that two loops should be nested inside each other. The result is that the outer loop controls the inner loop. This can be done as follows: Outer loop { Inner loop { Command block 11

12 An analogy to nested loops can be found in the earth which rotates around the sun. A rotation in one year is comparable with the outer loop and a rotation of the earth around its own axis within one day is comparable with the inner loop. In Java, a program to output the days and hours of a year (not a leap year) can be written with the following nested loop: for (int days=0; days<365; days++){ for (int hours=0; hours<24; hours++) { System.out.println("Day " + days); System.out.println("Hour " + hours); The first three outputs are: Day 0: Hour 0 Day 0: Hour 1 Day 0: Hour 2 The last three outputs are: Day 364: Hour 21 Day 364: Hour 22 Day 364: Hour 23 12