Procedural Programming

Transcription

1 Procedural Programming Lecture 2 - Summer Semester 2016 Control Structures Flow Charts Faculty of Engineering Institute of Computer Engineering

2 What we know so far... Programming is only one step in program development process We use computers to solve problems Therefore, we reduce complex problems into more simple parts applying problem-solving strategies Abstraction cascade: from system to model By algorithms we structure a solution approach into a sequence of instructions to follow for solving a problem Flow of changes to model state Programs are translations of algorithms that we have written in a programming language and which can be understood and executed by computers Instructions (the program) operate on data, which represents the model. Both are stored in the main memory of a computer. 2

3 How to cross the river? A farmer with a fox, a goose, and a sack of corn needs to cross the river. The boat can carry the farmer and only one item at a time. The fox cannot beleftonthesameshoreasthegoose,andthegoosecannotbelefton thesameshoreh as thesack of corn without t the farmer watching thi them. 3

4 How to cross the river? A farmer with a fox, a goose, and a sack of corn needs to cross the river. The boat can carry the farmer and only one item at a time.thefox cannot be leftonthesameshoreasthegoose,andthegoosecannotbeleftonthe same shore as the sack of corn without the farmer watching them. 4

5 Perform according to Program Development Process Buildingupa up self contained model of the problem situation: Restate the problem by collecting allfacts Reduce the problem to relevant parts 5

6 How to cross the river? Restate the problem A farmer with a fox, a goose, and a sack of corn needs to cross the river. The boat can carry the farmer and only one item at a time.thefox cannot be leftonthesameshoreasthegoose,andthegoosecannotbeleftonthe same shore as the sack of corn without the farmer watching them. 6

7 Collect all facts: Overview of model elements (variables) 2 shores 3 items: Fox Goose Sack of corn 1 farmer in a boat 7

8 Collect all facts: Relational constraints on model elements and their values (conditions) One shore is near, one shore is far. Any item and the boat can be only at one shore at a time. The farmer can carry one or none item in the boat. In order to load an item, it must be at the same shore as the boat and the boat must be empty. Goose and fox cannot be at the same shore alone (viz. without t farmer). Goose and sack of corn cannot be at the same shore alone (viz. without farmer). The boat can cross the river from near to far shore or vice versa. In the beginning the boat and all items are at the near shore (INIT STATE). In the end the boat and all items are at the far shore (GOAL STATE). 8

9 Collect all facts: Overview of methods to change model state The boat can cross the river. Items can be loaded and unloaded to the boat. The farmer can judge the criticality of a situtation at a shore. 9

10 Reduce the problem to relevant parts in your model Attribute / Elements: Boat Fox Goose Corn Values: Near shore Far shore Methods: Cross the river Load Unload Safe shore? 10

11 1.2 Solution Design

12 Perform according to Program Development Process Building upa self contained model of the problem situation: Restate the problem by collecting allfacts Reduce the problem to relevant parts Transferring modelof of problem situation into a desired state systematically: Create a step wise change of the model written as algorithm Document solution approach in a commonly accepted way 12

13 Decision making and program flow (1/2) The computer executes a sequences of instructions (program) to change data in its main memory (model state). t Change of data is done by assignment. Values can be the results of e.g. arithmetic operations (value operator value), use of constants and value copies from other addresses. Assignment or assignment statement sets the value (of a variable) stored in a storage cell denoted by an address. Address Content of storage cell Comments adr := con 1 Assignmentof cell Goose Goose with value 1 (here: 1 represents far shore) Storage cell for variable ibl Goose 13

14 Decision making and program flow (2/2) Sequences in algorithm or respective program execution can be interrupted by decisions as a result of checking conditions given in the model state. Depending on the outcome of a decision (usually a logical expression, which is evaluated) alternative ways of continuing model state changes are executed. Programs leave sequences based on (un)conditional jumps. Four basic ways of structuring statements (instructions) are considered: Sequence Decision Repetition Procedure Control Structures are ways of structuring statements controlling the order in which the computer executes statements. 14

15 Program control structures (1/4) Sequence a series of statements that are executed one after another. 15

16 Program control structures (2/4) Decision or selection (branch) is a conditional control structure, which executes different statements and thus yields alternative program flows depending on logical evaluation of certain conditions. 16

17 Program control structures (3/4) Repetition or iteration (loop) is a repetitive control structure, which repeats statements while certain conditions are met. 17

18 Program control structures (4/4) Procedure or subprogram (method) allows to structure the program by breaking it into smaller pieces and assemble them into the final form. 18

19 Flowchart A diagram that represents an algorithm Illustrates a step-wise solution to a given problem Arrows showing flow of control Notation Description Activity: a processing step, which can consist of several statements. Decision: where based on evaluating a condition (commonly a YES NO question) the algorithm continues in executing the respective branch. Input/Output: Information is received or send to user. Start/End State: Total of values the algorithm starts or ends with. 19

20 Flowchart for calculating the absolute value Calculating the absolute value (or modulus) n of a real number n is the non-negative value of n without regard to its sign. The absolute value of a number may be thought of as its distance from zero. 20

21 In the beginning was the GOTO (1/3) -- Example: calculating absolute values in 3AC Address Content of storage cell Comments adr := con 13 Begin of program adr := con if val > 0 then GOTO adr Conditional jump (based on decision) Adr := val * val Simply taking the next instruction the PC points to is one example for an unconditional jump next instruction End of program Storage cell for variable n Storage cell for variable factor 21

22 In the beginning was the GOTO (2/3) -- How to make use of identifiers An identifier (id) is a Address Content of storage cell label. In our 3AC example it exchanges the absolute address of a storage cell by a name. Its comparable to an URL of a website. Identifiers do not change the program but its readability adr := con adr factor := con if val > 0 then GOTO adr Adr := val * val next instruction factor 1 factor 22

23 In the beginning was the GOTO (3/3) Address Content of storage cell Example for n= 13 START adr number := con adr factor := con if val number > 0 then GOTO adr END adr number := val number * val factor END... next instruction... number factor 1 23

24 Perform according to Program Development Process Building upa self contained model of the problem situation: Restate the problem by collecting allfacts Reduce the problem to relevant parts Transferring modelof of problem situation into a desired state systematically: Create a step wise change of the model written as algorithm Document solution approach in a commonly accepted way 24

25 Step-by-Step Solution Design Problem-Solving Technique used here: Reduction Approach we use is comparable to Means-Ends Analysis (MEA) in which the solution of a problem can be described by finding a sequence of actions that lead to a desirable goal Reaching the goal depends on building up associations between particular model state changes and methods that will bring these changes about Represented by flowchart 25

26 Overview of relevant parts for: How to cross the river? Attribute / Elements: Boat Fox Goose Corn Values: Near shore Far shore Methods: Cross the river Load Unload Safe shore? 26

27 Creating a flowchart for: How to cross the river? START INIT STATE END GOAL STATE 27

28 Creating a flowchart for: How to cross the river (1/5) START CROSS THE RIVER CURRENT STATE END We cannot END here, all conditions are violated and nothing has been carried over. 28

29 Creating a flowchart for : How to cross the river? (2/5) START LOAD ITEM CROSS THE RIVER UNLOAD ITEM END CURRENT STATE We cannot END here, since the farmer has to judge which item is safe to carry over. 29

30 Flowchart: How to cross the river? (3/5) START LOAD ITEM CROSS THE RIVER YES SAFE SHORE? NO CURRENT STATE UNLOAD ITEM END We cannot END here, because not all items are carried over to the far shore. 30

31 Creating a flowchart for: How to cross the river? (3/5) START LOAD ITEM SAFE CURRENT STATE YES? NO CROSS GOAL THE RIVER YES STATE? NO UNLOAD ITEM END We cannot do a simple repetition, otherwise we would undo our advancement towards a solution. 31

32 Final flowchart for: How to cross the river? (5/5) YES START LOAD ITEM SAFE? NO Trick: In case of a safe far shore situation return with an empty boat (no items loaded) to near shore, otherwise loadcritical item and return to near shore. CROSS THE RIVER YES SAFE? NO YES UNLOAD ITEM GOAL STATE? END NO CROSS THE RIVER LOAD ITEM CROSS THE RIVER UNLOAD ITEM 32

33 Flowchart and Pseudocode START LOAD ITEM Pseudocode is a kind of structured English for describing algorithms. YES YES SAFE? CROSS THE RIVER UNLOAD ITEM GOAL STATE? END NO NO YES CROSS THE RIVER SAFE? NO LOAD ITEM CROSS THE RIVER UNLOAD ITEM It allows to focus on the logic of the algorithm, which describes the sequence of instructions to follow represented by control structures such as: Sequence Decision IF THEN ELSE ELSE Repetition WHILE DO, REPEAT UNTIL Procedure 33

34 Pseudocode for flowchart: How to cross the river? PROCEDURE RowBoatFromNearToFarShore LoadItem PROCEDURE RowBoatFromFarToNearShore REPEAT IF Safe LoadItem THEN CrossTheRiver UNTIL Safe ELSE CrossTheRiver LoadItem UnloadItem CrossTheRiver ENDPROCEDURE UnloadItem ENDIF ENDPROCEDURE START CALL RowBoatFromNeartoFarShore WHILE NotGoalState DO CALL RowBoatFromFarToNearShore CALL RowBoatFromNeartoFarShore ENDWHILE END 34

35 END of 2nd Lecture