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1 What makes up an algorithm The sequence (or Compound) instruction - PDF document

Feb 13, 2023 •165 likes •229 views

Not quite an algorithm Chair of Softw are Engineering Einfhrung in die Programmierung Introduction to Programming Prof. Dr. Bertrand Meyer October 2006 February 2007 Lecture 9: Control Structures I I ntro. to Programming, lecture 9:

  1. Not quite an algorithm Chair of Softw are Engineering Einführung in die Programmierung Introduction to Programming Prof. Dr. Bertrand Meyer October 2006 – February 2007 Lecture 9: Control Structures I I ntro. to Programming, lecture 9: Control structures I 4 5 properties of an algorithm Defines data to which process will be applied Every elementary step taken from a set of well-specified actions Describes ordering(s) of execution of these steps 2 and 3 based on precisely defined conventions, suitable for execution by an automatic computer For any data, guaranteed to terminate after finite number of steps I ntro. to Programming, lecture 9: Control structures I 2 I ntro. to Programming, lecture 9: Control structures I 5 The notion of algorithm Algorithm vs program General definition: “Algorithm” usually considered a more abstract notion, independent of platform, programming language etc. An algorithm is the specification of a process to be carried out by In practice, the distinction tends to fade: a computer � Algorithms need a precise notation � Programming languages becoming more abstract However: � In programs, data (objects) are just as important as algorithms � A program typically contains many algorithms and object structures I ntro. to Programming, lecture 9: Control structures I 3 I ntro. to Programming, lecture 9: Control structures I 6 1

  2. What makes up an algorithm The sequence (or Compound) instruction 1 Basic steps: � Feature call x . f ( a ) instruction 2 � Assignment � ... ... instruction n Sequencing of these basic steps: CONTROL STRUCTURES I ntro. to Programming, lecture 9: Control structures I 7 I ntro. to Programming, lecture 9: Control structures I 10 Control structures Semicolon as optional separator Definition: program construct that describes the instruction 1 ; scheduling of basic actions instruction 2 ; Three fundamental control structures: � Sequence ... ; � Loop � Conditional instruction n They are the “Control structures of Structured Programming” I ntro. to Programming, lecture 9: Control structures I 8 I ntro. to Programming, lecture 9: Control structures I 11 Not quite an algorithm Control structures as problem-solving techniques Sequence: “To achieve C from A, first achieve an intermediate goal B from A, then achieve C from B” Loop: solve the problem on successive approximations of its input set Conditional: solve the problem separately on two or more subsets of its input set I ntro. to Programming, lecture 9: Control structures I 9 I ntro. to Programming, lecture 9: Control structures I 12 2

  3. Correctness of a Compound As a function maximum ( a, b : INTEGER ): INTEGER Precondition of instruction 1 must hold initially -- The higher of a and b do Postcondition of each instruction i must imply if precondition of each instruction i+1 a > b then Final effect is postcondition of instruction n Result := a else Result := b end end I ntro. to Programming, lecture 9: Control structures I 13 I ntro. to Programming, lecture 9: Control structures I 16 Conditional instruction The conditional as problem-solving technique if PROBLEM SPACE - - Boolean_expression Condition then Region 2 Instructions - - Compound else Other_instructions Region 1 - - Compound end Use technique 2 Use technique 1 I ntro. to Programming, lecture 9: Control structures I 14 I ntro. to Programming, lecture 9: Control structures I 17 Computing the greater of two numbers Basic form if Condition then Instructions if else a > b Other_instructions then end max := a else max := b end I ntro. to Programming, lecture 9: Control structures I 15 I ntro. to Programming, lecture 9: Control structures I 18 3

  4. A variant of the conditional Nested structure if Condition then Instructions end I ntro. to Programming, lecture 9: Control structures I 19 I ntro. to Programming, lecture 9: Control structures I 22 A variant of the conditional Comb-like structure if Condition then Instructions end (Means the same as if Condition then Instructions else end with an empty “ else ” clause) I ntro. to Programming, lecture 9: Control structures I 20 I ntro. to Programming, lecture 9: Control structures I 23 Nesting Comb-like conditional if Condition 1 then if Condition 1 then Instructions 1 Instructions 1 elseif Condition 2 then else Instructions 2 if Condition 2 then elseif Condition 3 then Instructions 3 Instructions 2 else elseif if Condition 3 then ... Instructions 3 else ... else Instructions 0 end end end end I ntro. to Programming, lecture 9: Control structures I 21 I ntro. to Programming, lecture 9: Control structures I 24 4

  5. Comb-like structure End of lecture 9 I ntro. to Programming, lecture 9: Control structures I 25 I ntro. to Programming, lecture 9: Control structures I 28 What we have seen I ntro. to Programming, lecture 9: Control structures I 26 What we have seen � The notion of algorithm � Basic properties � Difference with “program” � The notion of control structure � Correctness of an instruction � Control structure: sequence � Control structure: conditional � Nesting, and how to avoid it I ntro. to Programming, lecture 9: Control structures I 27 5

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