Notes on Algorithms Pseudocode and Flowcharts Introduction Do you like hot sauce Here is an algorithm for how to make a good one Volcanic Hot Sauce from http recipeland com recipe v Volcanic Hot Sauce 1125 10 12 scotch bonnets or Habanero serrano jalapeno 6 cloves Garlic peeled and chopped 1 3 c Fresh lime juice 1 3 c Distilled white vinegar 2 tbl Dijon style mustard 2 tbl Olive oil 1 tsp Molasses 1 2 tsp Turmeric 1 tbl Salt or to taste 1 Combine the pepper garlic lime juice vinegar mustard oil molasses turmeric and salt in a blender and puree until smooth Correct the seasoning adding more salt or molasses to taste 2 Transfer the sauce to a clean bottle You can use it right away but the flavor will improve if you let it age for a few days Volcanic Hot Sauce will keep almost indefinitely refrigerated or at room temperature Just give it a good shake before using As you can see this algorithm is a really a recipe that is a set of step by step instructions that takes raw ingredients and produces a tasty result In general an algorithm can be described as a procedure to solve a problem In the context of computer programming an algorithm is defined as a well ordered collection of unambiguous and effectively computable operations that when executed produces a result and halts in a finite amount of time 1 Characteristics of an Algorithm Well ordered the steps are in a clear order Unambiguous the operations described are understood by a computing agent without further simplification Effectively computable the computing agent can actually carry out the operation Method for Developing an Algorithm 1 Define the problem State the problem you are trying to solve in clear and concise terms 2 List the inputs information needed to solve the problem and the outputs what the algorithm will produce as a result 3 Describe the steps needed to convert or manipulate the inputs to produce the outputs Start at a high level first and keep refining the steps until they are effectively computable operations 4 Test the algorithm choose data sets and verify that your algorithm works 1 definition from An Invitation to Computer Science Gersting Schneider via http www cs xu edu csci170 08f sect01 Overheads WhatIsAnAlgorithm html visited 19JUN2009 BJ Furman ME 30 Computer Applications Algorithms Pseudocode and Flowcharts doc 29DEC2010 Page 1 of 6 Structured Programming In 1966 computer scientists Corrado B hm and Giuseppe Jacopini demonstrated that all programs could be written using three control structures Sequence Selection and Repetition2 The sequence structure is the construct where one statement is executed after another The selection structure is the construct where statements can executed or skipped depending on whether a condition evaluates to TRUE or FALSE There are three selection structures in C 1 IF 2 IF ELSE 3 SWITCH The repetition structure is the construct where statements can be executed repeatedly until a condition evaluates to TRUE or FALSE There are three repetition structures in C 1 WHILE 2 DO WHILE 3 FOR Pseudocode or Program Design Language Consists of natural language like statements that precisely describe the steps of an algorithm or program Statements describe actions3 Focuses on the logic of the algorithm or program Avoids language specific elements Written at a level so that the desired programming code can be generated almost automatically from each statement Steps are numbered Subordinate numbers and or indentation are used for dependent statements in selection and repetition structures4 2 Corrado B and Jacopini G May 1966 Flow Diagrams Turing Machines and Languages with Only Two Formation Rules Communications of the ACM 9 5 366 371 3 Some programmers also include data declarations in their pseudocode I think this is a good idea because it helps you keep track of the variables that you will need in the algorithm and can help you think through what data types are needed 4 Some programmers will add an ending keyword on a separate line to make it explicit where a selection or repetition structure ends for example ENDIF ENDWHILE etc On the one hand this is good because it makes it clear where the selection or repetition block ends but on the other hand it adds to the length of the pseudocode and such statements will not translate into an actual line of code in C In a language like Pascal however they will BJ Furman ME 30 Computer Applications Algorithms Pseudocode and Flowcharts doc 29DEC2010 Page 2 of 6 Pseudocode Language Constructs5 Computation Assignment Compute var1 as the sum of x and y Assign expression to var2 Increment counter1 Input Output Input Get var1 var2 Output Display var1 var2 Selection Single Selection IF 1 IF condition THEN IF condition is true then do subordinate statement 1 etc If condition is false then skip statements 1 1 statement 1 1 2 etc Double Selection IF 2 IF condition THEN IF condition is true then do subordinate statement 1 etc If condition is false then skip statements and execute statements under ELSE 2 1 statement 1 2 2 etc 3 ELSE else if condition is not true then do subordinate statement 2 etc 3 1 statement 2 3 2 statement 3 4 SWITCH expression TO 4 1 case 1 action1 4 2 case 2 action2 4 3 etc 4 4 default actionx Repetition 5 WHILE condition while condition is true then do subordinate statements 5 1 statement 1 5 2 etc DO WHILE structure like WHILE but tests condition at the end of the loop Thus statements in the structure will always be executed at least once 6 DO 6 1 statement 1 6 2 etc 7 WHILE condition 5 See Pseudocode Standard at http users csc calpoly edu jdalbey SWE pdl std html for more examples BJ Furman ME 30 Computer Applications Algorithms Pseudocode and Flowcharts doc 29DEC2010 Page 3 of 6 FOR structure a specialized version of WHILE for repeating execution of statements a specific number of times 8 FOR bounds on repetition 8 1 statement 1 8 2 etc Pseudocode Example Express an algorithm to get two numbers from the user dividend and divisor testing to make sure that the divisor number is not zero and displaying their quotient using pseudocode 1 2 3 4 5 6 7 8 Declare variables dividend divisor quotient Prompt user to enter dividend and divisor Get dividend and divisor IF divisor is equal to zero THEN 4 1 DO 4 1 1 Display error message divisor must be non zero 4 1 2 Prompt user to enter divisor 4 1 3 Get divisor 4 2 WHILE divisor is equal to zero ENDIF Display dividend and divisor Calculate quotient as dividend divisor Display quotient