References. Iteration For. Iteration (Repetition) Iteration While. For loop examples

Transcription

1 References ENGG1811 Computing for Engineers Week 7 Iteration; Sequential Algorithms; Strings and Built-in Functions Chapra (Part 2 of ENGG1811 Text) Topic 19 (chapter 12) Loops Topic 17 (section 10.1) Strings Topic 16 (section 9.2) Built-in Functions Assignment requirements will be explained at the 10am Wed lecture (see second-last slide). ENGG1811 UNSW, CRICOS Provider No: 00098G1 W7 slide 1 ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 2 Iteration (Repetition) Very often need to execute repeatedly Loops are that can do this Process is called iteration Kinds of loop: For (iterate a fixed number of times) While (iterate as long as something is True) Do-Loop (general iteration) Iteration For For statement sets an integer loop variable to each value in a range and executes forming the loop body for each value: optional For intvar = start To finish Step amount Next intvar Loop body start and finish are expressions, evaluated once Step amount is optional (default 1) may use (but not change) the value of the loop variable loop must terminate (VBA checks consistency of start, finish and amount) ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 3 ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 4 For loop examples ' 1. factorial (assume n is set beforehand) fact = 1 For i = 1 To n fact = fact * i Next i ' fact = n! ' 2. sum = sum of the first n squares (reverse order!) sum = 0 For k = n To 1 Step -1 sum = sum + k*k Next k ' 3. Compound interest without exponentiation ' (principal P, interest rate r (%), n years) balance = P For y = 1 To n balance = balance * (1.0 + r*0.01) Next y While statement continues to execute as long as a Boolean expression is True While boolean-expression Loop guard Loop body * Loop guard is evaluated If it is True execute the loop body and go back to start of loop to re-test the guard Otherwise (i.e., it is False) exit loop and continue with rest of program Loop body must change state so that loop guard can eventually become False (else infinite loop) * Why not End While? Beats me. Iteration While ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 5 ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 6 1

2 Loop body must change state so that loop guard can eventually become False (else we have produced an infinite loop) Iteration termination MC Escher (1948) Handteckning [Drawing Hands]; lithograph While loop examples ' 1. largest power of 2 less than some limit (lim) pow2 = 1 While 2*pow2 <= lim pow2 = 2*pow2 ' 2. approximation to square root: k for any k >= 0 ' Algorithm: Newton-Raphson. For theory see Wikipedia or ' x 1 = x 0 f (x 0 ) / f '(x 0 ), where f (x) = x 2 k and thus f '(x) = 2x Dim x As Double ' current estimate Dim lastx As Double ' previous estimate x = k: lastx = 0 ' guesses While Abs(x lastx) > EPSILON lastx = x x = x - (x ^ 2 k) / (2 * x) ' x k Constant: tiny value representing acceptable error ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 7 ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 8 Pseudocode and refinement Pseudocode and refinement How did we come up with the square root solution? Initial version is statement of purpose set x to within EPSILON of k using Newton-Raphson algorithm Express algorithm using English and VB control structures (If, While etc): pseudocode Indent subsidiary Refine to subprograms and VB set initial guess and last guess to suitable starting values While current guess and last guess are too far apart recalculate guess using N-R formula Refined pseudocode: remember to update the variable holding the last guess before updating the current guess; Also choose a safe value for the initial guesses set current guess (x) to k [or 1, k/100, any > EPSILON] set last guess (lastx) to 0 While the abs val of the difference is > EPSILON update lastx update x using the formula (when loop exits, x is an excellent approximation to k) Demo: use this loop in a subprogram to show convergence ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 9 ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 10 Other kinds of loop Do-Loop statement is similar to While, but the loop body is always executed at least once: Do Loop body Loop While boolean-expression Loop guard Alternative form uses complemented guard: Do Loop Until boolean-expression Can terminate the loop early with the statement Exit Do (also Exit While and Exit For) Excel object model (simplified) VBA programs can interact with the data on a worksheet, create new sheets etc. Workbook, sheets, cells form a hierarchy of objects, manipulated like ordinary variables (can use and/or change) Simplest reference to cells is via top-level Application object: Application.ActiveCell Application.ActiveSheet.Cells(row,col) any integer expression for row and col row counts from 1 to 65535* col counts from 1 (= A) to 255* Can omit Application. Prefix * Excel accepts a wider range than this ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 11 ENGG1811 UNSW, CRICOS Provider No: 00098G W5 slide 12 2

3 Excel object model, cont. Each cell has a large number of attributes or properties, recording its contents and appearance. Most can be changed. Most important properties from our viewpoint are Value and Formula, the two attributes easily changed by the user Value is a variant (untyped value) Formula is a string (as it would appear in the formula bar) both can be assigned Worksheet Programming We can use values on the active worksheet as inputs to a program, and overwrite cells to show output and calculated values We can iterate over rows or columns of data We can detect empty cells to stop the iteration Example: running sum to the right of a column (defective subprogram over, to be corrected in the lecture) Developer Macros AddRunningSum ENGG1811 UNSW, CRICOS Provider No: 00098G W5 slide 13 ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 14 Example: Running sum ' WARNING: contains a deliberate error Sub AddRunningSum() Dim row As Integer ' index always integral Dim col As Integer Dim sum As Double ' numeric values normally real row = 2: col = 1: sum = 0 ' FYI, the : symbol separates multiple simple While ActiveSheet.Cells(row, col).value <> "" sum = sum + ActiveSheet.Cells(row, col).value ActiveSheet.Cells(row, col+1).value = sum ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 15 Debugging Loops If a program fails to terminate, press Ctrl-Alt-Break (Break may be labelled Pause), or Esc in the labs VBA interpreter pauses, shows program state: current execution point Hover mouse over variable to show current value Is there a general principle you can apply to all loops? ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 16 Procedures Functions and Subprograms are together called procedures Functions return a single value, should not change program state Subprograms can change state by altering variables outside the procedure altering files outside the application communicating with other applications changing the values of arguments passed by reference (next slide) Function argument list enclosed in parentheses, subprogram arg list isn t unless you use the Call statement (more VB inconsistency ) Parameters revisited Values passed to procedures are called arguments Variables defined to hold these values are called parameters Parameters are local variables, like ones declared using Dim Data can be passed in two different ways: by value (argument is expression, only value transferred) by reference (argument is variable, parameter acts as a temporary alias) keywords ByVal and ByRef specify which (ByRef is assumed if omitted) ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 17 ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 18 3

4 Reference Parameters Sub halve1(byval x As Double) x = x / 2 Sub halve2(byref x As Double) x = x / 2 y = 6.4: z = 6.4 halve1 y ' what is the value of y? halve2 z ' what is the value of z? The keyword ByRef (or none) passes a reference to the argument rather than its value. x becomes an alias for z and changes to x also affect z immediately. With ByVal, or if the argument is not a variable, x is an independent local variable initialised by the argument (6.4). Changing it has no effect outside the procedure. Example: Swapping values Sub Swap(ByRef x1 As Variant, ByRef x2 As Variant) Dim tmp As Variant tmp = x1 x1 = x2 x2 = tmp y = -23.5: z = 6 Swap y, z ' what are the values of y and z? We used the Variant type so the procedure can be used to exchange the contents of variables any type. This is the only recommended use for Variants. Unfortunately, the following does NOT work Swap ActiveSheet.Cells(1,1), ActiveCell Motivation: imagine exchanging the positions of an apple and an orange on the table in front of you. Now do it again using just one hand! ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 19 ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 20 Named Arguments Sequential Algorithms For procedures with long argument lists (especially built-in procedures), arguments can be specified in any order using the notation paramname:=argument Example (see MsgBox Function in VBA Help) reply = MsgBox(title:="Bummer", _ returns integer value representing button pressed: vbcancel or vbretry prompt:="no solution to your equation", _ buttons:=vbcritical + vbcancelretry) parameter name underscore allows line break within statement built-in VBA constants assist readability VBA is particularly useful for tasks such as Processing rows or columns on one pass Locating cells with particular characteristics Identifying extreme values (max/min) These are called sequential tasks, because they treat the data as a sequence of items, usually stored in cells Standard sequential algorithms can be adapted to solve these kinds of problems ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 21 ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 22 Simple Scans The simplest cases just process each element of a sequence in order (for convenience) Elements are independent End of sequence normally marked by empty cell Pseudocode: column scan set col to the required column number set row to the row where the sequence starts While ActiveSheet.Cells(row, col) is not empty apply required step to this cell move row one cell down State-based Scans Other cases accumulate history in variables during the scan Element order may or may not matter Variables need to be correctly initialised Examples: AddRunningSum, maximum or minimum calculation, run of equal values, merging ordered sequences, mode of an ordered sequence (you are not likely to need the last few). ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 23 ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 24 4

5 Maximum/minimum Max/min calculations maintain a best so far variable May also need to know the position of the (first or last occurrence of) the maximum Pseudocode for maximum of a column: set col to the required column number set row to the row where the sequence starts set max to the value in that cell While ActiveSheet.Cells(row,col) is not empty If cell value is greater than max Then set max to cell value End If move row one cell down Maximum/minimum, continued If the problem is to highlight all extreme (max or min) values, need two passes First pass finds the extreme value (prev slide) Second pass processes each cell whose value matches the extreme value Next week we will see how cell appearance can be altered (more general than conditional formatting) Macro Recorder used to capture relevant code ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 25 ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 26 Maximum/minimum, continued If the problem is to find min/max for only some cells, can t initialise from the first cell as usual use a Boolean variable to record whether a min/max has been found after the loop the variable tells you whether there was a min/max at all If you need to find out more than one piece of info about a list of values, consider whether you can perform multiple actions on a single pass example: min + max, or min/max + count etc Example: min of positive values only Problem: Find where the smallest positive value (if any) is in the column containing the active cell Pseudocode, first attempt set col to the column containing the active cell set row to the starting row set rowmin to zero ' = undefined minimum position While the cell at (row, col) contains a number If the cell value is positive Then If rowmin = 0 Or cell value < value at (rowmin, col) Then set rowmin to row ' now valid End If End If move row one cell down (Solution developed in lecture) ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 27 ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 28 Strings Literal Strings A string is a sequence of characters Strings in VBA can be of virtually any length (including zero, of course) Components are characters represented internally as codes with numeric value from 0 to 255 each code is interpreted by an output device as a graphic character or special action functions Chr and Asc convert between character and numeric code String constants are enclosed in double-quote characters " " (not single quotes) How to include a double-quote as a member of the string (the delimiter-as-data problem)? Write it twice, as in "Reference: ""Excel for Experts""" Empty string is denoted by "" ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 29 ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 30 5

6 String Operators & concatenates (joins) two strings "c" & "at" (= "cat") "cat" & "ch" & "-22" relational (comparison) operators compare individual characters, so "cat" < "dog" (because "c" < "d") "cat" < "cave" (because "t" < "v") "cat" < "catastrophe" (prefix) "Cat" < "cat" (case sensitive) can change to case-insensitive comparison (to make "Cat" = "cat") by placing Option Compare Text at the top of the module String Matching The Like operator allows a string expression to be tested against a pattern that includes wildcards strdata Like "pattern" The pattern consists of? any single character # any digit character * zero or more characters [list] any character in list [!list] any character not in list Important notation! Same notation as Access query partial match ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 31 ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 32 String Matching Examples Assume the following values str1 = "cat": str2 = "Route66": str3 = "2*3" Expression Value Reason str1 Like "cat" True ordinary chars OK str1 Like "c?t" True? matches the character a str1 Like "c" False must match whole of string str2 Like "*##" True ends in two digits str2 Like "[A-Z]*" True [A-Z] matches any capital str2 Like "*[!6]" False can't match non-6 at end str2 Like "*[!6]*" True (you work it out!) str3 Like "*3" True * matches 2* str3 Like "#[*]?" True to match special chars * [? literally, enclose in [ ] ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 33 Select Case Multiway selection with a common expression: VBA has a simpler way than many If any number of each of these can occur Select Case expression Case value Case value 1, value 2 Case value 1 To value 2 Case Is > value Case Like pattern Case Else End Select evaluated first, then matched against each label single value multiple values grouped inclusive range of values open range of values values matching pattern all unmatched values ENGG1811 UNSW, CRICOS Provider No: 00098G W5 slide 34 Select Case example Classify a single-character string c Select Case UCase(c) Case "A","E","I","O","U" sym = "vowel" Case "0" To "9" sym = "digit" Case "A" To "Z" sym = "consonant" Case "" sym = "empty" Case Like "??*" sym = "more than one char" Case Else sym = "punctuation" End Select use of UCase saves repeating upper and lower case letter rules ' or Case Like "[A-Z]" Built-in Functions VBA includes a range of mathematical, string and conversion functions Accept one argument (sometimes other than one) and return exactly one value Mathematical functions include Sin, Cos, Tan, Atn (arctangent), all in radians Abs, Sgn, Int (largest int ), Fix (truncate towards 0), Round (to any number of decimal places, including neg) Sqr ( ), Exp (e x ), Log (ln x) Rnd (pseudo-random number) Look up Math Functions in VBE's Help ENGG1811 UNSW, CRICOS Provider No: 00098G W5 slide 35 ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 36 6

7 Built-in Functions, cont. Can also use any* function available to worksheet formulas, using Application.WorksheetFunction.name(args ) Examples (Application. can be omitted): angle = WorksheetFunction.Radians(-90) payment = WorksheetFunction.PMT(rate, numperiods, _ curvalue) code = WorksheetFunction.Replace(text, start, 3, _ "***") Some expect Range objects (described in a later lecture) as their data sources Look up Worksheet Functions in VBE's Help * except if there s a direct equivalent in VBA, like Sin Function call Len(str) String and Character Functions Mid(str,start,len) Asc(str) Chr(code) LCase(str) Trim(str) Returns number of characters in str len-char substring of str, start; also Left(str,len) and Right(str,len) char code for first char in str Unit string containing char with code str with all upper-case chars converted to lower case (also UCase) str with spaces removed from both ends (also LTrim, RTrim) InStr(str,match) first position in str where match occurs, or 0 Replace(str,match,r epl) String(str,n) str with all occurrences of match replaced by repl (other args possible) n copies of first char of str ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 37 ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 38 Type Conversion Functions Format Function Can mix up numeric types (angle + intoffset is evaluated as Double) When explicit conversion is needed, use function: Function name Accepts as argument Returns CStr number String (also Format next slide) CLng number or string Long Integer (rounded if from Double) CDbl number or string Double Val string Integer or Double, ignores trailing junk Round(v,d) number v Rounded to d decimal places Hex integer Hexadecimal string form ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 39 Specialised formatting of numbers and dates Format(expr, fmt) returns a formatted string fmt mixes ordinary characters and formatting codes: Predefined code Displays 0 digit or zero # digit or nothing % percentage placeholder ( 100, show %) e+ e- E+ E- scientific single char or space < > force lower-case or upper-case ddd mmm yy hh nn date/time components (see text) Can also use predefined formats such as Scientific, Percent, Long Date etc. See VBE Help for details ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 40 12s2 Assignment 1 (ecas) Purpose: implement algorithms to avoid two aircraft colliding Framework sheet with simulation parameters (can be randomised) and results simulation module, calls your procedures ecas display mimics the real TCAS Tasks: four short functions and three longer subprograms (predict future positions, issue avoidance instructions, analyse incident) Required processes Make lots of objective decisions using Ifs in procedures (one subprog only:) process columns, read from/write to cells by name and by address, using layout Consts What you submit must be your own work, penalties apply Support and tools Test sheet for your four functions Comparison of your position prediction with ours Style assessor Forum (general questions), consultants (specific one-one help) Bonus-point challenges for more capable students Summary Reference parameters allow a subprogram to change the value of variables declared outside Use pseudocode to express algorithm prior to coding in VB Active sheet and its cells can be addressed directly Sequential algorithms process a row or column of data systematically Some algorithms accumulate values, others process cells locally and order-independently Strings store simple character sequences Built-in functions evaluate commonly-required quantities (string and numeric) Relatively complex tasks can usually be subdivided into easily managed subtasks and solutions combined ENGG1811 UNSW, CRICOS Provider No: 00098G W7 slide 42 7