Chapter 5 The Do Loop Statement

Transcription

1 The Do Loop Statement Loop structure that executes a set of statements as long as a condition is true. The condition is a Boolean expression. Executes at least once. The loop below iterates while sum is less than 10: sum = 0; Do sum += 2 Loop While sum < 10 Slide 1

2 Alternative Do Loop Executes only if the condition is initially true. May not iterate at all. The statement sum = 20 Do While sum < 10 sum += 2 Loop does not iterate at all because sum is initially greater than 10. Slide 2

3 Infinite Loops A loop that continues executing forever Can be caused by syntax or logic errors. For example: num = -1 Do num -= 1 'num is decreased by 1 While num < 0 Some errors result in an overflow causing a run-time error. Slide 3

4 The InputBox Function Displays a predefined dialog box that has a prompt, a text box, and OK and Cancel buttons, and then returns a string. Used to obtain information from the user. The function is used as part of an assignment statement: stringvar = InputBox(prompt, title) Clicking Cancel or leaving the text box blank returns Nothing. The Val() function can be used to convert string data to numeric data. Slide 4

5 Accumulator Variables A variable that is incremented by a varying amount. Often used for keeping a running total. Should be initialized when declared. Slide 5

6 Assignment Operators Operator Operation *= multiplication and then assignment /= division and then assignment \= integer division and then assignment ^= exponentiation and then assignment Slide 6

7 Chapter 6 Using Flags A flag, or sentinel, indicates when a loop should stop iterating. Often a constant. Code is easier to modify when sentinels are constants declared at the beginning of a procedure. Slide 7

8 The For Next Statement Loop structure that executes a set of statements a fixed number of times. Uses a counter to control loop iterations. The keyword Step can optionally be used to change the amount the counter is incremented or decremented. The loop below executes until num is equal to 10: For num As Integer = 0 To 10 i += num Next num Slide 8

9 The String Class Includes properties and methods. A String object is comprised of a sequence of characters with the first character at index position 0. String properties include: Chars(index) Length() Slide 9

10 String Methods String methods for manipulating a string include: ToUpper converts a string to all uppercase ToLower converts a string to all lowercase Trim removes spaces from the beginning and end of a string TrimEnd removes spaces from the end of a string TrimStart removes spaces from the beginning of a string PadLeft(len, char) adds a specified character to the beginning of a string until the string is len characters long PadRight(len, char) adds a specified character to the end of a string until the string is len characters long Slide 10

11 String Methods (cont.) String methods for manipulating a substring: Substring(startPos, numofchars) returns the substring that is numofchars in length and starts at startpos Remove(startPos, numofchars) deletes the substring that is numofchars in length and starts at startpos Replace(oldString, newstring) exchanges every occurrence of oldstring with newstring Insert(startPos, substring) inserts substring at startpos IndexOf(substring) returns the first position of substring Slide 11

12 String Concatenation Concatenation is joining two or more strings together. The String method Concat() joins two or more strings. It is a shared method and must be used with the String class, not an object of the class: s = String.Concat("this","and","that") The &= operator concatenates a string to an existing string: s = "thisand" s &= "that" The & operator concatenates strings: s = "this" & "and" & "that" Slide 12

13 Space(), vbtab, vbcrlf The Space() function returns a string of spaces. vbtab is a built-in constant that represents 8 spaces. vbcrlf is a built-in constant that represents a carriage return-linefeed combination. Slide 13

14 The Char Structure A simple form of a class. Char has two shared methods: ToUpper() ToLower() The methods must be used with the Char structure: newletter = Char.ToUpper(letter1) Slide 14

15 Unicode A digital code with representations for every character in every language and symbol. Two built-in functions for converting between characters and Unicode: AscW(char) ChrW(integer) Slide 15

16 Comparing Strings When relational operators (=, >, <, >=, <=, <>) are used to compare strings, their Unicode values determine the relationship between the strings. The Compare() method is a better choice to alphabetically compare strings: Compare(string1, string2, case-insensitive) returns 0 if string1 and string2 are the same. A positive number is returned if string1 is greater than string2 and a negative number if string1 is less than string2. case-insensitive should be true if the case of the strings should not be considered. Slide 16

17 The Like Operator Used to perform a textual comparison between two strings. Can be used to perform pattern matching. The pattern can include:? used in place of any single character * used in place of many characters # used in place of any single number [] used to enclose a list of characters - used to indicate a range of characters in a list, used to separate characters in a list Slide 17