Manipulating Text

In today’s lesson we will look at:

• why we might want to pick out parts of text strings

• some BASIC functions that can be used to chop up text

• There might be occasions when a particular piece of information can be presented in different ways.

• Think about a person’s name – you might want to display:

– the whole name – e.g. Andrew Virnuls

– just the forename – e.g. Andrew

– initial and surname – e.g. A Virnuls

• However, your user wouldn’t be very happy if you asked them for all the possible versions of their name!

Why Process Text?

• The simplest thing you can do with text strings is join them together – this is called concatenation

• We looked at this in lesson 2 – we can use the + operator to join words, e.g.

forename$ = “Andrew”

surname$ = “Virnuls”

fullname$ = forename$ + “ “ + surname$

Concatenation

• Sometimes you need to calculate the length of a string – e.g. to make sure that it will fit on the screen, or to check that it has the right number of characters (e.g. a bank sort code has 6 digits)

• There is a function called len() that tells you how long a string is, e.g.

input “Please enter your sort code: ”; sortcode$

chars = len(sortcode$)

if chars <> 6 then print “That doesn’t look right!”

String Length

• Sometimes you might want to take part of the string from the left hand end.

• You can do this using the left$() function – you give it the string and the number of characters you want, e.g.

input “What is your name? ”; forename$

initial$ = left$(forename$, 1)

print “Your first initial is ”; initial$

Characters from the Start

• There is a corresponding function called right$(), which gives you characters from the end of the string.

• For example...

input “What is your name? ”; forename$

last$ = right$(forename$, 1)

print “Your name ends with ”; last$

Characters from the End

• Should you want to take some characters from the middle of a string, there is a function called mid$()

• You tell it the string, where you want to start, and the number of characters, for example...

input “What is your name? ”; fore$

middle$ = mid$(fore$, 2, len(fore$)-2)

print “The middle of your name is ”; middle$

Characters from the Middle

• If you want to know if your string contains a particular character, you can use instr()

• You give it a string and a character, and it returns the position of the first occurrence of the character within the string, for example:

a$ = "hello world"

print instr(a$, " ")

• If the character doesn’t appear in the string, the output of instr() is 0.

Finding a Character

• What code could you use to separate a full name into a forename and a surname?

input "What is your full name? "; fullname$

space = instr(fullname$, " ")

forename$ = left$(fullname$, space)

surname$ = right$(fullname$, len(fullname$)-space)

print "Your forename is "; forename$

print "Your surname is "; surname$

• Using right$() can sometimes be a bit tricky because you are counting characters backwards from the end of the string.

Putting It All Together