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STRINGSSTRINGSString data type

Basic operations on strings

String functions

String procedures

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Definition of Strings

• character data– e.g., ‘A’, ‘B’, ‘C’, ‘x’, ‘y’, ‘z’, ‘!’, ‘*’, etc.

• sequences of characters– e.g., ‘computer programming’

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String Data Type

• string constant– a sequence of characters enclosed in single

quotation marks - ‘ ’– quotation marks themselves are not– e.g., ‘John’ ‘s computer’

• string variable– a predefined type identifier in Pascal– e.g., var Student : string[30]

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Basic Operations on Strings

• assignment

• input/output

• accessing individual characters of a string

• comparison

• initializing a string variable

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1. Assignment

• assigned value length < declared length– no problem

• assigned value length > declared length– the value is truncated to the declared length– only the leftmost characters are retained

• Exercise - what is the output of:– writeln(Name, class);

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2. Input/output

• Input– use readln statement– input string value : no need to enclosed in ‘ ’

except this case, ‘bbbABC Company’

• Output– use write, or writeln statements

leading spaces

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3. Accessing individual characters

• a string can be treated as a special array of characters

• example:var Name : string;

..

Name := ‘Jacky’;

writeln(Name[2]);

‘J’

‘a’

‘c’

‘k’

‘y’

Name[1]

Name[2]

Name[3]

Name[4]

Name[5]

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4. Comparison

• strings are compared with <, >, =, <=, >= and <>

• two characters are compared with one another according to their ASCII value

• two strings are compared with one another character by character

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5. Initializing a string variable

• an uninitialized string may contain

unpredictable value

• initialize a string variable with a

null string:

Sentence := ‘’no space in between

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String Functions

• length

• concat

• copy

• ord

• chr

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lengthlength

•length( <string> )

– returns the length of a <string>

– i.e., the number of characters in the string

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concatconcat

• concatenation

– the process of joining strings together to form longer string

• 2 methods:

– concat( <string1>, <string2>, ..., <stringN> )

– using concatenation operator, +

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copycopy

• copy( <string>, <start>, <size> )

– returns a substring with the special

<size> from the <string>, beginning at

position <start>

– used to cut off part of a string

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ord & chrord & chr

• ord( <character> )

– returns the ASCII value of a < character>

• chr( <code> )

– returns the ASCII character associated with that

<code>

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String Procedures

• some useful procedures are pre-declared and can be used by the program writers at any time they want– e.g., read, readln, write and writeln

• those concerning string are:– val– str

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valval

• val( <string>, <num>, <ecode> )– converts <string> to its numeric equivalent

• conversion– successful:

• <num> - numeric value

• <ecode> - the value 0

– failure:• <num> - set to 0

• <ecode> - the position of the 1st invalid character

• declaration

error code

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strstr

• str( <num>, <string> )– converts a numeric value, <num>, to its string

representation – assigns this string value to <string>

• field descriptors can be included to specify the string format– str( <num>:<w>:<d>, <string> )

widthno. of decimal place

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The End