Different types of string operations are,
A substring is the basic unit of access in a string. Unlike other array elements substrings have their own meaning. To access a substring we need the following informations,
- Name of the string.
- Position of the first character of the substring in the given string.
- length of the substring.
Algorithm SUBSTRING (SUBSTR,STR,INITIAL,LENGTH)
1. STRLEN=LENGTH(STR) 2. If STRLEN<(INITIAL+LEN) Then Exit 3. SET I=INITIAL AND J=0 4. REPEAT STEPS 5 TO 7 WHILE I<LEN 5. SUBSTR[J]=STR[I] 6. J=J+1 7. I=I+1 8. EXIT
Here substring SUBSTR of length LEN is extracted from STR, starting from the location INITIAL.
if string A=’WELCOME’, then after SUBSTRING(SUB,A,3,2), the value of SUB will be ‘LC’.
Indexing operation is used to find the position of the first appearance of a string pattern in a given string. It is also called pattern matching. INDEX operation returns a 0 if there is no such pattern in that string.
Algorithm INDEX (TEXT,PAT) – Naive Pattern Searching
1. LENSTR=LENGTH(TEXT) 2. LENPAT=LENGTH(PAT) 3. SET I=1 and MAX=LENSTR-LENPAT. 4. Repeat Step 5 to 7 while I<=MAX: 5. Repeat for J=1 to LENPAT: If TEXT[I+J-1] ≠ PAT[J], then: Go to Step 5. 6. SET INDEX=K, and EXIT. 7. I=I+1. 8. SET INDEX=0. 9. Exit.
Here we search for pattern PAT in string TEXT. LENSTR is the length of the TEXT and LENPAT is the length of the PAT.
if A=’MORNING’ then INDEX(A,’NI’) will return, 4, the first appearance of pattern ‘NI’ in the given string ‘MORNING’.
if A and B are two strings, then the concatenation of two strings, A//B is the string that consisting of the characters of A followed by the characters of B.
Algorithm CONCATENATE (STR1,STR2,ST3)
1. LEN1=LENGTH(STR1). 2. LEN2=LENGTH(STR2). 3. SET 1=0. 4. Repeat Steps 5 and 6 while I<LEN1-1: 5. STR3=STR1. 6. SET 1=1+1. 7. SET J=0. 8. Repeat Steps 9 to 11 while I<(LEN1+LEN2-2): 9. STR3[I]=STR2[I]. 10. J=J+1. 11.I=I+1. 12.Exit.
This algorithm concatenates ST2 to STR1 and form STR3.
if A=’GOOD’ and B=’MORNING’ then A//B will be ‘GOODMORNING’
Number of characters in a string is called the length of that string.
1. SET LEN=0 AND I=0. 2. Repeat Steps 3 to 4 while STRING[I] is not NULL: 3. LEN=LEN+1. 4. SET I=I+1. 5. Exit.
if A=”DECEMBER” then LENGTH(A) will return 8.
Text Processing Operations
Processing of printed matter such as letters and articles are called word processing. Basic operations associated with word processing are,
Word processing operations can be done using string operations.
INSERT(TEXT,POSITION,STRING) operation inserts STRING in TEXT so that STRING begins in POSITION.
For example INSERT(‘ABCDE’,2,’PQR’) = ‘APQRBCDE’
INSERT operation can be implemented using string operations as follows,
INSERT(T,K,S) = SUBSTRING(T,1,K-1) // S // SUBSTRING(T,K,LENGTH(T)-K+1)
That is the substring before the position K is concatenated with S and with the remaining part of the string T.
DELETE(TEXT,POSITION,LENGTH) delete the substring which begins at POSITION and has length LENGTH.
For example DELETE(‘ABCDEF’,2,3) = ‘AEF’
DELETE operation can be implemented using string operations as follows,
DELETE(T,K,L) = SUBSTRING(T,1,K-1) // SUBSTRING(T,K+L,LENGTH(T)-K-L+1)
That is the substring before position k is concatenated with the substring starting at position K+L.
REPLACE(TEXT,PATTERN1,PATTERN2) replaces the first occurrence of PATTERN1 by PATTERNP2 in TEXT.
For example REPLACE(‘PQRSTU’,’RS’,’X’) = PQXTU
REPLACE operation can be implemented using three-string operations as follows,
K=INDEX(TEXT,PATTERN1) T=DELETE(TEXT,K,LENGTH(PATTERN1) INSERT(TEXT,K,PATTERN2)
The first two steps deletes first occurance of PATTERN1 from TEXT and third step insert PATTERN2 in that position.