String Functions

Getting Numeric Equivalents of Chars

>> double('abc xyz')

ans =

97 98 99 32 120 121 122

>> double('ABC XYZ')

ans =

65 66 67 32 88 89 90

Converting Numbers Back to Strings

>> x = double('here is a text')

x = 104 101 114 101 32 105 115 32 97 32 116 101 120 116

>> xst = char(x)

xst =

here is a text

Converting to Upper Case

• Given a string st, convert all the chars to upper case. (Matlab has built-in functions upper & lower)

• Observation : the difference between the lower case and upper case of the same character is same for all characters.

>> double('a') - double('A')ans = 32>> double('f') - double('F')ans = 32

Algorithm

Given st :

stvector = numeric vector of st

For all elements in stvector do

if current char lower case

change the element to upper case

isLower Function

function result = isLower(ch)

if (double(ch) >= double(‘a’) &&

double(ch) <= double(‘z’))

result = true;

else

result = false;

end

toUpper Function

function upperst = toUpper(st)

diff = double('a') - double('A');stvector = double(st);for i=1 : length(st) if isLower(st(i)) stvector(i) = stvector(i) - diff; endendupperst = char(stvector);

Converting string into integer

function result = str2int( st )

result = 0;

for i = 1:length(st)

digitst = st(i);

digit = double(digitst) - double('0'); % digitst - '0' also works

result = 10 * result + digit;

end

Getting parts of a string

• In general : st(v), where v is a vector of locations that will be included.

• Examples :

>> st = 'abcdefgh‘;>> st( [1 3 6] )ans = acf>> st(2:5)ans =bcde

Categorizing characters

• ISLETTER True for letters of the alphabet.• ISSPACE True for white space characters.

>> isletter('s')ans = 1>> isletter('3')ans = 0>> isspace(' ')ans = 1>> isletter('a 3b 4e')ans = 1 0 0 1 0 0 1

deblank function

• DEBLANK(S) removes trailing blanks from string S.

>>st = ‘ abc abc ‘;>> ['---->' deblank(st) '<-----']

ans =

----> abc abc<-----

Getting Rid of Space Characters

• Let’s implement a function, “trim”, similar to deblank that also removes leading spaces

• Example: ‘ abc ‘ ‘abc’

Trim function algorithm

• Find the location of first non-space character

• Find the location of the last non-space character

• Return the result as the subset in between these two locations

Trim function

function result = trim( st )

first = 1;while (isspace(st(first))) first = first + 1;end

last = length(st);while (isspace(st(last))) last = last - 1;end

result = st(first:last);

Trim all spaces?

• Let’s write a function that removes all space characters.

• Example : ‘ ab cd e f’ ‘abcdef’

• As an intermediate step, let’s first list locations of all non-space characters

Listing locations of non-space characters

for i = 1: length(st) if ~isspace(st(i)) disp([i]) endend

• You can view this segment as generating numbers and processing them (here processing amounts to displaying on screen)

Crating a locations vector

• If we put these locations inside a vector, rather than displaying, we could use this vector to get all non-space characters out of our string

• Let’s first look at how we can create a new vector out of user input. We will keep putting elements until user enters –1.

Incrementally adding elements to a vector at command prompt

>> v??? Undefined function or variable 'v'.>> v(1) = 3;>> vv = 3>> v(2) = 5v = 3 5>> v(3) = 7v = 3 5 7

Creating a new vector of unknown size from user input

function v = createVect

nexti = 1;x = input('enter a number >');while (x ~= -1) v(nexti) = x; nexti = nexti + 1; x = input('enter a number >');end

Complete Program

• Now we can merge the segment that generates the indices of non-space characters with the segment that creates a new vector to come up with our final solution

trimAll function

function result = trimAll( st )

nexti = 1;for i = 1: length(st) if ~isspace(st(i)) nonspace(nexti) = i; nexti = nexti + 1; endend

result = st(nonspace);

Comparing Strings

• STRCMP determines if two strings are identical

• STRCMPI Compare strings ignoring case.

• STRNCMP Compare first N characters of strings.

• STRNCMPI Compare first N characters of strings ignoring case.

Examples

>> strcmp('abc' , 'aBc')

ans = 0

>> strcmpi('abc' , 'aBc')

ans = 1

>> strncmp('abcxyz' , 'aBcpqr', 3)

ans = 0

>> strncmpi('abcxyz' , 'aBcpqr', 3)

ans = 1

Implement strcmpfunction result = myStrCmp(st1, st2)

if (length(st1) ~= length(st2))

result = false;

return;

end

result = true;

for i = 1 : length(st1)

if (st1(i) ~= st2(i))

result = false;

%break

end

end

function result = myStrCmp(st1, st2)

if (length(st1) ~= length(st2))

result = false;

else

result = true;

for i = 1 : length(st1)

if (st1(i) ~= st2(i))

result = false;

%break

end

end

end

Implement strcmp

function result = myStrCmp(st1, st2)

if (length(st1) ~= length(st2)) result = false; return;end

i=1;while (i <= length(st1) && st1(i) == st2(i)) i = i + 1;end

result = ~(i <= length(st1));

Implementation

• Let’s implement strcmpi, strncmp, strncmpi ourselves using strcmp

Implement strcmpi

function result = myStrCmpi(st1, st2)

if (length(st1) ~= length(st2)) result = false; return;end

diff = 'a' - 'A';i=1;while ( (i <= length(st1)) && ... ( (st1(i) == st2(i)) || ... (isletter(st1(i)) && isletter(st2(i)) && abs(st1(i) - st2(i)) == diff ) ) ) i = i + 1;end

result = ~(i <= length(st1));

… (first part omitted )diff = 'a' - 'A'; i=1; equali = true;while (i <= length(st1) && equali) if (st1(i) == st2(i)) equali = true; elseif (isletter(st1(i)) && isletter(st2(i))) if (abs(st1(i) - st2(i)) == diff ) equali = true; else equali = false; end else equali = false; end i= i + 1;end

result = ~(i <= length(st1));

strcmpi implementation …

function result = myStrCmpi(st1, st2)

if (length(st1) ~= length(st2)) result = false; return;end

stu1 = upper(st1);stu2 = upper(st2);

result = strcmp(stu1, stu2);

Strncmp implementation

function result = myStrnCmp(st1, st2, n)

i=1;while (i <= n && i <= length(st1) && i <= length(st2)

&& st1(i) == st2(i)) i = i + 1;end

result = ~(i <= n);

Strncmp implementation

function result = myStrnCmp(st1, st2, n)

if (length(st1) < n || length(st2) < n) result = false; returnend

stn1 = st1(1 : n);stn2 = st2(1 : n);

result = strcmp(stn1, stn2);

Findstr function

FINDSTR Find one string within another. K = FINDSTR(S1,S2) returns the starting indices of any occurrences of the shorter of the two strings in the longer. FINDSTR is symmetric in its two arguments; that is, either S1 or S2 may be the shorter pattern to be searched for in the longer string. If you do not want this behavior, use STRFIND instead. Examples s = 'How much wood would a woodchuck chuck?'; findstr(s,'a') returns 21 findstr('a',s) returns 21 findstr(s,'wood') returns [10 23] findstr(s,'Wood') returns [] findstr(s,' ') returns [4 9 14 20 22 32]

Implement findstr …

• Assume the first string is the longer one.

• Use strcmp on subsets of the longer string

myFindStr function

function result = myFindStr( st, search )

nexti = 1;for j = 1 : length(st) - length(search) + 1 if (strcmp(search, st(j : j + length(search) - 1))) result(nexti) = j; nexti = nexti + 1; endend

Strrep function

STRREP Replace string with another.

S = STRREP(S1,S2,S3) replaces all occurrences of the string S2 in string S1 with the string S3. The new string is returned.

Example:

s1='This is a good example';

strrep(s1,'good','great') returns 'This is a great example'

strrep(s1,'bad','great') returns 'This is a good example'

strrep(s1,'','great') returns 'This is a good example'