how to stop while statement

function isitalike=comparestrings(a,b)

n=length(a);

m=length(b);

c=logical(0);

y=m;

z=n;

f=0;

if m>n

      temp=a;

      a=b;

      b=temp;

end

      for i=1:n

     c=strcmp(a(i),b(1))

          if c

              j=1;

              if a(i:n)>=length(b)

              for k=1:length(b)

              while a(i)==b(j)

          if j==length(b)

               if z>y

          disp('b is in a')

         f=1;

          else

              disp('a is in b')

              f=1;

              end

          end

           j=j+1;

               i=i+1;

              end

          end

              end

              end

          end

      if f==0

           if z>y

          disp('b is not in a')

          else

              disp('a is not in b')

           end

      end

The function above does not work properly I can't stop the while statement after i have found that a is in b or b is in a. This is what happens:

>> a

a =

agg

>> b

b =

sagggsa

>> comparestrings(a,b)

c =

       0

c =

       1

a is in b

??? Attempted to access b(4); index out of bounds because 

numel(b)=3.

Error in ==> comparestrings at 39

              while a(i)==b(j)

>>

Answer 1

[Walter Roberson]

What is your statement

if a(i:n)>=length(b)

intended to mean?

Your while loop continues indefinitely because you do not have an condition defined to escape from it with a "break" statement.

[Tor Fredrik Hove]

I wrote a(i:n)>=length(b) because i did not want the rest of a to be smaler then the whole b because then b could not be in a and it would be no use to try to find b in the string a. I thought the function would stop when length of b had been covered since I used for k=1:length(b) and if not b(j) was equal a(i) it would stop before because of the while condition which I used because then b could not be in a?

[Walter Roberson]

a(i:n) selects position i, i+1, i+2, .... n out of "a", giving a vector as a result as long as n >= i+1

You then compare those _characters_ to the length of b. That is going to generally give you a vector result, one value for each of the characters.

When you have a vector as the thing being tested in an "if" statement, the "if" is considered true only if _all_ of the elements of the vector are non-zero (true).

If you wanted to compare the _length_ of the rest of a, you could use

if length(a(i:n)) >= length(b)

Have another loop at the structure of your "while" statement. It is of the form

while a(i) == b(j)

if THAT %if #1

if THE_OTHER %if #2

else

end %end of if #2

end %end of if #1

i=i+1; j=j+1;

end %end of while

Notice there is no breaking out of the loop within the body of the loop, so the loop would only stop if it found an a(i) which was not equal to b(j) . However, if the two strings were the same right up to the end of one of them, you are going to increment i or j to point past the end of the "a" or "b" string (or both), resulting in the index out of range error you saw.

Remember, an outer "if" or "while" only affects whether you start an inner loop or not. Once you are within the inner loop, you stay within that loop until the conditions for that loop are satisfied, even if you change variables so completely that the condition of some outer loop or "if" was violated.

[Espen]

Thanks for the help!