C++ to Ruby: find substring in a string - c++

Question

I learned Rails and now would like to expand my knowledge of Ruby. So I'm doing some C++ exercises in Ruby. Specifically I need to find if a substring exists in a string. If it does I need it to return its starting index. If it doesn't exist have it return -1. I came up with a Ruby solution that's very similar to C++ and was wondering if there's a "better", more idiomatic solution in Ruby?
C++
int find(char str[], char sub_str[])
{
int str_length = strlen(str);
int sub_str_length = strlen(sub_str);
bool match = false;
for(int i=0; i<str_length; i++)
{
if(str[i] == sub_str[0])
{
for(int j=1; j<sub_str_length; j++)
{
if(str[i+j] == sub_str[j])
match = true;
else
{
match = false;
break;
}
}
if(match)
return i;
}
}
return -1;
}
Ruby
def find_sub_str(str, sub_str)
match = false
for i in 0...str.length
if str[i] == sub_str[0]
for j in 1...sub_str.length
if str[i+j] == sub_str[j]
match = true
else
match = false
break
end
end
if match == true
return i
end
end
end
return -1
end

Answer 1

You can use the index method of String. It returns nil on failure to match, which is more idiomatic Ruby than returning -1.
"SubString".index("String") # -> 3
"SubString".index("C++") # -> nil
You could wrap it in a test that returns -1 for nil if you really wanted this behavior.

Answer 2

Don’t use for in Ruby, it just calls each and doesn’t introduce scope. So for i in 0...str.length becomes (0...str.length).each do |i|.
Higher-order functions are your friend! Using each_cons & find_index makes things much cleaner (study Enumerable, it’s home to many useful methods):
def find_sub_str(str, sub_str)
str.chars.each_cons(sub_str.length).find_index do |s|
s.join == sub_str
end
end
find_sub_str('foobar', 'ob') #=> 2
Just use Ruby core’s index :):
'foobar'.index('ob') #=> 2
Both #2 & #3 return nil, not -1, when there is no match. This is best because nil is falsey in Ruby.

Answer 3

#how if we use this solution, it gets the job done in O(n)
given_string = "Replace me with your code!"
chars_given_string = given.split('')
chars_of_substr = "ith".split('')
is_substr = false
ptr = 0
char_given.each do |i|
if ( i == substr[ptr])
ptr += 1
else
ptr = 0
end
is_substr = true if ptr == substr.length
break if ptr == substr.length
end
puts is_substr