Help with this code

[José Manuel Suárez Sierra]

El lunes, 9 de enero de 2017, 14:09:09 (UTC+1), José Manuel Suárez Sierra  escribió:
> Hello, I am trying to make a code wich compares between 2 or several sequences (lists). It compares every element in a list with another list elements. For example, if we have a list_a=["a","b","c","d"] and list_b=["a","b"] I want to obtain a new list_c containing elements that match between these lists (a and b here), but, if for instance list_b were ["a","c"] the program must not store this data because they are not in same order.
> 
> Said this, I wrote this code but it doesnt work:
> 
> if __name__ == "__main__":
> 
> 
>     def compare(a, b):
> 
>         i = 0
>         j = 0
>         c1 = []
> 
>         while a[i] == b[j]:
>             c1.append(a[i])
>                 j = j+1
>                 i=i+1
> 
>         return c1
> 
> 
> 
> 
>     cadena_1=raw_input("Introduce list 1 \n")
>     cadena_2 = raw_input("Introduce list 2 \n")
> 
> 
>     transf1=list(cad_1) 
>     transf2 = list(cad_2)
> 
> 
>     print compare(transf1,transf2)
> 
> 
> 
> 
> Thank you

Thank you!

Here is my code, I dont understand why it doesnt work very well:
if __name__ == "__main__":


    cadena_1 = raw_input("Introduce la cadena 1 \n")
    cadena_2 = raw_input("Introduce la cadena 2 \n")
    # n=input("De cuantas letras quieres hacer la comparacion?\n")

    transf1 = list(cadena_1)  
    transf2 = list(cadena_2)
    l1=len(transf1)
    l2=len(transf2)
    c3=[]
    i=0
    j=0


    for transf1[i] in transf1:          
        for transf2[j] in transf2:   
            if transf1[i]==transf2[j]: 
                c3.append(transf1[i])   
                i=i+1                   
                j=j+1                   
            else:                      
                j=j+1                   

    print c3

This is the traceback:
line 18, in <module>
   for transf2[j] in transf2:
IndexError: list assignment index out of range

If I have initialized j=0 (such as i) why does it not work?
I want the script to read sequence 1 and compares every element inside it with elements in sequence 2 no mattering where it matches.

Thank you!