Doubt with matrix

[Peter Otten]

José Manuel Suárez Sierra wrote:

> Hello, I want to  go over matrix indexs with this code:
> def comparador2(a, b):
>     c3 = ["0"]  # variables
>     x = -1  # contador de letras aniadidas a c3
>     i = 0  # contador bucle secuencia a
>     j = 0  # contador bucle secuencia b
>     l1 = len(a)
>     l2 = len(b)
>     cont = []  # contador de ciclos poner primer termino a 0
>     k = -1  # contador de 0 y 1 aniadidos a cont
> 
>     if l1 > l2:  # metodo de la burbuja que elige la secuencia mas larga
>     REVISAR puede ser alreves

In Python there's a cool way to swap values without an explicit helper 
variable:

          a, b = b, a

>         aux = a
>         a = b
>         b = aux

OK, now you have swapped a and b, but not their respective lengths l1 and 
l2...
 
>     for a[i] in a:  # en la secuencia1 recorro los elementos

Do you really want to modify a[i] on every iteration? Most certainly not. 
Try to use the normal way with just a name, e. g.

      for entry_a in a:
          ...

Then use entry_a instead of a[i] for your comparison in the body of the for-
loop. With that approach it's impossible to get an IndexError.
If you still need the index to alter an entry in the list -- enumerate() is 
your friend:

for index_a, entry_a in enumerate(a):
    ...

Again it's unlikely that you will get an IndexError.
 
>             for b[j] in b :
> 
>                 if a[i] == b[j] and i <= l1 and j <= l2:  # Si el elemento
>                 i de la seq1 es igual que el elemento j de la seq2, y el
>                 numero de elementos en i y j es menor o igual que la
>                 longitud de las secuencias 1 y 2
>                     c3.append(a[i])  # se aniade el elemento comun a la
>                     lista c3 x = x + 1
>                     k = k + 1
>                     j = j + 1  # se pasa el elemento siguiente de la seq2
>                     i = i + 1  # se pasa el elemento siguiente de la seq1
>                     cont.append(1)
> 
> 
> 
>                 elif a[i] != b[
>                     j] and i <= l1 and j <= l2:  # si no coinciden estos
>                     elementos se pasa al siguiente elemento de la lista 2
>                     j = j + 1 k = k + 1
>                     cont.append(0)
>                     if cont[k] == 0 and cont[k - 1] == 1 and cont[k - 2]
>                     == 0 and k >= 2:
>                         i = i - 1
>                         j = j - 1
>                     else:
>                         k = k + 1
>                         cont.append(0)
>                         if i == l2:
>                             i = i + 1
>                             j = 0
> 
> 
>     return c3
> 
> and this issue is prompted:
> IndexError: list assignment index out of range
> 
> How could I fix it?
> 
> Thank you for your assistance