类型错误：字符串索引必须是整数

Question

I'm very new to coding and as a school project I was trying to create a Cesar code, I got it working but then I tried to reverse it, so it could decode. I thought it was as simple as subtracting the jump but then it came up with this:

entrada=input("Please input your Message:")
matriz= "abcdefghijklmnñopqrstuvwyz 1234567890"

salida=''

for letra in entrada:
 posicion = matriz.find(letra.lower())
 salto = posicion
 posicion = posicion-salto

 if posicion > len(matriz) -1:
  posicion=posicion-len(matriz)

 if posicion < len(matriz) +1:
  posicion = posicion+len(matriz)

 salida = salida+matriz[posicion]


print (salida)

After this, it came back with the error in line 18

Answer 1

salto = posicion
posicion = posicion-salto

after this your posicion is 0 whatever salto and posicion you have had before.

 if posicion < len(matriz) +1:
    posicion = posicion+len(matriz)

Here you get posicion which is greater than len(matriz), so matriz[position] results in IndexError

Answer 2

Firstly,

salto = posicion
posicion = posicion-salto

will make posicion 0, no matter what. Perhaps, you're missing a piece of code here. Also, your code is returning IndexError: string index out of range at line 17, salida = salida + matriz[posicion] . There is no type error.