How to create two y points for a same x?

Question

Hello, I would like to create a python routine to create "double points", where a same value of x would assume two values of y for any situation. Using the following data sequence for example:

1;5
2;2
3;4
4;10
5;6

Plotting this graph as the first column x and the second y would form a straight line.

The scenario I would like to arrive at would be the following:

1;5
1;2
2;2
2;4
3;4
3;10
4;10
4;6
5;6

And that would generate a rectangular graph. In this case, the value of x has its respective value of y, but this same value of x also has the value of y for its "x+1", and so on.

I tried to create something like a list, but without success

arquivo = open('vazmed.out', 'rt')
arq = open('vazmed2.out', 'wt')
lista = []
valor_prev = 0
for linha in arquivo:
    campo = linha.split(';')
    i = (campo[0])
    valor = float(campo[1])
    lista.append(valor)
n = len(lista)
for i in range(n-1):
    lista.append([lista[i:][:i], lista[i:][:i+1]])
    arq.write('{};{}\n' .format(i, lista[i]))
arq.close

I am new to python and this is part of my studies, so if someone can help me and explain how and why I would be grateful.

EDIT Code for transformation of positive values into 1 and negative into -1.

arquivo = open('vazdif.out', 'rt')
lista = []
valor_prev = 0
for linha in arquivo:
    campo = linha.split(',')
    cont = int(campo[0])
    vazdif = float(campo[2])
    if vazdif * valor_prev < 0:
        lista.append([lista[-1][0], - lista[-1][1]]) 
    if vazdif < 0:
        lista.append([cont, -1])                                         
    else:
        lista.append([cont, 1])
    valor_prev = vazdif                                                 
fou = open('res_id.out','wt')
for i in lista:
    fou.write('%4d,%8.4f\n' % (i[0],i[1]))
fou.close()

The output file of this code is as follows::

   0, -1.0000
   1, -1.0000
   2, -1.0000
   3, -1.0000
   4, -1.0000
   5, -1.0000
   5,  1.0000
   6,  1.0000
   7,  1.0000
   8,  1.0000
   9,  1.0000
  10,  1.0000
  11,  1.0000
  12,  1.0000
  13,  1.0000
  14,  1.0000
  14, -1.0000
  15, -1.0000
  16, -1.0000
  17, -1.0000
  18, -1.0000
  19, -1.0000
  20, -1.0000

As you can see, whenever the value changes from negative to positive, the i repeats with the value of i previous. However the i is not part of the data effectively, being only an index.

Important to note that it is only the index that will repeat, not the value itself.

Answer 1

Leafar’s answer is correct, but it will only work if you treat the data correctly.

Treating data from a line-separated file

texto = '''1;5
2;-2
3;4.5
4;10
5;6'''
# Para cada linha que tiver na string, gerar uma lista de valores separado por ;
lista = [i.split(';') for i in texto.splitlines()]
print(lista)
# [['1', '5'], ['2', '-2'], ['3', '4.5'], ['4', '10'], ['5', '6']]

Now just apply the logic offered by Leafar and treat the strings:

m = []
for i in range(len(lista)):
    # Primeiro valor registra como INT e segundo valor registra como FLOAT
    m.append([int(lista[i][0]), float(lista[i][1])])

    if i < len(lista)-1:
        m.append([int(lista[i][0]), float(lista[i+1][1])])
 print(m)
 # [[1, 5.0], [1, -2.0], [2, -2.0], [2, 4.5], [3, 4.5], [3, 10.0], [4, 10.0], [4, 6.0], [5, 6.0]]

And to pull the dice:

for i in m:
    print(f'index = {i[0]}; valor = {i[1]}')

Answer 2

Well, the main problem is that you are using the list index to 'save' the value x, but this is not possible when you have two elements with the same value x, since the list soh can have a value y saved in that Dice x.

The other problem is that you’re adding all the original points in the list before you start adding the new ones, which prevents you from sorting the points the way you put them in the example.

The command lista[i:][:i] Also it’s pretty strange. What you’re doing here is cutting all the first ones i-1 list elements and, given the list resulting from this cut, keep only the first i elements. In any case, this operation will result in a list, which does not seem to be what you want there. I recommend until you do some experiments giving print on different values of i as Indice to better understand how lists work.

Try to do something about it:

l = [[1,5], [2,2], [3,4], [4,10], [5,6]] # Troque pela leitura do arquivo...

m = []
for i in range(len(l)):
    m.append(l[i])

    if i < len(l)-1:
        m.append([l[i][0], l[i+1][1]])

# Escreva os elementos de m no arquivo...

This code results in m = [[1, 5], [1, 2], [2, 2], [2, 4], [3, 4], [3, 10], [4, 10], [4, 6], [5, 6]], which seems to be the result you expect.