Iteration using "while"

Question

I have a little trouble understanding while and I came across an exercise that asked me to do the following:

Read N sequences and find out how many f has in each sequence. I cannot use for in that matter.

My code went like this:

i = 0
faltas = 0

while True:
    registros = raw_input()
    if registros == '-':
        break

    while i < len(registros):
        if registros[i] == 'f':
            faltas += 1

        i += 1


    print faltas

The problem is that if I type a sequence that has less 'f’s than the previous one it does not show the amount of gaps in the sequence.

Ex:

.f.f.f.f
faltas = 4

.f.f..
faltas = 4 (onde deveria imprimir 2)

Answer 1

The first problem is that it initializes the control variables and fault accumulator at the beginning of the algorithm, but when it finishes entering the first time they are not reset, so for this to occur you need to put the initialization inside the main loop.

There is a problem in checking output, you have to parse only the first character to know if it is a dash, and not the whole text.

while True:
    registros = raw_input()
    if registros[0] == '-':
        break
    i = 0
    faltas = 0
    while i < len(registros):
        if registros[i] == 'f':
            faltas += 1
        i += 1
    print faltas

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.

Answer 2

You can use the method str.count to calculate the amount of f in its wake:

while True:
    sequencia = raw_input('Digite uma sequência: ')
    if sequencia:
        print('Possui {} faltas na sequência'.format(sequencia.count('f')))
    else:
        break

^{See working on Repl.it}

Answer 3

TL;DR

Try it like this:

def count_f(registros):
  faltas, i = 0, 0
  while i < len(registros):
    if registros[i] == 'f':
        faltas += 1

    i += 1
  return faltas

while True:
  registros = input()
  if registros == '-':
      break

  print(count_f(registros))

Testing:

[aasfff
3
werf
1
asdfgffff
5

See working on repl it.