Why doesn't else statement work instead of if (all previous cases aren't true)?

So I have a list mem that represents memory and in this loop I'm searching for dataSize number of empty spaces which are represented as "-" in memory.
Logic is following:
First search for "-" (empty slot), then mark it as starting location writeStart and then see how much more empty slots I have after that position and count it as writeSize. Found is then set on True if I have found enough slots or else loop breaks and returns to original loop that searches for another empty slot as writeStart.Then I use writeStart and writeSize as parameters to write in memory after this loop.
This code works correctly but if I switch third if statement with an else it won't work anymore. Why? Thank you in advance.

for i in range(len(mem)):
    if(mem[i] == "-"):
        writeStart = i
        writeSize = 0
        for j in range(i, len(mem)):
            if(mem[j] == "-"):
                writeSize += 1
            if(writeSize == dataSize):
                found = True
                break
            if((mem[j] != "-") & (writeSize != dataSize)): #if switched with an else: error
                i = j
                break
    if(found):
    break