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

Question

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

Answer 1

If by "third if" you mean the one that has both the mem[j] condition and the writeSize condition, the problem is that the else clause would only be the alternative to the writeSize == dataSize if, not to both of them.

I think you could restructure the code like this:

for j in range(i, len(mem)):
    if (mem[j] == "-"):
        writeSize += 1
        if (writeSize == dataSize):
            found = True
            break
    else:
        i = j
        break

because checking writeSize only matters if you increment it.

Answer 2

I think you can refer to this link :

What's the main difference between 'if' and 'else if'?

You need to use elif - the reason is explained well on the link