Chemical element symbols task-CodePudding

I have to make a task where we need to check if a symbol is a valid symbol (by the rules of the task)

The background information of the task which I probably won't be able to explain:

Example provided by the assignment:

def isvalid(symbol, element, length = None):
    elbool = False
    
    if symbol[0]!=symbol[0].upper():
        return False #first letter in the symbol has to be a capital letter
    
        
    for i in range(len(symbol)-1):
            
        if element.find(symbol[i 1])>element.find(symbol[i]): #checking if the order is correct
            elbool = True
        else:
            return False
    
    if length is not None:
        if len(symbol)!=length:
            return False
        else:
            elbool = True
           
        
    return elbool

Is my code now but it doesn't work for example with this one: isvalid('Rcm', 'Americium') because there is an m before the c and it counts that one.

So I think I need to split the element string from the last letter in the symbol so I don't have that problem but how do I do that?

Sorry if the question is a bit confusing.

CodePudding user response：

You need to use .find(needle, start_pos) to look for the character after a certain location in element. Also, you don't need to mess with indices and keep finding the previous and current character from symbol. Just keep track of the location of the current character for the next iteration.

You should also do a case-insensitive search, because, using your example, there is no "R" in "Americium". I do this by converting element to lowercase once, and then doing .find(c.lower(), ...) on each character in symbol

Finally, you forgot to check that all characters other than the first one in symbol are lowercase. I also added that to the for loop.

element = element.lower() # Lowercase element so that we can find characters correctly
lastfound = 0
for ix, c in enumerate(symbol):
    if ix > 0 and c.isupper():
        # Not the first character, and is uppercase
        return False

    thisfound = element.find(c.lower(), lastfound) # Find the location of this character
    if thisfound == -1: # character not found in element after location lastfound
        return False

    lastfound = thisfound # Set lastfound for the next iteration

A few other minor suggestions:

You can return False as soon as you find something wrong. Then, at the end of the function, just return True because the only way you reach the end is when nothing is wrong.
You can check if a character is lowercase with symbol[0].islower(). No need to do symbol[0] != symbol[0].upper().
You should check for the length requirement before you check for the order of characters, because that's the simpler check.

Applying all these:

def isvalid(symbol, element, length = None):
    if symbol[0].islower():
        return False

    if length is not None and len(symbol) != length:
        return False
    
    element = element.lower() # Lowercase element so that we can find characters correctly
    lastfound = 0
    for ix, c in enumerate(symbol):
        if ix > 0 and c.isupper():
            return False
        thisfound = element.find(c.lower(), lastfound) # Find the location of this character
        if thisfound == -1: # character not found in element after location lastfound
            return False

        lastfound = thisfound # Set lastfound for the next iteration
  
    return True

Using your tests:

>> isvalid('Zer', 'Zeddemorium')
True

>> isvalid('Zer', 'Zeddemorium', 2)
False

>> isvalid('di', 'Zeddemorium')
False

>> isvalid('Rcm', 'Americium')
True