CODEWITHSUNDEEP
python
OrionspC
OrionspC

Reputation: 21

Subtracting from a single element in a list?

I am quite new to Python learning how it works this is a program I've made and I cant seem to reduce the quantity

How do you subtract a specific number from a single element in a list?

    import csv
    stockfile = open("Stock list.txt")
    csvf = csv.reader(stockfile,delimiter = ",")
    #Sets csvf vaiable to open text file
    #Delimiter is what seperates your values

    code = []
    item = []
    price = []
    quantity = []
    quantity = list(map(int, quantity))


    for row in csvf:
    code.append(row[0])
    item.append(row[1])
    price.append(row[2])
    quantity.append(row[3])

    usersearch = input("Please enter a 7-digit code: ")
    #Asks the user to enter the unique item codes
    while usersearch not in code:   
    print("This is not a valid 7-digit code")
    usersearch = input("Please enter a 7-digit code: ")
        if usersearch in code:               
        print("The code has been found")
        index = code.index(usersearch)  
        print("The item you have chosen is: %s"%item[index])   
        print("The price is: £%s"%price[index])                 
        print("There are %s left in stock"%quantity[index])
        quantity[index] -= 1

I want to reduce the quantity[index] by 1

When I use quantity[index] -= 1 I get an error:

TypeError: unsupported operand type(s) for -=: 'str' and 'int'

These are the contents of the file:

0000001,PS4,300,50

0000011,Xbox One,300,50

0000111,Need for Speed,40,20

0001111,Just Cause 3,45,24

0011111,Dualshock 4,48,30

Never mind I figured it out, the quantity=list(map(int, quantity)) had to come just before the quantity[index] -= 1

Upvotes: 1

Views: 551

Answers (2)

Nathaniel Ford
Nathaniel Ford

Reputation: 21269

Python has a strong concept of 'names'. Using your example:

a = [12,34,43,21,11]

a is a 'name' that is pointing to a list of five numbers, or, more generally, a points to a place in memory where some data is being held - in this case an array. When you 'index into' an list, you are, in effect, being more specific with the name you're using. So the name a[4] points to a place in memory where an integer is held, 11.

So when you do this:

a[4] = "bob"

You are saying 'change the place in memory pointed to by the name a[4] to "bob"'. When you do this:

a[4] = a[4] - 3

You are saying 'change the place in memory pointed to by the name a[4] and change it to the result of the following evaluation: a[4] - 3.' That evaluation first looks to see what is already in a[4], then subtracts three. In this case that is 8 and that result is assigned to the location in memory referred to by a[4].

Note that 'getting specific' with names is dependent on what 'datatype' the original name is. Since your a is a list, Python knows how to index into it. Here is Python (in the REPL) trying to index into a few other common types:

>>> s = "abcde"
>>> s[4]
'e'
>>> b = 5
>>> b[5]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'int' object is not subscriptable
>>> c = { 'a':1, 'b':2, 'c':3, 'd':4, 'e':5 }
>>> c[4]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 4
>>> c['d']
4

Note that in that last one, the key d worked but 4 did not. It is important to know what you're indexing into when you're trying to get specific with what you're doing!

Upvotes: 1

Martijn Pieters
Martijn Pieters

Reputation: 1125388

You could use augmented assignment, -=:

a[4] -= 3

Or just re-assign the result of a subtraction back to the index (which is what -= does anyway):

a[4] = a[4] - 3

Upvotes: 5

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