Python：将项目添加为列表，该列表是字典中的值

Question

I can't figure out how to add certain values to a list for each individual key. I have a few types (b,m,t,d,c) which are the keys, and then I want to add costs of those items to a list that is the value of the dictionary each time I go through the loop. This is what I have so far:

a={}
allitemcostb=[]
allitemcostm=[]
allitemcostt=[]
allitemcostd=[]
allitemcostc=[]
n=4

while n>0:
    itemtype=raw_input("enter the item type-b,m,t,d,c:")
    itemcost=input("enter the item cost:")
    if itemtype="b":    
        allitemcostb.append(itemcost)
        a[itemtype]=allitemcostb
    if itemtype="m":
        allitemcostm.append(itemcost)
        a[itemtype]=allitemcostm
    if itemtype="t":
        allitemcostt.append(itemcost)
        a[itemtype]=allitemcostt
    if itemtype="d":
        allitemcostd.append(itemcost)
        a[itemtype]=allitemcostd
    if itemtype="c":
        allitemcostc.append(itemcost)
        a[itemtype]=allitemcostc
    else:
        print "Sorry please enter a valid type"
    n=n-1
print a

It keeps giving me error messages, whether it be something isn't defined, or improper syntax. Thanks

Answer 1

Instead of a[itemtype] = allitemcostb , which simply sets that key's value to a new cost, you need to create a list if that key doesn't exist yet or add it to the existing list if it does. Do this with the setdefault() method.

The following uses just a dictionary with itemtype:[itemcost, itemcost...] and no separate list s, dispenses with the manually-incremented while loop in favor of a for loop with an xrange , and replaces the large branching structure with a more direct structure (instead of "if it's a , do a ," it does "do whatever it is"). The line if itemtype in ('b', 'm', 't', 'd', 'c'): checks that the entered itemtype is a single-character string representing an available option. If the entered itemcost can't be converted to a float , the error is caught and the user is prompted to try again.

a={}
n=4

for i in xrange(n):
    itemtype = raw_input("enter the item type-b,m,t,d,c:")
    itemcost = raw_input("enter the item cost:")
    try:
        itemcost = float(itemcost)
    except ValueError:
        print "Sorry, please enter a valid cost."
        break
    if itemtype in ('b', 'm', 't', 'd', 'c'):
        a.setdefault(itemtype, []).append(itemcost)
    else:
        print "Sorry, please enter a valid type."

print a

Answer 2

try this:

a = {}
all_item_cost_b=[]
all_item_cost_m=[]
all_item_cost_t=[]
all_item_cost_d=[]
all_item_cost_c=[]
n = 4

while n > 0:
    item_type = input("enter the item type-b,m,t,d,c:")
    item_cost = input("enter the item cost:")
    if item_type == "b":
        all_item_cost_b.append(item_cost)
        a[item_type] = all_item_cost_b
    elif item_type == "m":
        all_item_cost_m.append(item_cost)
        a[item_type] = all_item_cost_m
    elif item_type == "t":
        all_item_cost_t.append(item_cost)
        a[item_type] = all_item_cost_t
    elif item_type == "d":
        all_item_cost_d.append(item_cost)
        a[item_type] = all_item_cost_d
    elif item_type == "c":
        all_item_cost_c.append(item_cost)
        a[item_type] = all_item_cost_c
    else:
        print("Sorry please enter a valid type")
    n = n - 1
print(a)

Give us a feedback. Don't forget to mark as answered, if this solves your problem. Cheers.

Answer 3

Here are two solutions.

The fist one is not so strict. it will allow the user to enter any value for the itemtype but not for the itemcost

a={}
n=4

while (n>0):
    itemtype = input("enter the item type-b,m,t,d,c:")
    itemcost = input("enter the item cost:")

    while(True):
        try:
            itemcost = float(itemcost)
            break;
        except ValueError:
            print ("Sorry, please enter a valid cost.")
            itemcost = input("enter the item cost:")

    if itemtype.lower() in "b m t d c".split():
        a[itemtype] = a.get(itemtype,list())+[itemcost]

    n-=1

print (a)

This second form will be strict for both user inputs and will keep prompting till the user enters the expected value

a={}
n=4

while (n>0):
    itemtype = input("enter the item type-b,m,t,d,c:")
    ##user enters a wrong value
    while(itemtype.lower() not in "b m t d c".split() ):
        print ("Sorry, please enter a valid item.")
        itemtype = input("enter the item type-b,m,t,d,c:")

    itemcost = input("enter the item cost:")
    ##user enters a wrong value
    while(True):
        try:
            itemcost = float(itemcost)
            break;
        except ValueError:
            print ("Sorry, please enter a valid cost.")
            itemcost = input("enter the item cost:")

    a[itemtype] = a.get(itemtype,list())+[itemcost]

    n-=1

print (a)