当值通过循环输入时，不会拾取字典 0 键值

Question

At runtime it does't access value at index[0] and give error of 'avg' not defined How do i fix it. It require two decimal value but it give only one decimal value. CODE BELOW:

    n = int(input())
    student_marks = {}
    for i in range(n):
        name, *line = input().split()
        scores = list(map(float, line))

        student_marks[name] = scores
        print(scores)
        print(student_marks)
    query_name = input()
    if query_name == name:
        print(query_name)
        avg=0
        avg=sum(scores)/3
    print(avg)

OUTPUT:

    4
    dd 3 34 2 2
    [3.0, 34.0, 2.0, 2.0]
    {'dd': [3.0, 34.0, 2.0, 2.0]}
    g 3 4 5 6
    [3.0, 4.0, 5.0, 6.0]
    {'dd': [3.0, 34.0, 2.0, 2.0], 'g': [3.0, 4.0, 5.0, 6.0]}
    d 3 4 534 34
    [3.0, 4.0, 534.0, 34.0]
    {'dd': [3.0, 34.0, 2.0, 2.0], 'g': [3.0, 4.0, 5.0, 6.0], 'd': [3.0, 4.0, 534.0, 34.0]}
    e 3 4 4 4
    [3.0, 4.0, 4.0, 4.0]
    {'dd': [3.0, 34.0, 2.0, 2.0], 'g': [3.0, 4.0, 5.0, 6.0], 'd': [3.0, 4.0, 534.0, 34.0], 'e': [3.0, 4.0, 4.0, 4.0]}
    dd
    Traceback (most recent call last):
      File "C:/Users/Priya/Desktop/1.py", line 15, in <module>
        print(avg)
    NameError: name 'avg' is not defined

Answer 1

Instead of:

if query_name == name:

Perhaps you want to do:

if query_name in student_marks:    # Check if the query name is in the previously provided student details.

Because:

name, *line = input().split()

if query_name == name:

Means if you enter the query name as the name of the last student provided, then only it will go inside if . (I am 99.99% sure this is not what you want.)

In your input, students are: ['dd', 'g', 'd', 'e' ]. If you provide the query name as 'e' , then only it will go inside if . Also if you provide n = 0, then it will throw an error because for loop will not run and name will not exist. (I am 99.99% sure this is not what you want.)

---

Also:

avg=sum(scores)/3

Should be:

avg=sum(student_marks[query_name])/4   # You have four marks and not three.

---

So, change:

query_name = input()
if query_name == name:
    print(query_name)
    avg=0
    avg=sum(scores)/3
print(avg)

To:

query_name = input()
if query_name in student_marks:
    print(query_name)
    avg=sum(student_marks[query_name])/len(student_marks[query_name])
    print(avg)
else:
    print('Student details does not exist.')

Answer 2

Modify your code from this line.

query_name = input()

if query_name in student_marks:
    scores = student_marks[query_name]
    avg = sum(scores) / len(scores)
    print(avg)
else:
    print(f'Student {query_name} does not exist')

A bit of explanation on how python variables are scoped:
A variable created inside an if-else clause or a loop is accessible outside the block.

Therefore, the reason python can't find the variable avg is because it never got created, which means the if condition was not satisfied. While coding, try debugging by simply putting print statements and print out the values to see why the condition is not satisfied.

The variable name gets assigned a new value in each iteration of the for loop. When the control leaves the loop, the value of name is the last value entered by the user (name = 'e' for the example provided in the question) .

You need to check if the query_name is present among all the names entered by the user, which can be found from the keys of the dict student_marks . A simple membership check using the in operator does the trick.

Answer 3

Put the avg=0 statement outside the if because avg is not defined when if statement is not true.

So whenever the if statement is false, 'avg' is not defined that is coming in the 'print(avg)' statement.

n = int(input())
student_marks = {}
for i in range(n):
    name, *line = input().split()
    scores = list(map(float, line))

    student_marks[name] = scores
    print(scores)
    print(student_marks)
query_name = input()
avg=0
if query_name == name:
    print(query_name)
    avg=sum(scores)/3
print(avg)

Another way is to provide a else statement and assign a value to avg there but i guess you dont need that.

Hope it helps!