Python：如何在使用for循环和文件时订购列表。 （家庭作业）

Question

Basically I need the names of the students to be attributed to their respective grades in the order of the list. So this is what I have so far:

def main():

Define myFile and open/read grades.txt file

myFile = open("grade.txt", "r")

Define the number of students and grade percentages for assignments, quizzes and exams

numStudents = int(myFile.readline())
prctgs = myFile.readline().split()
students = ["Amy", "Jack", "Arron", "Zack", "Jen", "Jane"]

Create for loop to go through each of the students' scores

for i in range(numStudents):

Store each student's grades

stdntGrades = myFile.readline().split()

Calculate grade percentage

grade = 0
for j in range(len(prctgs)):
    grade = grade + float(prctgs[j]) * int(stdntGrades[j])
    print("Student #", i+1, ": ", " %.2f" % grade, sep="")

myFile.close()

main()

'students' is the list in question, I had also tried to directly input the names into the file but that failed miserably

The 'grade.txt' file has the following in it:

6
.3 .1 .6
90 89 78
96 92 79
85 100 94
87 92 96
81 88 93
85 91 99

Note: This is my fifth program, so I am a complete beginner.

Answer 1

The students' names should be part of the data, not the code:

6
.3 .1 .6
Amy 90 89 78
Jack 96 92 79
Aaron 85 100 94
Zack 87 92 96
Jen 81 88 93
Jane 85 91 99

Then read them in along with the data:

line = file.readline().split()
student = line[0]
prctgs = line[1:]

Answer 2

Lets assume we have 2 strings ...

weights = "0.3 0.1 0.3"
grades =  "90   87  92"

You already know that you can split these up into lists:

weights.split() #['0.3', '0.1', '0.3]
grades.split() #['90', '87', '92']

Now you say to yourself -- "Wouldn't it be nice if I could get the matching numbers at the same time?" Well, you can. That's what zip is built for:

total = 0
for weight,grade in zip(weights.split(),grades.split()):
    total += float(weight)*float(grade)

This can even be shortened using sum if you'd like:

total = sum( float(w)*float(g) for w,g in zip(weights.split(),grades.split() )

So, this gives you one person's total. Now, we can use the same concept when working with the file. We want to "zip" the person's name with their grade information. Here's how:

with open('grade.txt') as data: #fancy way of opening files...
    npeople = int(next(data)) #read first line, make an integer
    weights = [float(x) for x in next(data).split()] #get the weights as a list of floats
    people = ['jack','jill','mary','little miss muffet','kermit','animal']
    for person, grade_info in zip(people,data):
        grade = sum( w*float(g) for w,g in zip(weights,grade_info.split() )
        print person,grade

Another trick that I've used here is next(fileobject) . That's just a fancy way of saying fileobject.readline() . In fact, in python3.x, fileobject.readline() no longer exists ... Might as well get used to using next now :).

The 4 builtin functions that I've used here are zip , next , float and sum . float and sum are pretty obvious how they work so they don't take much work to explain. zip and next are a little more tricky. Read the linked docs until you understand them. They're super useful. I've also used list-comprehensions for building some lists. They're super useful as well. Definitely something you want in your python utility belt.