对于循环不停止

Question

I have a for loop in a function that won't stop. Here is the entire function which is used for finding a person's grades, adding them together, then displaying the average onscreen:

float Student::average() { 
   cout << "How many grades would you like to enter? (Up to ten)\n";
   float x;
   cin >> x;

   cout << "What is your first grade?";
   cin >> grade[0];
   int i = 1;
   for (i; i = x; i++) {
      cout << "What is the next number?\n";
      cin >> grade[i];
   }
   averageGrade = std::accumulate(grade, grade+10, 0.0);
   averageGrade = averageGrade / 10;

   return averageGrade;
}

And here is the for loop on its own:

for (i; i = x; i++) {
   cout << "What is the next number?\n";
   cin >> grade[i];
}

An error also outputs (but still allows the program to run) saying:

1>c:\\users\\hastudent\\documents\\visual studio 2008\\projects\\weapons\\weapons\\weapon.cpp(25) : warning C4244: '=' : conversion from 'float' to 'int', possible loss of data

1>c:\\users\\hastudent\\documents\\visual studio 2008\\projects\\weapons\\weapons\\weapon.cpp(30) : warning C4244: '=' : conversion from 'double' to 'float', possible loss of data

The program gets up to the point where it ask for the next number. Then you input the number and it keeps asking you that forever.

Answer 1

The for loop should read:

for (int i = 1; i < x; i++)

What you have now ( i = x ) is the assignment of x to i , whereas you probably intended a comparison . In this case, the correct comparison to use is "less-than" ( i < x ).

Answer 2

Your for loop ran forever because:

• i = x is an assignment, rather than an equality test.
• its value is the value of the left-hand-side
• you gave it non-zero values
• and a non-zero value is true in C++.

If you want an equality test, use ==. However, in this case, you want a < thest.

Answer 3

Have you tried printing the value of i in the for loop? You start at x, and keep incrementing.

That should probably be:

for(int i=0; i<x; i++) { /* do something */ }

Usually you start counting from 0, not 1.

The main issue is that where you do i=x you want to put instead the condition that must hold at each iteration of the loop (that is called the loop invariant ), that is i < x if you start with i=0, or i==x as others have suggested, if you start counting from i=1.

Answer 4

The second part of the for loop is the "When to continue" condition, not "when to stop".

Since you start with index 1 (unconventional but not unheard of in C++) you should iterate up to the number in question:

for(i = 1; i <= x; ++i)

Note that even if the condition were "when to quit" = is assignment in C++ while == is equality comparison.

Answer 5

在FOR循环中使用！=（比较运算符）而不是=（赋值运算符）。