for loop structure

This program will calculate the average grade for 4 exams using a for loop by prompting the user for exam grades, one at a time, then calculate the average and display the result.

public class ExamsFor4 {

public static void main(String[] arguments) {


int inputNumber; // One of the exams input by the user.
int sum = 0;     // The sum of the exams.
int i;       // Number of exams.
Double Avg;      // The average of the exams.

TextIO.put("Please enter the first exam: ");        // get the first exam.
inputNumber = TextIO.getlnInt();    

for ( i = 1; i <= 4; i++ ) {  

    sum += inputNumber;                 // Add inputNumber to running sum.
    TextIO.put("Please enter the next exam: ");     // get the next exam.   
    inputNumber = TextIO.getlnInt();

        if (i == 4) {
            Avg = ((double)sum) / i;
            TextIO.putln();
            TextIO.putln("The total sum for all " + i +" exams is " + sum);
            TextIO.putf("The average for the exams entered is %1.2f.\n", Avg);
            break;

        }
} 

}   // end main ()
}  // end class ExamsFor4

My result:

Please enter the first exam: 100
Please enter the next exam: 99
Please enter the next exam: 98
Please enter the next exam: 97
Please enter the next exam: 96
The total sum for all 4 exams is 394
The average for the exams entered is 98.50.

This would be correct except for the last print out of: 'Please enter the next exam: 96' I tried putting the IF statement between the 'sum' line and the TextIO.put 'Enter next exam', but that isolates it.

Thanks, from a Network Dude trap in a Programmer's world.开发者_如何转开发

You have what is called an off-by-one error, compounded by the fact that you're convoluting your loop logic unnecessarily.

With regards to the loop, I recommend two things:

• Don't loop for (int i = 1; i <= N; i++); it's atypical
  • Do for (int i = 0; i < N; i++); it's more typical
• Instead of checking for the last iteration to do something, refactor and take it outside of the loop

Related questions

• What is exactly the off-by-one errors in the while loop?

See also

• Wikipedia/Off-by-one error

On Double Avg

In Java, variable names start with lowercase. Moreover, Double is a reference type, the box for the primitive double. Whenever possible, you should prefer double to Double

See also

• Java Language Guide/Autoboxing
• JLS 5.1.7 Boxing Conversion and 5.1.8 Unboxing Conversion
• Effective Java 2nd Edition, Item 49: Prefer primitives to boxed primitives

Related questions

• What is the difference between an int and an Integer in Java/C#?
• Java: What’s the difference between autoboxing and casting?
• Why does int num = Integer.getInteger(“123”) throw NullPointerException?
• Why does autoboxing in Java allow me to have 3 possible values for a boolean?
• Is it guaranteed that new Integer(i) == i in Java? (YES!!!)
• When comparing two Integers in Java does auto-unboxing occur? (NO!!!)
• Java noob: generics over objects only? (yes, unfortunately)

Rewrite

Here's a way to rewrite the code that makes it more readable. I used java.util.Scanner since I don't think TextIO is standard, but the essence remains the same.

import java.util.*;

public class ExamsFor4 {
    public static void main(String[] arguments) {
        Scanner sc = new Scanner(System.in);
        final int NUM_EXAMS = 4;
        int sum = 0;

        for (int i = 0; i < NUM_EXAMS; i++) {
            System.out.printf("Please enter the %s exam: ",
                (i == 0) ? "first" : "next"
            );
            sum += sc.nextInt();
        }
        System.out.printf("Total is %d%n", sum);
        System.out.printf("Average is %1.2f%n", ((double) sum) / NUM_EXAMS);
    }
}

An example session is as follows:

Please enter the first exam: 4
Please enter the next exam: 5
Please enter the next exam: 7
Please enter the next exam: 9
Total is 25
Average is 6.25

Note that:

• Only necessary variables are declared
  • The loop index is local only to the loop
• There are no cluttering comments
  • Instead, focus on writing clear, concise, readable code
• If it makes sense to make something final, do so
  • Constants in Java is all uppercase

Related questions

• Why does (360 / 24) / 60 = 0 in Java
  • Because it performs integer division. This is why the cast to (double) prior to the division in above code is necessary, so that it performs floating point division.
• How does the ternary operator work?
  • This is the ?: operator in above code, also known as the conditional operator.
  • See also: JLS 15.25 Conditional Operator ?:

Change your end condition to be strictly less than 4 and put the code that prints out the total and average outside the loop.

You should probably put the if-statment outside the for-loop. That way you don't need the if-statement. Second the statement in the loop should be < 4 instead of <= 4.

public class ExamsFor4 {

public static void main(String[] arguments) {


int inputNumber; // One of the exams input by the user.
int sum = 0;     // The sum of the exams.
int i;       // Number of exams.
Double Avg;      // The average of the exams.

TextIO.put("Please enter the first exam: ");        // get the first exam.
inputNumber = TextIO.getlnInt();    

for ( i = 1; i < 4; i++ ) {  

    sum += inputNumber;                 // Add inputNumber to running sum.
    TextIO.put("Please enter the next exam: ");     // get the next exam.   
    inputNumber = TextIO.getlnInt();
} 

Avg = ((double)sum) / i;
TextIO.putln();
TextIO.putln("The total sum for all " + i +" exams is " + sum);
TextIO.putf("The average for the exams entered is %1.2f.\n", Avg);
break;

}   // end main ()
}

Just making few changes in your code makes it work. But you should follow cleaner approach as proposed in some of answers.

public class ExamsFor4 {

  public static void main(String[] arguments) {
    int inputNumber; // One of the exams input by the user.
    int sum = 0;     // The sum of the exams.
    int i;       // Number of exams.
    double Avg;      // The average of the exams.

    TextIO.put("Please enter the first exam: ");        // get the first exam.
    inputNumber = TextIO.getlnInt();    
    sum += inputNumber;

    for ( i = 1; i < 4; i++ ) {  

      TextIO.put("Please enter the next exam: ");     // get the next exam.   
      inputNumber = TextIO.getlnInt();
      sum += inputNumber;                 // Add inputNumber to running sum.

    } 

    Avg = ((double)sum) / i;
    TextIO.putln();
    TextIO.putln("The total sum for all " + i +" exams is " + sum);
    TextIO.putf("The average for the exams entered is %1.2f.\n", Avg);

  }   // end main ()
}  // end class ExamsFor4

import java.util.Scanner;
public class ExamsFor4 {

public static void main(String[] arguments) {

    int sum = 0; // The sum of the exams.
    int i = 1; // Number of exams.
    double avg = 0; // The average of the exams.

    Scanner in = new Scanner(System.in);
    System.out.print("Please enter the first exam: ");
    sum += in.nextInt();
    i++;
    while(i<=4){
        System.out.print("Please enter the next exam: ");
        sum += in.nextInt();
        if(i==4)
            break;// this line is so that it wont increment an extra time.
        i++;
    }
    System.out.println("The total sum for all " + i +" exams is " + sum);
    avg = ((double)sum/i);
    System.out.println("The average for the exams entered is" + avg);
} // end main ()
} // end class ExamsFor4