如何结合switch和if else语句

Question

I'm taking an online java class and the teacher has asked for the following: Write a menu program that ask a user for a number indicating the four basic math equations(addition, subtraction, multiplication, division). Using a if/else structure to do the required operation, ask the user for inputs and solve the equation. I am new to this, but I don't see the point of the if/else, but I guess that is irrelavent.

If I use case 1, 2, 3, 4 as below to determine the operation, how do I refference the case with a 'If statement'?

int userSelection;
double x;        
double y;        
double answer;

switch ( userSelection ) {
   case 1:
        TextIO.putln("Let's add! ");
        TextIO.putln();
        TextIO.putln("Enter the first number: ");
        x = TextIO.getlnDouble();
    TextIO.putln("Enter the second number: ");
        y = TextIO.getlnDouble();
        break;

//I thought I could use the 'If' like this

        if (case 1);  // I don't know how to refference this correctly
        answer = x + y;
        TextIO.putln( x 'plus' y 'equals' answer);

Thanks in advance!

Answer 1

Here's the presudo code for a rewrite of a CASE statement:

IF (Case==1) THEN
  *What to do when Case==1.*
ELSE IF (Case==2) THEN
  *What to do when Case==2.*
ELSE IF (Case==3) THEN
  *What to do when Case==3.*
ELSE IF (Case==4) THEN
  *What to do when Case==4.*
END IF

You'll have to code this in Java though. :) Good luck with your assignment.

Answer 2

该switch用于针对单个给定的变量/表达式，然后使用它的值，但由于您没有测试单个变量/表达式，这不适用，您应该使用if-elseif-else结构。

Answer 3

The switch case statements is a structural alternative to using nested if..else statements where you want to run differnet logic for different values of a numerical or enumerical data type. It does not combine with if..else and the 2 are different approaches to solve conditional problems. Which one is better suited depends on the condition you are evaluating.

If I understand your code correctly, then I think you are looking for something like this:

switch ( userSelection ) {  
case 1:  
    TextIO.putln("Let's add! ");  
    TextIO.putln();  
    TextIO.putln("Enter the first number: ");  
    x = TextIO.getlnDouble();  
    TextIO.putln("Enter the second number: ");  
    y = TextIO.getlnDouble();  
    answer = x + y;    
    TextIO.putln( x + " plus " + y + " equals " + answer);   
    break;  
case 2:  
    ... //oh, I don't know. but I would guess: let's subtract!  
    break; 
case 3:  
    ... //let's multiply!
    break; 
case 4:  
    ... //let's divide! 
    break; 

}

Answer 4

You're switching on userSelection so userSelection already has the value you need; you can reference the value by referencing the userSelection variable:

if (userSelection == 1) {
    // add
} else if (userSelection == 2) {
   // subtract
} else if (userSelection == 3) {
   // multiply
} else if (userSelection == 2) {
   // divide
}

Answer 5

It would appear to me that your instructor does not want you to use the switch statement to begin with, but simply use the if flow control structure:

if(booleanExpression)
{

}
else if(booleanExpression2)
{

}