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Switch

Oct 30, 20249 min read

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Switch Statements

Multiple equality comparisons

  • In some situations, your program will need to test if a variable is equal to one of several values, and perform a different action based on which value the variable matches

  • For example, you have an int variable named month containing a month number, and want to convert it to a string with the name of the month. This means your program needs to take a different action depending on whether month is equal to 1, 2, 3, … or 12:

    “A flowchart representation of the mapping between month number and name”

  • One way to do this is with a series of if-else-if statements, one for each possible value, like this:

    Console.WriteLine("Enter the month as a number between 1 and 12.");
int month = int.Parse(Console.ReadLine());
string monthName;
if(month == 1)
{
    monthName = "January";
}
else if(month == 2)
{
    monthName = "February";
}
else if(month == 3)
{
    monthName = "March";
}
else if(month == 4)
{
    monthName = "April";
}
else if(month == 5)
{
    monthName = "May";
}
else if(month == 6)
{
    monthName = "June";
}
else if(month == 7)
{
    monthName = "July";
}
else if(month == 8)
{
    monthName = "August";
}
else if(month == 9)
{
    monthName = "September";
}
else if(month == 10)
{
    monthName = "October";
}
else if(month == 11)
{
    monthName = "November";
}
else if(month == 12)
{
    monthName = "December";
}
else
{
    monthName = "Error!"; // Invalid month
}
Console.WriteLine("The number " + month + " corresponds to the month " + monthName + ".")

  • This code is very repetitive, though: every else if statement is almost the same, with only the number changing. The text “if(month ==” is copied over and over again.

Syntax for switch statements

  • A switch statement is a simpler, easier way to compare a single variable against multiple possible values

  • It is written like this:

    switch (<variable name>)
{
    case <value 1>:
        <statement block 1>
        break;
    case <value 2>:
        <statement block 2>
        break;
    …
    default:
        <statement block n>
        break;
}

  • First, the “header” of the switch statement names the variable that will be compared

  • The “body” of the switch statement is enclosed in curly braces, and contains multiple case statements

  • Each case statement gives a possible value the variable could have, and a block of statements to execute if the variable equals that value. Statement block 1 is executed if the variable is equal to value 1, statement block 2 is executed if the variable is equal to value 2, etc.

  • The statement “block” within each case is not enclosed in curly braces, unlike if and else if blocks. Instead, it begins on the line after the case statement, and ends with the keyword break.

  • The default statement is like the else statement: It defines code that gets executed if the variable does not match any of the values in the case statements.

  • The values in each case statement must be literals, not variables, and they must be unique (you cannot write two case statements with the same value)

Example switch statement

  • This program has the same behavior as our previous example, but uses a switch statement instaed of an if-else-if statement:

    Console.WriteLine("Enter the month as a number between 1 and 12.");
int month = int.Parse(Console.ReadLine());
string monthName;
switch(month)
{
    case 1:
        monthName = "January";
        break;
    case 2:
        monthName = "February";
        break;
    case 3:
        monthName = "March";
        break;
    case 4:
        monthName = "April";
        break;
    case 5:
        monthName = "May";
        break;
    case 6:
        monthName = "June";
        break;
    case 7:
        monthName = "July";
        break;
    case 8:
        monthName = "August";
        break;
    case 9:
        monthName = "September";
        break;
    case 10:
        monthName = "October";
        break;
    case 11:
        monthName = "November";
        break;
    case 12:
        monthName = "December";
        break;
    default:
        monthName = "Error!"; // Invalid month
        break;
}
Console.WriteLine("The number " + month + " corresponds to the month " + monthName + ".")

  • Since the variable in the switch statement is month, each case statement means, effectively, if (month == <value>). For example, case 1: has the same effect as if (month == 1)

  • The values in each case statement must be int literals, since month is an int

  • The default statement has the same effect as the final else in the if-else-if statement: it contains code that will be executed if month did not match any of the values

switch with multiple statements

  • So far, our examples have used only one line of code in each case

  • Unlike if-else, you do not need curly braces to put multiple lines of code in a case

  • For example, imagine our “months” program needed to convert a month number to both a month name and a three-letter abbreviation. The switch would look like this:

    string monthName;
string monthAbbrev;
switch(month)
{
    case 1:
        monthName = "January";
        monthAbbrev = "Jan";
        break;
    case 2:
        monthName = "February";
        monthAbbrev = "Feb";
        break;
    // and so on, with all the other months...
}

  • The computer knows which statements are included in each case because of the break keyword. For the “1” case, the block of statements starts after case 1: and ends with the break; after monthAbbrev = "Jan";

Intentionally omitting break

  • Each block of code that starts with a case statement must end with a break statement; it will not automatically end at the next case statement

    • The case statement only defines where code execution starts when the variable matches a value (like an open {). The break statement defines where it ends (like a close }).

  • However, there is one exception: A case statement with no body (code block) after it does not need a matching break

  • If there is more than one value that should have the same behavior, you can write case statements for both values above a single block of code, with no break between them. If either one matches, the computer will execute that block of code, and then stop at the break statement.

  • In a switch statement with this structure:

    switch(<variable>)
{
    case <value 1>:
    case <value 2>:
        <statement block 1>
        break;
    case <value 3>:
    case <value 4>:
        <statement block 2>
        break;
    default:
        <statement block 3>
        break;
}

    The statements in block 1 will execute if the variable matches value 1 or value 2, and the statements in block 2 will execute if the variable matches value 3 or value 4.

  • For example, imagine our program needs to tell the user which season the month is in. If the month number is 1, 2, or 3, the season is the same (winter), so we can combine these 3 cases. This code will correctly initialize the string season:

    switch(month)
{
    case 1:
    case 2:
    case 3:
        season = "Winter";
        break;
    case 4:
    case 5:
    case 6:
        season = "Spring";
        break;
    case 7:
    case 8:
    case 9:
        season = "Summer";
        break;
    case 10:
    case 11:
    case 12:
        season = "Fall";
        break;
    default:
        season = "Error!"
        break;
}

    If month is equal to 1, execution will start at case 1:, but the computer will continue past case 2 and case 3 and execute season = "Winter". It will then stop when it reaches the break, so season gets the value “Winter”. Similarly, if month is equal to 2, execution will start at case 2:, and continue until the break statement, so season will also get the value “Winter”.

  • This syntax allows switch statements to have conditions with a logical OR, equivalent to an if condition with an ||, like if(x == 1 || x == 2)

  • For example, the “seasons” statement could also be written as an if-else-if with || operators, like this:

    if(month == 1 || month == 2 || month == 3)
{
    season = "Winter";
}
else if(month == 4 || month == 5 || month == 6)
{
    season = "Spring";
}
else if(month == 7 || month == 8 || month == 9)
{
    season = "Summer";
}
else if(month == 10 || month == 11 || month == 12)
{
    season = "Fall"
}
else
{
    season = "Error!"
}

Scope and switch

  • In C#, the scope of a variable is defined by curly braces (recall that local variables defined in a method have a scope that ends with the } at the end of the method)

  • Since the case statements in a switch do not have curly braces, they are all in the same scope: the one defined by the switch statement’s curly braces

  • This means you cannot declare a “local” variable within a case statement — it will be in scope (visible) to all the other case statements

  • For example, imagine you wanted to use a local variable named nextMonth to do some local computation within each case in the “months” program. This code will not work:

    switch(month)
{
    case 1:
        int nextMonth = 2;
        monthName = "January";
        // do something with nextMonth...
        break;
    case 2:
        int nextMonth = 3;
        monthName = "February";
        // do something with nextMonth...
        break;
    //...
}

    The line int nextMonth = 3 would cause a compile error because a variable named nextMonth already exists — the one declared within case 1.

Limitations of switch

  • Not all if-else-if statements can be rewritten as switch statements

  • switch can only test equality, so in general, only if statements whose condition uses == can be converted to switch

  • For example, imagine we have a program that determines how much of a fee to charge a rental car customer based on the number of miles the car was driven. A variable named mileage contains the number of miles driven, and it is used in this if-else-if statement:

    decimal fee = 0;
if(mileage > 1000)
{
    fee = 50.0M;
}
else if(mileage > 500)
{
    fee = 25.0M;
}

  • This if-else-if statement could not be converted to switch(mileage) because of the condition mileage > 1000. The switch statement would need to have a case for each number greater than 1000, which is infinitely many.

Graph View

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