R Decision Making

The R language provides the following types of decision-making statements:

• if statement
• if...else statement
• switch statement

if statement

An if statement consists of a Boolean expression followed by one or more statements.

The syntax is as follows:

if(boolean_expression) {
    // 布尔表达式为真将执行的语句
}

If the Boolean expression boolean_expression is true, the code inside will be executed; if it is false, it will not be executed.

Executing the above code produces the following result:

[1] "X 是一个整数"

if...else statement

An if statement can be followed by an optional else statement, which executes when the Boolean expression is false.

The syntax is as follows:

if(boolean_expression) {
    // 如果布尔表达式为真将执行的语句
} else {
    // 如果布尔表达式为假将执行的语句
}

If the Boolean expression boolean_expression is true, the code inside the if block is executed. If the Boolean expression is false, the code inside the else block is executed.

Executing the above code produces the following result:

[1] "包含 example"

If there are multiple conditions to evaluate, you can use if...else if...else:

if(boolean_expression 1) {
    // 如果布尔表达式 boolean_expression 1 为真将执行的语句
} else if( boolean_expression 2) {
    // 如果布尔表达式 boolean_expression 2 为真将执行的语句
} else if( boolean_expression 3) {
    // 如果布尔表达式 boolean_expression 3 为真将执行的语句
} else {
    // 以上所有的布尔表达式都为 false 时执行
}

Executing the above code produces the following result:

[1] "第二个 if 包含 example"

switch statement

A switch statement allows testing a variable for equality against a list of values. Each value is called a case.

The syntax is as follows:

switch(expression, case1, case2, case3....)

switchThe statement must follow the rules below:

• switchIn the statement,expressionis a constant expression, which can be an integer or a string. If it is an integer, it returns the value at the corresponding case position; if the integer is not within the range of positions, it returns NULL.
• If multiple values match, the first one is returned.
• expressionIf it is a string, it corresponds to the value of the variable name in the case; if there is no match, no value is returned.
• switch has no default argument available.

The following example returns the third value:

Executing the above code produces the following result:

[1] "taobao"

If it is a string, the value corresponding to the string variable is returned:

Executing the above code produces the following result:

[1] "www.example.com"

If the integer is not within the range, NULL is returned.

Other extensions