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To execute a block of code once or a several number of times based on a condition, you would need Control structures. Groovy supports the usual 

• Conditional statements: if-else, "nested" if then else if, switch, try-catch-finally
• Looping statements: for, for in, while loop
• Branching statements: break, continue, return

See here for more information on the Control structures.

On this page:

Conditional statements

Conditional statements execute a set of statements only if the condition is true

...

//Simple if
if(x==2){
  false
}

//Simple if else
if(x == 2){
  x = x + 2
}
else{
  x = x - 2
}

//Nested if
if (x) {
    x = x + 1
} else if (y) {
    y = y + 1
} else {
    0
}

//Assign and test in nested expression
if ((x = 3)) {
 return true
}

//Optional return statement
def sysName = "Windows"
if (sysName.contains("Windows"))
 "We're on Windows."
 else
 "Oh, well we are on Mac"

Switch statement 

The switch statement executes one statement from multiple conditions. It is like if-else-if ladder statement. The switch statement in Groovy can handle any kind of switch value and different kinds of matching can be performed.

...

def myMethod() {
 throw new IllegalArgumentException()
}
def log = []
try {
 myMethod()
} catch (Exception e) {
 log << e.toString()
} finally {
 log << "finally"
}
log.size() == 2 

Looping statements

Looping repeats the execution of a block of code multiple times. The loops available in Groovy are covered below.

while loop

In a while loop the boolean test is evaluated, and if it's true, the body of the loop is then executed.

def list = [1, 2, 3]
while (list){
  list.remove(0)
}
list

Note: There is no do-while loop in Groovy

for/for-in loop

for / for-in loop is used to iterate a part of the program several times. If the number of iteration is fixed, it is recommended to use for loop. In Groovy the for loop is much simpler and works with any kind of array, collection, Map, etc.

// Normal for loop
  for (i = 0; i < 5; i++) {
    i;
  }

// iterate over a range
def x = 0
for ( i in 0..9 ) {
    x += i
}
x == 45

// iterate over an array
def array = (0..4).toArray()
x = 0
for ( i in array ) {
    x += i
}
x == 10

//iterate over a list
def x = 0
def list = [10,12,13,14]; 
  for(j in list){
    x += j;
}
x == 49
 
//iterate over a map
def x = 0
  def customers = [
    0 : "David",
    1 : "Elle",
    2 : "Peaches"
  ];

for(cust in customers){
    x += cust.key
  }
x ==  3

//iterate over values of a Map
def x = ""
  def customers = [
    0 : "David",
    1 : "Elle",
    2 : "Peaches"
  ];

for(cust in customers){
    x += cust.value
  	x += ","
  }
x == "David,Elle,Peaches"

// iterate over the characters in a string
def text = "abc"
def list = []
for (c in text) {
    list.add(c)
}
list == ["a", "b", "c"]

...

Continue statement - The continue statement complements the break statement. Its use is restricted to while and for loops, explained above.

Return statement - The last line of a method in Groovy is automatically the return statement. For this reason, an explicit return statement can be left out.