# Control Flow

Examples in V
``````a := 10
b := 20
if a < b {
println('\$a < \$b')
} else if a > b {
println('\$a > \$b')
} else {
println('\$a == \$b')
}

// if can be used as an expression:
num := 777
s := if num % 2 == 0 {
'even'
}
else {
'odd'
}
println(s) // "odd"``````
``````// V has only one looping construct: for.
numbers := [1, 2, 3, 4, 5]
for num in numbers {
println(num)
}
names := ['Sam', 'Peter']
for i, name in names {
println('\$i) \$name')  // Output: 0) Sam
}                         //         1) Peter

// The for value in loop is used for going through elements of an array. If an index is required, an alternative form for index, value in can be used.
// Note, that the value is read-only. If you need to modify the array while looping, you have to use indexing:
mut numbers2 := [1, 2, 3, 4, 5]
for i, num in numbers2 {
println(num)
numbers2[i] = 0
}

// This form of the loop is similar to while loops in other languages.
// The loop will stop iterating once the boolean condition evaluates to false.
// Again, there are no parentheses surrounding the condition, and the braces are always required.
mut sum := 0
mut i := 0
for i <= 100 {
sum += i
i++
}
println(sum) // "5050"

// The condition can be omitted, this results in an infinite loop.
mut num := 0
for {
num++
if num >= 10 {
break
}
}
println(num) // "10"

// Finally, there's the traditional C style for loop. It's safer than the `while` form because with the latter it's easy to forget to update the counter and get stuck in an infinite loop.
// Here i doesn't need to be declared with mut since it's always going to be mutable by definition.
for i := 0; i < 10; i++ {
// Don't print 6
if i == 6 {
continue
}
println(i)
}``````
``````// A match statement is a shorter way to write a sequence of if - else statements.
// When a matching branch is found, the following statement block will be run, and the final expression will be returned.
// The else branch will be evaluated when no other branches match.
os := 'windows'
print('V is running on ')
match os {
'darwin' { println('macOS.') }
'linux'  { println('Linux.') }
else     { println(os) }
}

number := 1
s := match number {
1    { 'one' }
2    { 'two' }
else {
println('this works too')
'many'
}
}

// A match statement can also be used to branch on the variants of an enum by using the shorthand .variant_here syntax.
enum Color {
red
blue
green
}
fn is_red_or_blue(c Color) bool {
return match c {
.red { true }
.blue { true }
else { false }
}
}``````
``````Last Run  :
================ V panic ================
module: builtin
function: get()
file: /home/ubuntu/code_runner/root_volume/v/vlib/builtin/array.v
line: 153
message: array.get: index out of range (i == -1, a.len == 0)
=========================================
| 0x56059535eb5a | /code/root/v/v(panic_debug+0x131)
| 0x56059535d740 | /code/root/v/v(array_get+0xaf)
| 0x5605953a71c3 | /code/root/v/v(compiler__CGen_insert_before+0x60)
| 0x5605953cc01b | /code/root/v/v(compiler__Parser_match_statement+0x1c4)
| 0x56059539477c | /code/root/v/v(compiler__Parser_statement+0x458)
| 0x56059538fde4 | /code/root/v/v(compiler__Parser_parse+0xe69)
| 0x5605953d0089 | /code/root/v/v(compiler__V_parse+0x15b)
| 0x5605953d0306 | /code/root/v/v(compiler__V_compile+0x261)
| 0x5605953f4eb6 | /code/root/v/v(main__main+0x649)
| 0x5605953f788b | /code/root/v/v(main+0x68)
| 0x7fee2618cb97 | /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xe7)
| 0x56059535ceea | /code/root/v/v(_start+0x2a)
``````
``````// A defer statement defers the execution of a block of statements until the end of scope or until surrounding function returns.