add doc for foreach

114-For/for-0.gop

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+# For loop
+// Go+ has only one looping keyword: for, with several forms.

114-For/for-1.gop

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+# 1、for/<-
+// This is the most common form. You can use it with a slice, map, numeric range or custom iterators.
+//
+// The for value <- arr/map form is used for going through elements of a slice or a map.
+//
+// If an index is required, an alternative form for index, value <- arr can be used.
+
+names := ["Sam", "Peter"]
+for i, name <- names {
+	println i, name
+}
+
+m := {"one": 1, "two": 2}
+for key, val <- m {
+	println key, val
+}
+for key, _ <- m {
+	println key
+}
+
+for val <- names {
+	println val
+}
+for val <- m {
+	println val
+}

114-For/for-2.gop

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+# 2、Range for
+// You can use range expression (start:end:step) in for loop.
+
+for i <- :5 {
+	println i
+}
+
+for i <- 1:5 {
+	println i
+}
+
+for i <- 1:5:2 {
+	println i
+}

114-For/for-3.gop

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+# 3、for/<-/if
+// All loops of for/<- form can have an optional if condition.
+
+numbers := [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+for num <- numbers if num%3 == 0 {
+	println num
+}
+
+for num <- :10 if num%3 == 0 {
+	println num
+}

114-For/for-4.gop

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+# 4、Condition for
+// The condition can be omitted, resulting in an infinite loop. You can use break or return to end the loop.
+
+sum := 0
+i := 1
+for i <= 100 {
+	sum += i
+	i++
+}
+println sum
+
+for {
+	if sum > 0 {
+		break
+	}
+}

114-For/for-5.gop

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+# 5、C for
+// 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.
+
+for i := 0; i < 10; i += 2 {
+	if i == 6 {
+		continue
+	}
+	println i
+}

114-For/for-6.gop

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+# break and continue
+// How do you get out of an infinite for loop without using the keyboard or
+// turning off your computer? That’s the job of the break statement. It exits the
+// loop immediately, just like the break statement in other languages. Of course,
+// you can use break with any for statement, not just the infinite for statement.
+//
+// Go+ also includes the continue keyword, which skips over rest of the body of a
+// for loop and proceeds directly to the next iteration. Technically, you don’t need
+// a continue statement.
+
+for i := 1; i <= 100; i++ {
+	if i%3 == 0 && i%5 == 0 {
+		println("FizzBuzz")
+		continue
+	}
+	if i%3 == 0 {
+		println("Fizz")
+		continue
+	}
+	if i%5 == 0 {
+		println("Buzz")
+		break
+	}
+	println(i)
+}

114-For/for-7.gop

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+# Labeling Your “for” Statements
+// By default, the break and continue keywords apply to the for loop that
+// directly contains them. What if you have nested for loops and you want to exit
+// or skip over an iterator of an outer loop? Let’s look at an example. We’re going
+// to modify our string iterating program to stop iterating through a string as
+// soon as it hits a letter “l”
+
+samples := []string{"hello", "apple_π!"}
+outer:
+	for _, sample := range samples {
+		for i, r := range sample {
+			println(i, r, string(r))
+			if r == 'l' {
+				continue outer
+			}
+		}
+		println()
+	}

121-For-Each/for-each-1.gop

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+// In Go+ there is no foreach loop instead, the for loop can be used as “foreach“. There is a keyword range, you can combine for and range together and have the choice of using the key or value within the loop.

121-For-Each/for-each-2.gop

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+# The for-range Statement
+// What makes a for-range loop interesting is that you get two loop variables.
+// The first variable is the position in the data structure being iterated, while the
+// second is value at that position. The idiomatic names for the two loop variables
+// depend on what is being looped over. When looping over an array, slice, or
+// string, an i for index is commonly used. When iterating through a map, k (for
+// key) is used instead.
+//
+// The second variable is frequently called v for value, but is sometimes given a
+// name based on the type of the values being iterated.
+//
+// If you don’t need to access the key, use an underscore (_) as the variable’s name. This tells Go+ to ignore the
+// value.
+
+evenVals := []int{2, 4, 6, 8, 10, 12}
+for _, v := range evenVals {
+	println(v)
+}

121-For-Each/for-each-3.gop

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+# Iterating Over Maps
+// There’s something interesting about how a for-range loop iterates over a
+// map.
+
+m := map[string]int{
+	"a": 1,
+	"c": 3,
+	"b": 2,
+}
+for i := 0; i < 3; i++ {
+	println("Loop", i)
+	for k, v := range m {
+		println(k, v)
+	}
+}