Note
- An if expression has a value.
- A block has a value—the value of its last expression.
- The Scala for loop is like an “enhanced” Java for loop.
- for ( i -> 0 to n)
- for ( i -> 0 until n)
- Semicolons are (mostly) optional.
- Need when: more than one statements on a single line
- The void type is Unit.
- Avoid using return in a function.
- With a recursive function, you must specify the return type.
- Beware of missing = in a function definition.
- Because the procedure doesn’t return any value, we omit the = symbol
- It is a common error to accidentally omit the = in a function definition.You then get an error message at the point where the function is called, and you are told that Unit is not acceptable at that location.
- Exceptions work just like in Java or C++, but you use a “pattern matching” syntax for catch.
- Scala has no checked exceptions.
- Variable Arguments
- The remedy is to tell the compiler that you want the parameter to be considered an argument sequence. Append : _*, like this:
val s = sum(1 to 5: _*) // Consider 1 to 5 as an argument sequence
- The remedy is to tell the compiler that you want the parameter to be considered an argument sequence. Append : _*, like this:
Exercises
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The signum of a number is 1 if the number is positive, –1 if it is negative, and 0 if it is zero. Write a function that computes this value.
Answer:
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5def (i:Int)={
if (i>0) 1 else {
if (i<0) -1 else 0
}
} -
What is the value of an empty block expression {}? What is its type?
Answer: Unit -
Come up with one situation where the assignment x = y = 1 is valid in Scala.
(Hint: Pick a suitable type for x.)Answer: x is type of Unit
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3var y = 1
var x = {}
x = y = 1 -
Write a Scala equivalent for the Java loop
for (int i = 10; i >= 0; i–) System.out.println(i);Answer:
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5for(i<- 0 to 10) println(10-i)
for(i <- 0 to 10 reverse)print(i) -
Write a procedure countdown(n: Int) that prints the numbers from n to 0.
Answer:
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9def countdown(n:Int) {
for(i<- 0 to n reverse) print(i)
}
// another version:
def countdown(n:Int) {
0 to n reverse foreach print
} -
Write a for loop for computing the product of the Unicode codes of all letters in a string. For example, the product of the characters in “Hello” is 825152896.
Answer:
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var result: Long = 1
for(c<-str) result *= c.toLong
result
}
- Solve the preceding exercise without writing a loop. (Hint: Look at the StringOps Scaladoc.)
Answer:
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var result: Long = 1
str.foreach(result *= _.toLong)
result
}
Write a function product(s : String) that computes the product, as described in the preceding exercises.
same with above
Make the function of the preceding exercise a recursive function
Answer:
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if(str.length == 1) return str.head.toLong
else str.head.toLong * productUcode2(str.tail)
}Write a function that computes xn, where n is an integer. Use the following recursive definition:
• xn = y2 if n is even and positive, where y = xn / 2.
• xn = x· xn – 1 if n is odd and positive.
• x0 = 1.
• xn = 1 / x–n if n is negative.
Don’t use a return statement.Answer:
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if (n==0) 1.toDouble else {
if (n < 0) 1/computesXn(x, -n) else {
if (n%2 == 0) {
val y = computesXn(x, n/2); y*y
} else {
x*computesXn(x, n-1)
}
}
}
}
//
def computesXn(x:Double, n:Int):Double = {
if (n==0) 1.toDouble
else if (n>0 && n%2==0) {
val y = computesXn(x, n/2); y*y
}
else if (n>0 && n%2==1) {
x*computesXn(x, n-1)
}
else 1/computesXn(x, -n)
}
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