F # Tutorials: recursion

So far we've learned about if, for, and while, so we can try to write some simple programs.

Let's try to write a program that calculates the sum from 1 to the specified number. First defines a sumtotal function that reads the specified value from the keyboard as an input parameter.

#light
let　SumTotal　n　=
　　　　　　　　 let　mutable　ttl　=　0
　　　　　　　　 for　i　=　1　to　n　do
　　　　　　　　　　　　　 ttl　<-　ttl　+　i
　　　　　　　　 ttl
let　s　=　stdin.ReadLine()
let　num　=　System.Int32.Parse(s)
printfn　"%d"　(SumTotal　num)

Using mutable, you can make the TTL a variable variable.

There is a line of code that has only TTL, and the same function as C # 's return TTL.

The input of the keyboard can be done by stdin. ReadLine implementation.

Since it is not known how to transform from string to numerical, use the int32.parse of the. NET Framework first.

While this is the requirement, you should avoid using the re-assignment of variables as much as possible in F #. Let's try rewriting the code. This seems to work only with recursion. You should use the REC keyword when defining recursive functions.

let　rec　SumTotal　n　=
　　　　　　　　 if　n　=　1　then　1
　　　　　　　　 else　n　+　SumTotal　(n-1)
let　s　=　stdin.ReadLine();
let　num　=　System.Int32.Parse(s)
printfn　"%d"　(SumTotal　num)

Look at this kind of code, is not the mathematical definition of summation is very similar ah?

x　=　1　：　f(x)　=　1
x　>　1　：　f(x)　=　x　+　f(x-1)

With such a definition it is easy to get the SumTotal function mentioned above.

If the code in the Else section is rewritten as:

else n + sumtotal n-1

This can cause infinite loops and cause stackoverflow. The same is true for PRINTFN parameters in the last line. It seems that the operators of F # are prioritized in sequence, or somewhat biased with the feeling.