CODEWITHSUNDEEP
stringcastingschemesubstring
Jeremiah
Jeremiah

Reputation: 75

Scheme strings manipulation and defining procedure

I need help solving this problem as I have no idea how to solve this

Define a procedure pick that takes a non-empty string s and a non-negative integer i, and returns a substring of length one consisting of the character at index i in s. Assume that i is a legal index for the string s.

~ (pick "cat" 0)

"c"

~ (pick "cat" 1)

"a"

~ (pick "cat" 2)

"t"

Upvotes: 0

Views: 1124

Answers (2)

soegaard
soegaard

Reputation: 31147

One solution is to use substring.

> (substring "Scheme" 1 3)
"ch"

Here the letters of the word "Scheme" are numbered.

  • The letter S has index 0.
  • The letter c has index 1.
  • The letter h has index 2.
  • The letter e has index 3.
  • The letter m has index 4.
  • The letter e has index 5.

The expression (substring "Scheme" 1 3) makes a new string with the letters numbered from 1 to 2. The index 3 is the first index not in the new string.

Since you want to pick out just one character, try this:

> (substring "Scheme" 1 2)
"c"
> (substring "Scheme" 2 3)
"h"

Now you can define your own function:

(define (pick a-string i)
    (substring a-string ??? ???))

What should the ??? be?

Upvotes: 1

Óscar López
Óscar López

Reputation: 236004

Try this, it's simple using standard Scheme procedures:

(define (pick str idx)
  (string (string-ref str idx)))

Here's how it works: string-ref returns a character from the string at the given index, and the string procedure turns the character into a string of length one.

There's no real need to use the substring procedure (because we only need a single character), but that's a valid alternative, too.

Upvotes: 1

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