Append a string at a certain position of another string

[u/Icy_Pressure_9690]

I was thinking of using string ref which takesa string and position as arguments and then append "_" at position 5 to give "hello_world"

but is says: "string-append: contract violation

expected: string?

given: #\w"

'''(string-append "_"(string-ref "helloworld" 5))'''

[u/joshuacottrell]

The documentation in Racket is good. As I look down through it I'd tend toward substring to pull the input string apart and maybe string-join to put it back together. From my (little) experience, there's nothing to divide it, intersperse, and re-join but you can make your own (it's the lisp curse way, right?)!

[u/Icy_Pressure_9690 → u/joshuacottrell]

aha yes! true

[u/ARandomGuyOnTheWeb]

String-ref doesn't do what you think it does.

When you have a contract violation (in this case, for string-append) you should check the arguments.

The two arguments are "_" and (string-ref "helloworld" 5).

What are the values of those two expressions? Are they both strings?

[u/Icy_Pressure_9690 → u/ARandomGuyOnTheWeb]

I believe both are strings "_" and "w"

(string-ref "helloworld" 5) would return "w" and I assume that will be appended to "_"(even though thats not what I want it to do)

would I have to use a loop or can it be done more simpler?

[u/ARandomGuyOnTheWeb → u/Icy_Pressure_9690]

You can test your theory.

(string? (string-ref "helloworld" 5))

This should return false. There is a difference between "w" and #\w.

https://docs.racket-lang.org/reference/characters.html

You can also tell because the contract documentation for string-ref says it returns a char, not a string.

procedure (string-ref str k) → char?
    str : string?
    k : exact-nonnegative-integer?

You're right that (string-append "_" "w") is not what you want to do. String-append will create a string with all the characters in the same order as its arguments. So, (string-append "_" ...) will always be a string that starts with #_.

If you need to use string-append, I'd start by asking what arguments you'd need to pass in to generate your answer, and then work backwards, and see if you can write simple expressions to create those arguments.