Replacing only 1 string in a text

[WebAtelier- mt]

How can I replace only 1 string in a text?

For example "Hello world". When I use the function /replace all text/segment/replacement/ and set "l" to "x" the text would be "Hexxo worxd"

But when I only want to change the third "l" that doesn't work. The text should be for example "Hello worxd"

Even when I use the segment start function I can only replace all occurrences...

[Taifun]

you can write your own custom procedure to replace only the n th character

http://appinventor.mit.edu/explore/ai2/support/blocks/procedures.html

Taifun 

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Trying to push the limits of App Inventor! Snippets, Tutorials and Extensions from Pura Vida Apps by Taifun. 

[Abraham Getzler]

See attached for how to do it.

The $ in the function names are a homage to BASIC, which uses $

in its text function names.

ABG

[Taifun]

thank you ABG!

@Hossein: what about adding a method like that into your TextUtils extension?

https://amerkashi.wordpress.com/2017/03/09/textutils-extension/

Taifun

[WebAtelier- mt]

Thanks,

but I'm looking for some more diffuicult solution.

I have a list "1,2,3,4,5,6,7,8,9,0,a,b,c,d,,A,B,C,D,?,(,(" and a second with "a,b,c,d,e,f,g,h,i,j,...1,2,3,4,5..."

I now want to replace the things in the first list with the things in a second list. For example 1 becomes a, 2 becomes c etc.

I've already done it wit a text that I sent to a list. Then I replaced the things. (If I only have letters, it works)

But there's somehow a problem, if numbers and letter are mixed. So it comes to a wrong result...

Any idea?

[Italo]

You forgot the screenshot of your blocks. You say it works with letters but not with numbers, so there must be something to fix but without looking at your blocks, probably no one can help.

[Abraham Getzler]

This looks like a substitution cipher, where the two lists are just columns 1 and 2 in a map table.

There is a very handy lookup in pairs block that can do single letter translations from such a map.

result procedure ENCODE(text_input, map) returns(text_output)

 result text_output = ""

 for i = 1 to length(text_input)

   set local letter to segment(text_input,i,1)

   set text_output to JOIN(text_output, lookup_in_pairs(map,letter))

end for

return text_output

end procedure ENCODE

Use a spreadsheet to prepare your map,

export it as a csv file, upload it to Media drawer,

load it from file //map.csv at startup.

ABG