Styjun

What can we do to stop prior company from asking us questions?

To not tell, not take, and not want

How does the mv command work with external drives?

Should I tutor a student who I know has cheated on their homework?

How did the Bene Gesserit know how to make a Kwisatz Haderach?

Why do remote companies require working in the US?

Bold, vivid family

How do I go from 300 unfinished/half written blog posts, to published posts?

In excess I'm lethal

Example of a Mathematician/Physicist whose Other Publications during their PhD eclipsed their PhD Thesis

How do I transpose the 1st and -1th levels of an arbitrarily nested array?

What happens if you roll doubles 3 times then land on "Go to jail?"

How to solve a differential equation with a term to a power?

Received an invoice from my ex-employer billing me for training; how to handle?

What flight has the highest ratio of time difference to flight time?

Would a galaxy be visible from outside, but nearby?

If a black hole is created from light, can this black hole then move at speed of light?

What is "(CFMCC)" on an ILS approach chart?

Which kind of appliances can one connect to electric sockets located in a airplane's toilet?

Is 'diverse range' a pleonastic phrase?

Why do airplanes bank sharply to the right after air-to-air refueling?

Interfacing a button to MCU (and PC) with 50m long cable

Real integral using residue theorem - why doesn't this work?

Unreliable Magic - Is it worth it?

Printing first letter of an atom Prolog

1

Print the first letter of two atoms in a list. I can't even get the first letter of one of the two atoms in the list to print.

grab_letter([],[]).
grab_letter([A],[B]) :- A = [First|_], B = [Second|_]. 

?- grab_letter([apple,pie]).

true ? 

How do I get it to print "a"?

prolog

share|improve this question

edited Mar 21 at 17:06

false

10.4k773151

asked Mar 21 at 4:18

SuguruDabSuguruDab

64

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  atom_chars/2 is your friend !
  
  – joel76
  Mar 21 at 8:20
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  What about ''? There is an atom that does not have a first character.
  
  – repeat
  Mar 21 at 20:05
  

  
  
  
  

add a comment | 

1

Print the first letter of two atoms in a list. I can't even get the first letter of one of the two atoms in the list to print.

grab_letter([],[]).
grab_letter([A],[B]) :- A = [First|_], B = [Second|_]. 

?- grab_letter([apple,pie]).

true ? 

How do I get it to print "a"?

prolog

share|improve this question

edited Mar 21 at 17:06

false

10.4k773151

asked Mar 21 at 4:18

SuguruDabSuguruDab

64

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  atom_chars/2 is your friend !
  
  – joel76
  Mar 21 at 8:20
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  What about ''? There is an atom that does not have a first character.
  
  – repeat
  Mar 21 at 20:05
  

  
  
  
  

add a comment | 

Print the first letter of two atoms in a list. I can't even get the first letter of one of the two atoms in the list to print.

grab_letter([],[]).
grab_letter([A],[B]) :- A = [First|_], B = [Second|_]. 

?- grab_letter([apple,pie]).

true ? 

How do I get it to print "a"?

prolog

share|improve this question

edited Mar 21 at 17:06

false

10.4k773151

asked Mar 21 at 4:18

SuguruDabSuguruDab

64

grab_letter([],[]).
grab_letter([A],[B]) :- A = [First|_], B = [Second|_]. 

?- grab_letter([apple,pie]).

true ? 

share|improve this question

edited Mar 21 at 17:06

false

10.4k773151

asked Mar 21 at 4:18

SuguruDabSuguruDab

64

share|improve this question

edited Mar 21 at 17:06

false

10.4k773151

asked Mar 21 at 4:18

SuguruDabSuguruDab

64

edited Mar 21 at 17:06

false

10.4k773151

edited Mar 21 at 17:06

false

10.4k773151

asked Mar 21 at 4:18

SuguruDabSuguruDab

64

asked Mar 21 at 4:18

SuguruDabSuguruDab

64

1 Answer
1

active

oldest

votes

3

The ISO Prolog standard specifies a sub_atom/5 built-in predicates that can decompose an atom into a sub-atom. The first argument is the atom, the second argument is the number of characters before the sub-atom, the third argument is the length of the sub-atom, the fourth argument is the number of characters after the sub-atom, and the fifth argument is the sub-atom. For example:

| ?- sub_atom(apple, 0, 1, _, First).

First = a
yes

| ?- sub_atom(pie, 0, 1, _, First). 

First = p
yes

You can call this predicate from your code that processes the list containing the atoms. Can you give it a try and edit your question with the updated code?

Using in alternative the also standard atom_chars/2 predicate, as suggested in a comment, is not a good idea as it results in creating a temporary list (that will eventually be garbage-collected) just to access the first character.

share|improve this answer

edited Mar 21 at 8:45

answered Mar 21 at 8:40

Paulo MouraPaulo Moura

12.8k21426

add a comment | 

Your Answer

StackExchange.ifUsing("editor", function ()
StackExchange.using("externalEditor", function ()
StackExchange.using("snippets", function ()
StackExchange.snippets.init();
);
);
, "code-snippets");

StackExchange.ready(function()
var channelOptions =
tags: "".split(" "),
id: "1"
;
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function()
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled)
StackExchange.using("snippets", function()
createEditor();
);

else
createEditor();

);

function createEditor()
StackExchange.prepareEditor(
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader:
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
,
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
);



);

draft saved

draft discarded

Sign up or log in

StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);

Sign up using Google

Sign up using Facebook

Sign up using Email and Password

Post as a guest

Name

Email

Required, but never shown

StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f55273722%2fprinting-first-letter-of-an-atom-prolog%23new-answer', 'question_page');

);

Post as a guest

Name

Email

Required, but never shown

3

The ISO Prolog standard specifies a sub_atom/5 built-in predicates that can decompose an atom into a sub-atom. The first argument is the atom, the second argument is the number of characters before the sub-atom, the third argument is the length of the sub-atom, the fourth argument is the number of characters after the sub-atom, and the fifth argument is the sub-atom. For example:

| ?- sub_atom(apple, 0, 1, _, First).

First = a
yes

| ?- sub_atom(pie, 0, 1, _, First). 

First = p
yes

You can call this predicate from your code that processes the list containing the atoms. Can you give it a try and edit your question with the updated code?

Using in alternative the also standard atom_chars/2 predicate, as suggested in a comment, is not a good idea as it results in creating a temporary list (that will eventually be garbage-collected) just to access the first character.

share|improve this answer

edited Mar 21 at 8:45

answered Mar 21 at 8:40

Paulo MouraPaulo Moura

12.8k21426

add a comment | 

3

The ISO Prolog standard specifies a sub_atom/5 built-in predicates that can decompose an atom into a sub-atom. The first argument is the atom, the second argument is the number of characters before the sub-atom, the third argument is the length of the sub-atom, the fourth argument is the number of characters after the sub-atom, and the fifth argument is the sub-atom. For example:

| ?- sub_atom(apple, 0, 1, _, First).

First = a
yes

| ?- sub_atom(pie, 0, 1, _, First). 

First = p
yes

You can call this predicate from your code that processes the list containing the atoms. Can you give it a try and edit your question with the updated code?

Using in alternative the also standard atom_chars/2 predicate, as suggested in a comment, is not a good idea as it results in creating a temporary list (that will eventually be garbage-collected) just to access the first character.

share|improve this answer

edited Mar 21 at 8:45

answered Mar 21 at 8:40

Paulo MouraPaulo Moura

12.8k21426

add a comment | 

The ISO Prolog standard specifies a sub_atom/5 built-in predicates that can decompose an atom into a sub-atom. The first argument is the atom, the second argument is the number of characters before the sub-atom, the third argument is the length of the sub-atom, the fourth argument is the number of characters after the sub-atom, and the fifth argument is the sub-atom. For example:

| ?- sub_atom(apple, 0, 1, _, First).

First = a
yes

| ?- sub_atom(pie, 0, 1, _, First). 

First = p
yes

You can call this predicate from your code that processes the list containing the atoms. Can you give it a try and edit your question with the updated code?

Using in alternative the also standard atom_chars/2 predicate, as suggested in a comment, is not a good idea as it results in creating a temporary list (that will eventually be garbage-collected) just to access the first character.

share|improve this answer

edited Mar 21 at 8:45

answered Mar 21 at 8:40

Paulo MouraPaulo Moura

12.8k21426

| ?- sub_atom(apple, 0, 1, _, First).

First = a
yes

| ?- sub_atom(pie, 0, 1, _, First). 

First = p
yes

share|improve this answer

edited Mar 21 at 8:45

answered Mar 21 at 8:40

Paulo MouraPaulo Moura

12.8k21426

answered Mar 21 at 8:40

Paulo MouraPaulo Moura

12.8k21426

answered Mar 21 at 8:40

Paulo MouraPaulo Moura

12.8k21426