Help please

[Derian]

i've been trying to find out what's wrong with my program(to invert a string) :

program invert;
 var y,i,c : integer;
 cad,aux,suma: string;
begin
writeln('entry char');
readln(cad);
suma:=' ';
aux:=' ';
y:=length(cad);
for i:=1 to y do
 cad:=aux[y-i+1];                                            (line 13)
for c:=1 to y do
suma:= suma + aux[c];
write('the inverted string is',suma);
end.

Project "project1"  succesfully built


the error what i get is : Project project1 raised exception class 'External:SIGSEV'.
In file
'C:\Users\Derian\AppData\Local\Temp\project1.lpr' at line 13

[Derian]

in line 13 it says  cad [  i ] :=aux[y-i+1];

[engkin]

Notice that:

Code: [Select]

aux:=' ';
means any access to aux[?] is going to cause an error.

Before you get something from aux, you need to put something in first. Maybe you mean to use "cad" instead of "aux"?

[Derian]

aux is an empty string , where i am going to put the invert string

[engkin]

But on line 13, aux is empty and you are reading from it!

[taazz]

as it is now you try to put it in cad. an empty string means no data at all if you want to have something give it a length first eg call setlength(aux,length(cad)); that will give it enough memory for you to put things in it.

[Derian]

well i tried this :
program invert;
 var y,i,c : integer;
 cad,aux,suma: string;
begin
writeln('entry char');
readln(cad);
suma:=' ';
y:=length(cad);
for i:=1 to y do
 cad[ i ]:=aux[y-i+1];
for c:=1 to y do
suma:= suma + aux[c];
write('the inverted string is',suma);
end.
 
i get the same error in the same line

[engkin]

@Taazz
It is not empty, it has a space. We did not see it because the code is not enclosed in [ code ] section. So aux has a length for one char

@Derian
This line: cad[ i ]:=aux[y-i+1]; means take one char from aux at y-i+1 and put it in cad at i. Is that what you want?

[Derian]

eg
if cad = hello
cad[1]:= aux[5-1+1]
then the 1st char from cad would be the last for aux
cad[2]:=aux[5-2+1]
2nd char would be the 4th chat of aux .....
 i try to get this
cad = hello
aux = olleh

[engkin]

if cad = hello
cad[1]:= aux[5-1+1]
then the 1st char from cad would be the last for aux

That's not correct. An assignment ":=" changes the left side not the right side. cad[1] will get changed if there is any thing in aux at [5-1+1] or [5], not the opposite. AND you have another problem, aux has only one char. So using [5] on aux will give you an error.

[ChrisF]

Replace

Code: [Select]

for i:=1 to y do
 cad[ i ]:=aux[y-i+1];

with

Code: [Select]

setlength(aux,y);
for i:=1 to y do
 aux[ i ]:=cad[y-i+1];

No need to say it's a very complicated way to get this result...

[Derian]

 program invert;
 var y,i,c : integer;
 cad,aux: string;
begin
writeln('entry char');
readln(cad);
y:=length(cad);
setlength(aux,y);
for i:=1 to y do
 aux[ i ]:=cad[y-i+1];
writeln('the invert string is',aux)
end.

now i get another error :     unit1.pas(1,1) Fatal: Syntax error, "UNIT" expected but "PROGRAM" found
       

[engkin]

Start a new application:
Project - New Project - choose Simple Program, then paste your code in.

[Derian]

this is my final code:
program project3;
 var y,i : integer;
 cad,aux,inv: string;
begin
writeln('entry char');
readln(cad);
y:=length(cad);
setlength(aux,y);
for i:=1 to y do
aux[ i ]:=cad[y-i+1];
inv:=aux;
writeln('the invert string is ',inv)
end.
 
it runs perfectly now , thank you all , sorry if ive been annoying                 

[engkin]

Well done! and welcome to Lazarus/FPC.