How to get one character at a specified index and put it in a mutablestring

Question

I am new in cocoa programing and i am stock.
I am trying to get a character in a NSString and i want to put it in a NSMutableString with the command appendString.
For now it's not working. I try with different approach but i didn't find the right way to do that. It's probably very simple but i just don't see hit.
I try with characterAtIndex and with getCharater. And now i am thinking to use charcaterRange but i don't know how to articulate an NSRange.
Maybe somebody could show an example of of what i have to do to do that.
I put this <<<<<<<<<<<<<<<< to show where in my code it's not working
Thanks a lot for your help it's very appreciate.
*The header:*
#import <Cocoa/Cocoa.h>
@interface Controleur : NSObject
NSMutableArray *touche_presser;
NSMutableArray *typedevariable;
NSMutableString *formule;
int calculencours;
int i;
int n;
IBOutlet NSTextField *affichagede_laformule;
IBOutlet NSTextField *informationdesupport;
-(void) awakeFromNib;
-(IBAction)touche_clear:(id)sender;
-(IBAction)touche_0:(id)sender;
-(IBAction)touche_1:(id)sender;
-(IBAction)touche_2:(id)sender;
-(IBAction)touche_3:(id)sender;
-(IBAction)touche_4:(id)sender;
-(IBAction)touche_5:(id)sender;
-(IBAction)touche_6:(id)sender;
-(IBAction)touche_7:(id)sender;
-(IBAction)touche_8:(id)sender;
-(IBAction)touche_9:(id)sender;
-(IBAction)touche_plus:(id)sender;
-(IBAction)touche_moins:(id)sender;
-(IBAction)touche_multiplier:(id)sender;
-(IBAction)touche_diviser:(id)sender;
-(IBAction)touche_egale:(id)sender;
-(NSString *) constructionde_laformule;
-(NSNumber *) analysed_unerafale: (NSString *) elementd_uneformule;
@end
*The Methode:*
-(NSNumber *) analysed_unerafale: (NSString *) elementd_uneformule;
// Ici, j'analyse une partie de la formule et retourne le résultat (nsnumber)
// dans la variable resultatde_l_analyse_de_larafale
int ia;
int nombrede_caractere_dans_element_de_laformule;
NSNumber *resultat_actuel;
resultat_actuel = [[NSNumber alloc]initWithInt:0];
NSNumber *resultat_precedant;
resultat_precedant = [[NSNumber alloc]initWithInt:0];
NSNumber *resultatde_l_analyse_de_larafale;
resultatde_l_analyse_de_larafale = [[NSNumber alloc]init];
NSMutableString *caractere_x;
caractere_x = [[NSMutableString alloc]init];
NSMutableString *operateur;
operateur = [[NSMutableString alloc]init];
nombrede_caractere_dans_element_de_laformule = [elementd_uneformule length];
ia = 0;
for (n=1; n <= nombrede_caractere_dans_element_de_laformule; n = n+1)
ia++;
char y = [elementd_uneformule characterAtIndex:ia]; <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< here
NSString *x = [[NSString alloc] initWithUTF8String: y]; //[NSString stringWithFormat:@"%@", y]; <<<<<<<<<<<<< and here
[caractere_x setString:@""];
[caractere_x appendString:x];
return resultatde_l_analyse_de_larafale;
@end
P.S.
Excuse my bad english i am french speaking

Answer 1

I also couldn't figure out what your code is doing, but hope this sample code will help:
NSString *firstString = @"Canada";
NSMutableString *secondString = [NSMutableString stringWithCapacity:10];
int index = random() % [firstString length];
// get a substring of length 1 at the index position of firstString
NSString *oneCharString = [firstString substringWithRange:NSMakeRange(index, 1)];
// append the single-character substring to secondString:
[secondString appendString:oneCharString];
NSLog(@"firstString=%@ index=%d secondString=%@", firstString, index, secondString);
I was testing the above when your first reply was posted. It uses the same method Et suggested, but I didn't want to just throw it away.