Objective C Char Array Issue

Question

Hey guys so I'm trying to learn objective c as an independent study for my school and one of my projects just to practice is making a calculator program where the user inputs a string of numbers and operators and the program breaks it apart and does the calculation. Right now I have the input and I'm trying to have it find the operators and put them all in an array so that it can sort them to find order of operations and use the indices to find the different terms; however, when I try to print out the array to see if its holding the chars, it outputs some weird symbols like the apple logo or an upside down question mark. I've made an SSCCE of my problem -- does anyone know whats going on?

int main(int argc, const char * argv[])
{
@autoreleasepool {

    NSString *calculation;
    char str[50] = {0};
    int count = 0;

    NSLog(@"What would you like to calculate?");
    scanf("%s", str);
    calculation = [NSString stringWithUTF8String:str];

    for (int i = 0; i < [calculation length]; i++) {
        NSRange range = NSMakeRange (i, 1);
        if([[calculation substringWithRange: range] isEqualToString: @"*"] ||
           [[calculation substringWithRange: range] isEqualToString: @"/"])
            count++;
    }

    char operators[count];
    for (int i = 0; i < count; i++) {
        for (int j = 0; j < [calculation length]; j++) {
            NSRange range = NSMakeRange (j, 1);
            NSString *s = [s initWithString:[calculation substringWithRange: range]];
            if([s isEqualToString:@"*"]){
                operators[i] = '*';
                break;
            }
            if([s isEqualToString:@"/"]){
                operators[i] = '/';
                break;
            }
        }
        NSLog(@"%c", operators[i]);
    }

}
return 0;
}

Why aren't you using -characterAtIndex?

Richard J. Ross III
– Richard J. Ross III

2012-11-15 19:49:34 +00:00
Commented Nov 15, 2012 at 19:49 — Richard J. Ross III
– Richard J. Ross III, Commented Nov 15, 2012 at 19:49

NJones · Accepted Answer · 2012-11-15 20:31:22Z

1

To answer your question about what is going on in your code. The "weird symbols" you are seeing are the un-initialized values in memory at your operators[] array.

To see the reason the char[] is left with garbage values look at this section of your code:

    ...
    NSString *s = [s initWithString:[calculation substringWithRange: range]];
    if([s isEqualToString:@"*"]){
        operators[i] = '*';
        break;
    }
    ...

You are sending initWithString to s. s is not yet allocated. This results in s being a nil. And since sending a message like isEqualToString: to a nil will essentially evaluate NO even when an @"*" is in that range your assignment to operators[i] will never happen.

Easy fix though, just replace the s assignment with this:

NSString *s = [calculation substringWithRange:range];

answered Nov 15, 2012 at 20:31

NJones

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3 Comments

user529758 Over a year ago

'This results in s being a nil' - only under ARC.

Matt Cooper Over a year ago

Thanks! Thats what I originally tried but for some reason it wasn't working. It's all good now.

NJones Over a year ago

@MattCooper I'm glad this helped. I generally try not to over-advise self described learning exercises, But, RichardRoss's advice to use -characterAtIndex: is good advice. It's more efficient, it doesn't create a new NSString each time, it simply returns a unichar which you can compare with ==. And since composed character sets are unlikely to come up in a simple calculator(the main reason -characterAtIndex: is frowned upon) it should be safe.

user529758 · Accepted Answer · 2012-11-15 20:05:19Z

0

You don't need anything fancy for this. But: use stuff for what it is there for.

#import <Foundation/Foundation.h>

int main()
{
    NSAutoreleasePool *pool = [NSAutoreleasePool new];

    char str[128];
    printf("Enter formula:\n");
    scanf("%s", str); // this is DAMN unsafe, btw

    NSMutableArray *arr = [NSMutableArray array];

    char *s = str;
    while (*s) {
        if (*s == '*') {
            [arr addObject:@"*"];
        } else if (*s == '/') {
            [arr addObject:@"/"];
        }
        s++;
    }
    printf("\n");

    [arr enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        NSLog(@"%@", obj);
    }];

    [pool drain];
    return 0;
}

answered Nov 15, 2012 at 20:05

user529758

3 Comments

SpaceDust__ Over a year ago

does your code care about order of operations? what output will be for this string @"3*6/2*4" ?

user529758 Over a year ago

@SpaceDust The output will be * / *. But come on, why don't you try it?

user529758 Over a year ago

@SpaceDust I didn't get what you mean by that.

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