How to Convert NSMutableArray objects to const char in xcode - objective-c

Question

To develop a calculator in xcode I am using a c class for converting infix to postfix expression and its evaluation. But I have an NSString in my View controller class and I need to pass this NSString to a C class where the conversion and evaluation happens. How can I do this?

Answer 1

I think you need to convert NSString to cString. It can be done by
[str UTF8String]

Answer 2

Assuming you have an NSMutableArray containing NSString objects, and you want to convert this to a C array containing C strings, you need to allocate memory for a C array of char * of suitable size (e.g., count of the NSMutableArray + maybe 1 extra if you want a NULL terminator for the array, otherwise you need to pass along the array's element count everywhere). Then for each element of the NSMutableArray, populate the corresponding index in the C array with the C string returned by UTF8String of the NSString object.
Note that the C strings returned by UTF8String are “autoreleased”, so if you need to keep them around for longer than the current autorelease context, you need to duplicate them (e.g., strdup) and then free them (and the C array, which you need to free in any case) after you're done with them.
For example:
#include <Foundation/Foundation.h>
#include <stdio.h>
#include <string.h>
void printCStringsInArray(const char **arr) {
int i = 0;
while (*arr) { printf("%d: %s\n", i++, *arr++); }
}
int main () {
const char **carr;
#autoreleasepool {
NSMutableArray *arr;
const char **p;
arr = [NSMutableArray arrayWithObjects:#"11", #"+", #"12", nil];
p = carr = malloc(sizeof(*carr) * ([arr count] + 1));
for (NSString *s in arr) {
*p++ = strdup([s UTF8String]);
}
*p = NULL; // mark the end of carr
}
printCStringsInArray(carr);
{ // free the C strings and array
const char **p = carr;
while (*p) { free(*p++); };
free(carr);
}
return 0;
}
This prints:
0: 11
1: +
2: 12
edit: Of course if you just want to call some C function taking individual strings as const char * from your otherwise Objective-C code, you don't need to do anything this complicated, just convert each string on the fly with UTF8String and use the NSMutableArray for iteration, etc. But then one might wonder what it is that you can do with C strings that you couldn't do directly with NSStrings.

Answer 3

It's easy because you can use NSString's UTF8String method, but you have to handle memory allocation: allocate a block to contain the string.Choose if to wrap it into a NSData so that it will be released automatically when out of the pool block, or if to free it manually:
#autoreleasepool
{
NSArray* strings= #[ #"Hello", #"Hey" ]; // Your strings
// This could be also a NSMutableArray if you need it
NSMutableArray* cstrings=[NSMutableArray new]; // C strings
for(NSString* string in strings) // Iterate over all the strings
{
const char* temp=[string UTF8String]; // Get C string
const unsigned long length=strlen(temp); // Allocate memory for it
char* string=(char*)malloc((length+1)*sizeof(char));
strcpy(string,temp); // Copy it
// Store it inside NSData
[cstrings addObject: [NSData dataWithBytesNoCopy: string length: (length+1)*sizeof(char)]];
}
}