Problem: 

How to change the values of variables while you are debugging with LLVM in XCode.
For Example, you put a breakpoint in your code and while running it, you reach that breakpoint. So now you must have used the 'p' and 'po' commands in LLVM of XCode which help a lot while debugging by printing the values of passed variables.
But in this article, we will tell you how to change those values:

Solution:

We can use expr command provided. It can be used as:

expr stringToBeChangedWhileDebugging = @"New Changed String"

What is 'expr' command?

(lldb) help expr
Evaluate a C/ObjC/C++ expression in the current program context, using variables currently in scope. This command takes 'raw' input (no need to quote stuff).
Syntax: expression --
Command Options Usage: expression [-f ] [-G ] [-d ] [-u ] -- expression [-o] [-d ] [-u ] -- expression
   -G <gdb-format>  ( --gdb-format <gdb-format> )
        Specify a format using a GDB format specifier string.

   -d <boolean>  ( --dynamic-value <boolean> )
        Upcast the value resulting from the expression to its dynamic type
        if available.

   -f <format>  ( --format <format> )
        Specify a format to be used for display.

   -o  ( --object-description )
        Print the object description of the value resulting from the
        expression.

   -u <boolean>  ( --unwind-on-error <boolean> )
        Clean up program state if the expression causes a crash, breakpoint
        hit or signal.
Examples:
expr my_struct->a = my_array[3]
expr -f bin -- (index * 8) + 5
expr char c[] = "foo"; c[0]
IMPORTANT NOTE: Because this command takes 'raw' input, if you use any command options you must use ' -- ' between the end of the command options and the beginning of the raw input.
'expr' is an abbreviation for 'expression'


If you try to change the text of a UILabel:

'expr myLabel.text = @"I can't be changed like this :(" 

then you will get the following error:

error: property 'text' not found on object of type 'UILabel *'

Actually, lldb handles dot syntax much better than gdb. gdb just assumes you're treating it like a C-struct, which fails. lldb will correctly access properties, but only if they're actually declared with @property

So to change to change the text of a UILabel, you can use:
expr (void)[myLabel setText:@"I can be changed now :)"] 

Dot-Syntax doesn't work for po either. instead of po myLabel.text you have to use:

 po [myLabel text] 

 Using 'p' to change values while debugging:

You can also use p as a shortcut for expr. 
Example:

(lldb) p stringToBeChangedWhileDebugging = @"New Changed String"