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i have seen some code where programmers include a for loop inside a for loop.for example
for(i=0; i<n; i++){
for(j=0;j<n;j++)
Why is this important?
Please tell me when do I need to include this nested loop in my code.
You dont use nested loops because they are important. You use it because the program requires it. Try to understand the logic of the program you are referring. Then you will understand why they are used. You can read about nested loops here.
It is NOT important. You simply use nested for loops when you feel you need to. Some good examples would be sorting algorithms. You can't sort an entire vector in one loop, so you need a nested loop to ensure that you sort the entire vector, not just the elements from the first iteration.
Here is a list with some sorting algorithms. Take a look at how they use nested for loops to see why it is sometimes required.
Another good example of when to use nested loops is when you need to iterate through a matrix. A single for loop would let you check only the first line from the matrix, so you add a secondary loop to iterate through every single line, not just one.
It is not important to use nested loop in every program. It depends upon the requirement of the program.
You can understand the concept of nested programming HERE
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I'm building a two-pass assembler in C.
A part of its job is to be able to work on matrices.
Let's say that there is the following line:
mov m[r2][r5], XYZ
mov is the operation.
and m[r2][r5] and XYZ are the operands.
I need to find out if an operand is a matrix. and get the:
1. matrix name.
2. row.
3. column.
How is it possible?
Tried to use sscanf without any success.
Thanks in advance!
Unfortunately writing an assembler is not as easy as using the scanf. Simplifying: You need to divide the input stream into the tokens, then you need to parse it and build the semantic tree, then you need to do the semantic analysis, reduce the tree (by evaluating constant expressions, finding addresses etc etc), and eventually generate the machine code.
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Why do we use pointers in C Programming?
In general pointers are able to access the address where the int/float/char etc... is stored.
Are there any other uses?
It depends on what you try to achieve:
you can change the value of a variable inside a function
you can pass a struct to a function without having to copy all its fields - think of a function that receives a struct.
you can point to a specific variable/struct and point to it from other structs
and many other advantages (advantages is purpose dependant and it depends on whats your program is doing).
Pointers are quite basic C and there is a lot of material online you should get yourself familiar with them and the advantages will pop up themselves.
The reason is that pointers are used to bodge into C some vital features which are missing from the original language: arrays, strings, & writeable function parameters. They can also be used to optimize a program to run faster or use less memory that it would otherwise. A few tasks these days, such as programming microcontrollers, still need this.
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I have a pointer of pointers, and I want a loop to go thorough them and store its value into something else. Is there any way to do that?
e.g:
char **variable;
Now I want to read that into another variable:
char **variable2
i thought of doing something like this:
for(i = 0;i <LENGTH_OF_VARIABLE-1;i++){
variable2[i] = variable[i+1]
}
But that is not possible in c, right?
Now you might ask why not variable2 = variable? well variable2 should store only parts of the variable, not all of them.
EDIT: Variable's size is not known, and its dynamic(read from the command line). AND no it doesn't contain '\0' at the end. Cause its processed to remove such a character and then passed to a function that I am implementing.
If you already putting anything to your **variable, does that mean that you have allocated memory correctly?
I think it will better for you to revise and understand how simple one dimensional array works, after understanding that, move to double arrays. Then take a look how pointers work and learn how to allocate memory. After understanding this steps i have mentioned above, take a look at double pointers and allocation of memory in case of double pointers.
here you go.
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If I can do a cycle with for and while both, which should I choose?
[for]
int num = 10;
int i;
for (i=0; i<num; i++)
{
/* ... */
}
[while]
int num = 10;
int i = num;
while ( i-- )
{
/* ... */
}
The choice between for and while is just matter of clarity:
K&R . Chapter 1. The For statment:
The choice between while and for is arbitrary, based on which seems
clearer. The for is usually appropriate for loops in which the initialization and
increment are single statements and logically related, since it is more compact
than while and it keeps the loop control statements together in one place.
Performance, of course, depends on the implementation in the language used. However, in most cases and with most compilers, the generated code from both loops will pretty much be the same.
A rule of thumb would be to use while when you don't know exactly how many times you want to iterate. In your example, it doesn't make much difference. Use what's clearer to you.
And yes, declarations in the for loop are only allowed in newer versions of C (C99).
Usually, you'd use a for loop for something like that because you know before you start the loop how many times you need the loop to run. while loops are more for when you don't know how many times you're going to have to repeat, and something that the user inputs or some random int will change it
In the example above, you should use a for loop since it clearly expresses the intent of what you intend. "Do this loop num times". Also, it is much simpler for the compiler to potentially optimize for loops into vector assembly operations.
You should use a while loop when you have a clearly expressible condition like
"Read lines until EOF" or some other condition that doesn't have an easily enumerable solution.
Also, I consider it poor practice to depend on the fact that 0 is false. You should clearly write the condition that exits the loop.
Every program solve in while loop that can be solved in for loop,but for loop suitable for:-
1--> If i know how many time the loop Will execute and while loop is suitable for if i don't know how many time the loop will be executed.
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I want to obfuscate code just for fun. I'm looking at code from the international obfuscated c contest: http://www.ioccc.org/ And I seriously just have no idea how to even start reverse engineering some of this code to make anything of sense.
What are some common obfuscation techniques and how do you make sense of obfuscated code?
There is a lot of different techniques to obfuscate code, here is a small, very incomplete list:
Identifier mangling. Either you will find people using names like a, b, c exclusively, or you find identifiers that have absolutely nothing to do with the actual purpose of the variable/function. Deobfuscation would be to assign sensible names.
Heavy use of the conditional evaluation operator ? :, replacing all occurences of if() else. In most cases that's a lot harder to read, deobfuscation would reinsert if().
Heavy use of the comma operator instead of ;. In combination with 2. and 4., this basically allows the entire program to be one single statement in main().
Recursive calls of main(). You can fold any function into main by having an argument that main can use to decide what to do. Combine this with replacing loops by recursion, and you end up with the entire program being the main function.
You can go the exact opposite direction to 3. and 4., and hack everything into pieces by creating an insane amount of functions that all do virtually nothing.
You can obfuscate the storage of an array by storing the values on the stack. Should you need to walk the data twice, there's always the fork() call handy to make a convenient copy of your stack.
As I said, this is a very incomplete list, but generally, obfuscation is usually the heavy, systematic abuse of any valid programming technique. If the IOCCC were allowing C++ entries, I would bet on a lot of template code entering, making heavy use of throwing exceptions as an if replacement, hiding structure behind polymorphism, etc.