Confusion With Pointers

**ObjectWithBrain** · 07-13-2011

Im a noob to programming in general, and one of the biggest confusions I've had is with pointers in C. They make perfect sense to me conceptually, but syntactically they're incredibly confusing. I was writing a program that takes in an unsigned short and prints it in binary one digit at a time, and it worked. But when I went to try to store it in a pointer to a char[] I couldn't get it to work. Ive looked at at least five different tutorials on pointers and from what I read, the code I have should work, but it doesn't.

I get no compile errors or even warnings, but all it prints is a single newline. I left in the code I had originally for just printing it one character at a time. If you uncomment the two printf statements in printBin(), you'll see that the loop runs properly and does what it's supposed to do.

I think this must be a simple problem, but I can't figure it out by myself. Thank you in advance for any help you can give me.

Code:

#include <stdio.h>

char buffer[17];

char* printBin(unsigned short);

int main (int argc, const char * argv[]){
    
    char* buffer2 = &buffer[0];
    
    buffer2 = printBin(360);
    
    printf("%s\n", buffer2);
    
    return 0;
}

char* printBin(unsigned short number) {
    int shift;
    int bit;
    char* binString = &buffer[0];
    
    for (shift = 15; shift >= 0; shift--, binString++) {
        
        bit = 1 << shift;
        
        if ((number & bit) == bit) {
            sscanf(binString, "1");
            //printf("1");
        }
        else {
            sscanf(binString, "0");
            //printf("0");
        }
        
    }
    //sscanf(binString, "hello");
    return binString;
}

**tabstop** · 07-13-2011

Why do you think scanf will print something?

And if you want to put something somewhere, why not use the equals sign, since that's what it's for?

**AndrewHunter** · 07-13-2011

Code:

for (shift = 15; shift >= 0; shift--, binString++) {
        
        bit = 1 << shift;
        
        if ((number & bit) == bit) {
            sscanf(binString, "1");
            //printf("1");
        }
        else {
            sscanf(binString, "0");
            //printf("0");
        }
        
    }
    //sscanf(binString, "hello");
    return binString;
}

Where does binString point to after your loop. Is it the begginning of buffer?
Also you might want to take a look at what sscanf does

**ObjectWithBrain** · 07-13-2011

Ohh wow, I dont know what I was thinking using sscanf. I totally forgot what it does. Thanks for that.
So I should switch out sscanf() for = and then decrement binString by 16?
I'll give that a try. Thanks guys.

**AndrewHunter** · 07-13-2011

Don't forget your are saving a char to your buffer, not a string so: '1' instead of "1".

**ObjectWithBrain** · 07-13-2011

Didn't work. My code is now as follows:

Code:

#include <stdio.h>

char buffer[17];

char* printBin(unsigned short);

int main (int argc, const char * argv[]){
    
    char* buffer2 = &buffer[0];
    
    buffer2 = printBin(360);
    
    printf("%s\n", buffer2);
    
    return 0;
}

char* printBin(unsigned short number) {
    int shift;
    int bit;
    char* binString = &buffer[0];
    
    for (shift = 15; shift >= 0; shift--, binString++) {
        
        bit = 1 << shift;
        
        if ((number & bit) == bit) {
            binString = "1";
            //printf("1");
        }
        else {
            binString = "0";
            //printf("0");
        }
    }
    binString -= 16;
    
    return binString;
}

**ObjectWithBrain** · 07-13-2011

'1' saves an int to my buffer and gives me an incompatible integer to pointer conversion.

**AndrewHunter** · 07-13-2011

Originally Posted by AndrewHunter

Don't forget your are saving a char to your buffer, not a string so: '1' instead of "1".

Check the post I gave you. ^^^

**AndrewHunter** · 07-13-2011

You need to dereference your pointer to save a value to it.

Code:

if ((number & bit) == bit) {
             *binString= '1';
            //printf("1");
        }
        else {
            *binString = '0';
            //printf("0");

**AndrewHunter** · 07-13-2011

In addition, you could have binky explain pointers to you. And then after you are done laughing you could read this tutorial.

**ObjectWithBrain** · 07-13-2011

oh! it all clicks! it works now. thank you so much Andrew, you've saved me such a headache =)

My final code is

Code:

#include <stdio.h>

char buffer[17];

char* printBin(unsigned short);

int main (int argc, const char * argv[]){
    
    char* buffer2 = &buffer[0];
    
    buffer2 = printBin(360);
    
    printf("%s\n", buffer2);
    
    return 0;
}

char* printBin(unsigned short number) {
    int shift;
    int bit;
    char* binString = &buffer[0];

    
    for (shift = 15; shift >= 0; shift--, binString++) {
        
        bit = 1 << shift;
        
        if ((number & bit) == bit) {
            *binString = '1';
            //printf("1");
        }
        else {
            *binString = '0';
            //printf("0");
        }
    }
    binString -= 16;
    
    return binString;
}

And my output is:
0000000101101000
as it should be =)

**quzah** · 07-13-2011

Since no one bothered pointing this out, I will:

Originally Posted by ObjectWithBrain

'1' saves an int to my buffer and gives me an incompatible integer to pointer conversion.

That's not at all what it does:

Code:

char* printBin(unsigned short number) {
    int shift;
    int bit;
    char* binString = &buffer[0]; /* point at the buffer (which is global,
                                     so I don't know why you are using
                                     pointers at all...) */
    
    for (shift = 15; shift >= 0; shift--, binString++ /* increment pointer */) {
        
        bit = 1 << shift;
        
        if ((number & bit) == bit) {
            binString = "1"; /* point at a string literal,
                                you are no longer in 'buffer' */
            //printf("1");
        }
        else {
            binString = "0"; /* same as above */
            //printf("0");
        }
    }
    binString -= 16;
    
    return binString;
}

First you point at a buffer. Then you point yourself at a string literal, so you are no longer pointing at that buffer. Then you increment that pointer, making it point to the nul character the string literal contains (since it is only 1 non-nul character, either 1 or 0). Then you go make it point at the start of a string literal again the next time through the loop.

In the end, you don't get anything useful added to your buffer.

Quzah.

**AndrewHunter** · 07-13-2011

It seemed like it was just an exercise in pointers to me, albeit a weird one. Also I think all those things were addressed in the above posts. Did you see the OP's final code posted?

**quzah** · 07-13-2011

Originally Posted by AndrewHunter

Did you see the OP's final code posted?

Yes, but no one had described what was actually taking place, so I thought I would.

Quzah.

**AndrewHunter** · 07-13-2011

Good point. I will keep that in mind.

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