Infinite loop error with my binary to decimal method in C?

Question

I am getting an infinite loop with my method and I am unsure why. I am working on a method that is suppose to convert the bits from a decimal to a binary form. I can't see what is happening since I am getting an infinite loop when I run this method. I was hoping I could get some help why.

Here is my code:

int binToDec(char* bin)
{       
        int i = 0;

        int result = 0; // (1) start the decimal result at 0.   
        while(bin != "\n");// (2) remove the most significant binary digit(leftmost) and add it to the result.
        {

        if(bin[i] == '1')
        {       
                result = result * 2 + 1;
        }
        else if(bin[i] == '0')
        {       
                result *= 2;
        } 
        printf("%d\n", result);
        i++;    
        }       // (3) If all binary digits have been removed, you're done. Stop.
                // (4) Otherwise, multiply the result by 2 and go back to step 2.
        return result;
}
/**
 * Create two functions that do binary to decimal conversion and back. Their signatures (aka prototypes)
 * should look like:
 * int binToDec(char* bin);
 * char* decToBin(int dec);
 *
 * For both functions, remember that your string will not hold 0s and 1s, but the characters ‘0’ and ‘1’. 
 * Use the offset to determine the binary value.      
 */
    char* decToBin(int dec)
{

        int i;
        double z;
        for(i = 0; i < dec; i++)
        {
                 z = pow(2, i);
                printf("The bit is %d \n", z);
        }
        char *c = (char*) malloc(dec * sizeof(z));

        while(dec % 2 != 0) //As long as the quotient is not 0, continue to divide the newest quotient by 2.
        {
                c[i] +=  dec % 2 + '0';
                dec = dec / 2; //Divide the value by 2 and record the remainder.        
        i++;
        }
        return c;
}
int main()
{
        int num;
        char *ptr;
        ptr = (char*) malloc(num * sizeof(decToBin(11001)));

        printf("Call to binToDec given 1001 result in: %d\n", binToDec("11001"));
        printf("Call to decToBin given 9 results in : %s\n", decToBin(11001));
        free(ptr);
        return 0;
}

Let me know. The infinite loop takes place in the first method.

I apologize, this is the wrong method. Let me post the correct one — Adan Vivero
– Adan Vivero, Commented Oct 14, 2019 at 16:37
while(bin != "\n"); has two problems. 1. You can't use != to compare strings, you have to use strcmp(). 2. The ; at the end of the line is making this a loop with no body. — Barmar
– Barmar, Commented Oct 14, 2019 at 16:40
One of the first things you should have learned about strings is that you don't use == and != to compare them. — Barmar
– Barmar, Commented Oct 14, 2019 at 16:41

Ingo Leonhardt · Accepted Answer · 2019-10-14 17:11:22Z

1

Look at this line:

while(bin != "\n");

Even if the ; wasn't there the condition would never become true, just because you can't compare strings like that. It had to be

while( strcmp(bin, "\n") != 0 )

But looking at what should obviously have been the loop body you don't increment the pointer bin but an integer i. So finally, your condition should be

while( strcmp(bin+i, "\n") != 0)

or simply

while( bin[i] != '\n' )

... and without the `;' of course

And as @barmar mentioned correctly, if you call binToDec() with a string that contains no newline at all, you still have an infinite loop. So because bin should consist of '0' and '1' only, I would suggest:

  while( bin[i] == '0' || bin[i] == '1' )

or, if you want to support 'formatted' binary strings (e.g. with a space after every 8 digits)

  while( bin[i] != '\0' && bin[i] != '\n' )

your loop body would already be fine for that as you don't do anything if bin[i] is neither '0' nor '1'

edited Oct 14, 2019 at 17:11

answered Oct 14, 2019 at 16:44

Ingo Leonhardt

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2 Comments

Barmar Over a year ago

But his input strings don't end with newline, so the condition will never become true.

Ingo Leonhardt Over a year ago

@barmar thanks, I must admit I didn't look at that. I will edit.

Vlad from Moscow · Accepted Answer · 2019-10-14 17:11:35Z

This loop

while(bin != "\n");
                 ^^^

is indeed an infinite loop because the string literal "\n" in most cases has a different address than a string literal pointed to by the pointer bin. Moreover even if to write

while( "\n" != "\n");

then the loop can be also infinite because the compiler (depending on its options) can store identical string literals as different objects.

This statement

ptr = (char*) malloc(num * sizeof(decToBin(11001)));

does not make sense because the pointer ptr is not used and the variable num was not initialized.

And this declaration

char *c = (char*) malloc(dec * sizeof(z));

does not make sense. For example why sizeof( double ) (the type of the variable z is double) is used? And the returned string from the function is not zero terminated.

Also in these calls

printf("Call to binToDec given 1001 result in: %d\n", binToDec("11001"));
                               ^^^^                             ^^^^^

printf("Call to decToBin given 9 results in : %s\n", decToBin(11001));
                              ^^^                             ^^^^^

there are typos.

It seems you mean the following

#include <stdio.h>

unsigned int binToDec( const char *bin )
{       
        unsigned int result = 0; // (1) start the decimal result at 0.   

        for ( ; *bin != '\0'; ++bin ) // (2) remove the most significant binary digit(leftmost) and add it to the result.
        {
            result = 2 * result + ( *bin == '1' );
        }       // (3) If all binary digits have been removed, you're done. Stop.
                // (4) Otherwise, multiply the result by 2 and go back to step 2.

        return result;
}

int main(void) 
{
    const char *bin = "11001";

    printf( "Call to binToDec given %s result in: %u\n", bin, binToDec( bin ) );

    return 0;
}

The program output is

Call to binToDec given 11001 result in: 25

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