not using strcmp properly in C

Question

I wrote this method to find the biggest power of 2 that will fit in a given decimal. The decimal is in char array format to avoid input overflow error with number storage. Powers of 2 are calculated with pow(2, power) format of float ie. 8.000000 This number is then sent to a method to remove the period and the 0's that trail. ie. 8.000000 turns into 8

1  #include <string.h>
2  #include <stdio.h>
3  #include <stdlib.h>
4  #include <memory.h>
5  #include <math.h>
6
7   int i;
16
17  void removeFloatZeros(char *floatvalue)
18  {
19      char *ptr = strchr(floatvalue, '.');
20      *ptr = '\0';
21  }
22
45
173 char *decimalToBinary(char *decimal)
174 {
176     int x;
177     double power = 0;
178     char *binary = malloc(sizeof(char *) * 1024);
179     char *twosPower = malloc(sizeof(char *) * 1024);
180
181     /* What is the greatest power of 2 that will fit into the decimal? */
182     for(x = 0; x <= 30; x++)
183     {
184         power = pow(2.0, x);
185         snprintf(twosPower, 1023, "%f", power);
186         removeFloatZeros(twosPower);
189         printf("strcmp(decimal, twosPower) = %d\n", strcmp(twosPower, decimal));  
190         memset(twosPower, '\0', 1023);
191     }
214 }
215
216 int main(int argc, char*argv[])
217 {
218     char *dec1 = argv[1];
219     decimalToBinary(dec1);
220     return 1;
221 }
222

For example if I input 20 into argv[1] it will output:

strcmp(decimal, twosPower) = -1
strcmp(decimal, twosPower) = -1  
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = -1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = -1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = -1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = -1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = -1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = -1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = -1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = -1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = 1
strcmp(decimal, twosPower) = -1

Where am I going wrong with this? Also, ignore the ending condition for the for loop. It's supposed to output all 1's before the 6th iteration and all -1's on the 6th and after the 6th iteration.

Answer 1

strcmp return values:

A zero value indicates that both strings are equal.

A value greater than zero indicates that the first character that does not match has a greater value in str1 than in str2.

And a value less than zero indicates the opposite.

Your input: 20 Your first iteration of the loop: twosPower = "1" strcmp("20", "1")

The first character doesn't match and has a smaller value in str2 ("1") than in str1 ("2") -> return negative value.

The rest of the iterations should explain themselves...

Also, Edit:

printf("strcmp(decimal, twosPower) = %d\n", strcmp(twosPower, decimal)); 

Your printf format string states the opposite of what you're doing in the parameter.

Edit:

str1    str2    
1       20  First char that differs is '1' vs. '2'. '1' (ASCII 49) is smaller than '2' (ASCII 50), 49 - 50 = -1 = return value
2       20  First char that differs is '\0' vs. '0'. '\0' (ASCII 0) is smaller than '0' (ASCII 48), 0 - 48 = -48 = return value
4       20  First char that differs is '4' vs. '2'. '4' (ASCII 52) is greather than '2' (ASCII 50), 52 - 50 = 2 = return value
8       20  First char that differs is '8' vs. '2'. '4' (ASCII 56) is greather than '2' (ASCII 50), 56 - 50 = 6 = return value
16      20  First char that differs is '1' vs. '2'. '1' (ASCII 49) is smaller than '2' (ASCII 50), 49 - 50 = -1 = return value

... and so on ...

Maybe this output helps a bit more

Furthermore, your method of finding the greatest power of 2 in a number is flawed since strcmp's return value is just dependent on the FIRST char that differs. So strcmp("2", "16") and strcmp("200000000", "16") would always return the same thing.

Answer 2

This cleaned up version of your code produces the output shown:

$ ./xx 20
strcmp(1, 20) = -1
strcmp(2, 20) = -48
strcmp(4, 20) = 2
strcmp(8, 20) = 6
strcmp(16, 20) = -1
strcmp(32, 20) = 1
strcmp(64, 20) = 4
strcmp(128, 20) = -1
strcmp(256, 20) = 5
strcmp(512, 20) = 3
strcmp(1024, 20) = -1
strcmp(2048, 20) = 52
strcmp(4096, 20) = 2
strcmp(8192, 20) = 6
strcmp(16384, 20) = -1
strcmp(32768, 20) = 1
strcmp(65536, 20) = 4
strcmp(131072, 20) = -1
strcmp(262144, 20) = 6
strcmp(524288, 20) = 3
strcmp(1048576, 20) = -1
strcmp(2097152, 20) = 57
strcmp(4194304, 20) = 2
strcmp(8388608, 20) = 6
strcmp(16777216, 20) = -1
strcmp(33554432, 20) = 1
strcmp(67108864, 20) = 4
strcmp(134217728, 20) = -1
strcmp(268435456, 20) = 6
strcmp(536870912, 20) = 3
strcmp(1073741824, 20) = -1
$

Suffice to say that there are a lot of small changes.

#include <string.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>

static void removeFloatZeros(char *floatvalue)
{
    char *ptr = strchr(floatvalue, '.');
    *ptr = '\0';
}

static void decimalToBinary(char *decimal)
{
    int x;
    double power = 0;
    char *twosPower = malloc(sizeof(char *) * 1024);

    /* What is the greatest power of 2 that will fit into the decimal? */
    for(x = 0; x <= 30; x++)
    {
        power = pow(2.0, x);
        snprintf(twosPower, 1023, "%f", power);
        removeFloatZeros(twosPower);
        printf("strcmp(%s, %s) = %d\n", twosPower, decimal, strcmp(twosPower, decimal));  
        //printf("strcmp(decimal, twosPower) = %d\n", strcmp(twosPower, decimal));  
        memset(twosPower, '\0', 1023);
    }
    free(twosPower);
}

int main(int argc, char*argv[])
{
    for (int i = 1; i < argc; i++)
        decimalToBinary(argv[i]);
    return 0;
}

Showing the compared values makes things a lot easier to understand. You need to free memory (or use automatic arrays). You need to use headers. The static declarations aren't 100% necessary, but mean I don't get any warnings when I compile under my über-fussy compiler settings.

But the key change is printing out the values that are being compared - it makes sense of the numbers from strcmp().

(Consider adding an appropriate number of leading zeroes before doing your comparison.)