Printing bitmap in C

Question

I am trying to create a bitmap of 100 1s and 0s.

Below is what i came out so far.. I having issue printing the bitmap or rather i do not know how to print the bitmap.

I want to display the bitmap that consist of all the 1 and 0 that i had set. for index 0 to 99

int main()
{

    unsigned int bit_position, setOrUnsetBit, ch;
    unsigned char bit_Map_array_index, shift_index;

    unsigned char bit_map[100] = { 0 };

    do
    {
        printf("Enter the Bit position (bit starts from 1 and Ends at 100) \n");
        scanf("%d", &bit_position);

        printf(" Do you want to set/unset the Bit (1 or 0) \n");
        scanf("%d", &setOrUnsetBit);


        bit_Map_array_index = (bit_position - 1) / 8;


        shift_index = (bit_position - 1) % 8;

        printf("The bit_position : %d shift Index : %d\n", bit_position, shift_index);

        if (setOrUnsetBit)
        {
            bit_map[bit_Map_array_index] |= 1 << shift_index; //set 1
        }
        else
        {
            bit_map[bit_Map_array_index] &= ~(1 << shift_index); //set 0
        }


        printf(" Do You want to Continue then Enter any Number"
            "and for Exit then enter 100\n");
        scanf("%d", &ch);



    } while (ch != 100);

    //I wan to print bitmap here after exiting

    system("pause");
    return 0;
}

I have very little experience in C programming... So correct me wherever i am wrong.

Thanks in advance.

try this it might help: https://stackoverflow.com/questions/2525310/how-to-define-and-work-with-an-array-of-bits-in-c — Omer Dagan, Mar 11 '18 at 14:13
Please post a [mcve] so we can reproduce the problem. Without an example of the actual input, the desired output, the actual output and with code missing key parts, like the `#include` statements, we can only guess as to what the reality is. — user3629249, Mar 12 '18 at 17:52
when compiling, always enable the warnings, then fix those warnings. ( for `gcc`, at a minimum use: `-Wall -Wextra -Wconversion -pedantic -std=gnu11` ) — user3629249, Mar 12 '18 at 17:54
regarding these kinds of statements: `scanf("%d", &bit_position);` the variable `bit_position` is declared as 'unsigned' so the format specifier should be: `%u` When calling any of the `scanf()` family of functions, always check the returned value (not the parameter values) to assure the operation was successful — user3629249, Mar 12 '18 at 17:57
when working with external input, especially from the user, always validate the input to assure the data is valid. For instance, with `bib_position`, check that the entered value is in the range 1...100 — user3629249, Mar 12 '18 at 18:03

score 1 · Accepted Answer · answered Mar 11 '18 at 18:35

You are working with bytes, not bits. You have 100 bytes, set each byte to 0 or 1. There is no need for shifting values:

unsigned char bytes[100];
for(int i = 0; i < sizeof(bytes); i++)
    bytes[i] = rand() % 2;

for(int y = 0; y < 10; y++)
{
    for(int x = 0; x < 10; x++)
    {
        int i = y * 10 + x;
        printf("%d ", bytes[i]);
    }
    printf("\n");
}

If you are working with bits, then you can use

unsigned char data[13];

Because 13 is 13 * 8 bits, or 104 bits. You only need 100 bits. How you set and get the bits, depends on the format you choose. A bitmap file, for example, is padded so each line is multiple of 4-bytes. In general you can set the values as:

if(bytes[i])
    data[byteindex] |= (1 << shift);
else
    data[byteindex] &= ~(1 << shift);

To get the value back:

int value = (data[byte] & shift) > 0;

So did you intend to work with bits or bytes? See also this answer about 1-bit bitmap file format https://stackoverflow.com/a/49216392/4603670 — Barmak Shemirani, Mar 12 '18 at 20:25

Printing bitmap in C

1 Answers1