printf of a for loop is skipping only 0

I have hit a roadblock in one of my tasks.

This is the yet to be finished code of my conversion table.

int main(int argc, char *argv[]) {

    int i = 0; //celsius = 0, fahrenheit = 0;

    printf("Celsius\tFahrenheit \n");

    for (i = -20 ; i<=300 ; i= (i+20)) //iteration from -20 to 300 °C by +20
    {
        printf("%7.d\t%10d \n",i,i);
    }


    return 0;
}

The Output looks like this, which is exactly as expected by the teacher, minus the missing 0 (and soon to be implemented conversion)

Celsius Fahrenheit
    -20        -20
                 0
     20         20
     40         40
     60         60
     80         80
    100        100
    120        120
    140        140
    160        160
    180        180
    200        200
    220        220
    240        240
    260        260
    280        280
    300        300

Could anyone please tell me why the 0 keeps getting skipped and how to prevent that from happening in the future?

>Solution :

The problem is the conversion specifier %7.d: for an integer conversion, the precision field that follows the . is the minimum number of digits to produce (padding with leading zeroes). If you do not specify a ., the default value is 1 but with a . the number is 0 if no digits follow the .. The effect of this 0 precision is the value 0 is converted to an empty string. Remove the . to fix this problem.