If you just want to round the number for output purposes, then the "%.2f" format string is indeed the correct answer. However, if you actually want to round the floating point value for further computation, something like the following works:

#include <math.h>

float val = 37.777779;

float rounded_down = floorf(val * 100) / 100;   /* Result: 37.77 */
float nearest = roundf(val * 100) / 100;  /* Result: 37.78 */
float rounded_up = ceilf(val * 100) / 100;      /* Result: 37.78 */

Notice that there are three different rounding rules you might want to choose: round down (ie, truncate after two decimal places), rounded to nearest, and round up. Usually, you want round to nearest.

As several others have pointed out, due to the quirks of floating point representation, these rounded values may not be exactly the "obvious" decimal values, but they will be very very close.

For much (much!) more information on rounding, and especially on tie-breaking rules for rounding to nearest, see the Wikipedia article on Rounding.

You will have to input the data as string, then check it only has one decimal place.

It is not possible to input a floating-point value and then check for decimal places; since the floating point values are stored internally with binary places and hold an approximation of the value that was input, in most cases.

The input code could look like:

char temp[20];
if ( 1 != scanf("%19s", temp) )
     return EXIT_FAILURE;

// code to check for decimal place goes here - I'm purposefully not 
// showing it as this looks like homework!

// once we have checked that the input is correct, then convert to double
grade[i] = strtod(temp, NULL);