in C how do I prevent the input of the wrong data type?

read the input as string (not scanf with %d or whatelse) and do your check by yourself (check if it is a float number, i.e. it fits the syntax

[-|+]D[.D] or [-|+].D

[-|+] starting optional minus or (|) plus sign

followed by one or more Digits, optionally followed by a dot and another sequence of digits, or -/+ optional followed by a dot and a sequence of digits...

or something like this...

but if you want to "avoid" the user can press a letter and see it like a understand input... you can not use simply the getchar or scanf function, since you need some kind of control on the keypressing event in the console.

this can depend on what operating system you're programmin in.

If you write in C, you read the input into a string. If you expect a number, use the sscanf() to parse a number. sscanf() returns the number of matches, so if sscanf(mystring, "%d", &ival) returns zero, there is no number in the input. Instead of terminating the program, you should provide an error message like Error: number expected, 'xxx' found where xxx is the string entered by the user. This is an area where you want to be careful about buffer overflow!

Instead of constructing a double to cin you're going to ought to create a string. The string will learn in with cin and you'll seem at each man or woman that's within the string for non-numeric characters. Once you're making the verify you're going to ought to convert the string quantity right into a decimal importance on your double variable.