Want to learn how to input double precision floating point numbers in C? This beginner-friendly tutorial walks you through the process step-by-step, showing how to use fgets and sscanf to get user input, validate it, and handle errors like a pro. With clear code examples and a simple program demo, you’ll see how to check if input succeeds or fails. Perfect for C programming newbies or anyone looking to sharpen their skills. Subscribe for more practical coding tips and tutorials!
Introduction 00:00:00
Overview of Inputting Doubles 00:00:02
Program Setup and Includes 00:00:44
Main Function Explanation 00:01:13
Get Input Function Introduction 00:02:10
Defining Get Input Function 00:02:34
Character Buffer Creation 00:02:54
Using fgets for Input 00:03:49
Parsing Input with sscanf 00:06:33
Handling Success and Failure 00:08:07
Running the Program 00:09:05
Testing with Valid Input 00:09:20
Testing with Invalid Input 00:10:05
Conclusion and Call to Action 00:11:50
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Hey there!
In this video I’m going to show you how to input a double precision floating point number
in C from the user into your program and decide whether or not it has succeeded or failed.
Okay so for starters I just want you to know that this video is not about generating makefiles.
own make files or how to compile link execute or even the basics of c see my other videos for now
i’m just going to assume that you know how to make a make file here it is basically very quickly
but i’m just i’m just compiling a simple program here’s my program here it’s called main.c it
doesn’t really have anything else to it i’m going to start off by importing a couple includes or a
few includes that’ll just help me get some stuff that i need so here’s an include for the standard
input output here’s an include for the standard library here’s an include that
helps with c string functions here’s an include that helps with limits not
actually sure every single one of those includes is needed but we’ll see anyway
here’s my main program or I guess my main function since this program only is
just one module notice how we have a double called value here and then we
make a call to a function called get input and we give a pointer to to the
double as an argument to get input so basically get input should be able to just return to us
the value by reference and then get input will be able to tell us whether or not it has succeeded
as in has the user entered valid input by its return type or not not its return type but its
return value so the result is going to be a long from get input and that’ll indicate
runs we’re just going to print you know get input returned and then print the return value that it
returned and then it’s going to print the double that it inputted from the user and then a new line
so nothing uh nothing too big of a deal let me show you the main workhorse function that has
all the logic you need to input from the user and check that the input was valid so this is called
get input i’m sticking it on top of the main function because i don’t want to deal with
because I don’t want to deal with prototypes right now.
It’s a better idea to put prototypes up at the top of your source code file
if your source code is even a little bit complicated.
But in this case, it’s just one other function.
So I will just put get input on top of main.
That way, when the compiler is scanning the source code,
it will have already seen that get input exists before main calls it,
and it’ll compile.
So first thing I’m going to do is say that get input returns along
get input returns along like we just saw and it takes in a pointer to a double no references just
a pointer then we have to create a character buffer and what that’s going to be for is basically
you know the user is going to type some characters and i want that to go into the buffer so um
a line max is the number of characters that the buffer can hold and that’s uh i think where we
I think where we have limits, I think limits is providing that for us.
But basically, you know, you could stick a number here if you wanted to like an eight
character, eight kilobyte buffer, if you wanted like eight, one, nine, two, or just whatever.
I’m just putting line max so that it sort of aligns with the system’s idea of what
the maximum size of a line should be.
Okay.
So then I don’t know why I have it say grab side one.
I’ll just say grab input here.
Let me change my solution real fast.
Grab input.
That’s probably because that was part of a program.
that’s probably because that was part of a program in the past so then the function that I’m going to
actually use is called fget s which basically is saying let’s uh let’s input a string a c string
from a file and we’re providing the target for the input so that’s going to be the character string
right there and then we’re going to provide um the maximum number of characters that we’ll accept
length of the buffer and then what file are we going to input from standard
input if you don’t know standard input see my other videos but it’s basically a
special file handle that means when the program first launches you know the
operating system sort of gives it a special file handle for standard input
standard output standard error and so this will be basically the user typing
on the terminal so f get s will essentially take input from standard
terminal until they you know hit like a new line or they disconnect the input
buffer it will take up to this many characters line max which should match
the buffer whatever value you put in the buffer and then it will write the
characters into the buffer that’s why you have to match the length of the
buffer so basically here this function call will return something if it returns
a zero then that means we have failed so if it returns null then we’re going to return zero
meaning fail so get input is kind of a boolean even though we don’t have booleans in c and it’s
basically going to return a zero on a failure and a one on success so like you know zero is a falsy
value and one is a truely value so we’re not going to use system style return codes where zero is
codes where zero is success we’re going to use boolean style return codes where a zero is failure
and one is uh success so you can see i’ve kind of chained two commands here i’ve decided to say all
right um we’ll first grab some input into the buffer array with f get s if that is null then
we immediately return zero because this this logical or operator says if one or the other thing
is equal to a true then just go ahead and evaluate to true so here I’m asking like did the call
equal null if the answer is yes then that part will evaluate to true and then it’ll basically
short circuit the rest of that logical expression and just return zero right away and then if the
previous command succeeds like the fget s succeeds then it will look at the buffer and try to scan it
for valid input.
So the left side is just taking raw input.
The right side is parsing for certain data type.
So you can see sscanf,
we’re scanning a buffer that we put into the first argument.
So that’s at that point, by the time sscanf gets called,
the buffer should be filled
with whatever input the user typed.
So we’re scanning that
and then here’s a little formatting argument,
which is very similar to,
use the same tokens basically as printf. So, you know, here we’re saying percent lf. So we’re
looking for a long float, which means a double precision floating point number. And that
percentage just means like, here’s a special token that comes after it to describe what sort of
value we’re looking to scan for. So then if we’re successful, if it actually scans,
the double. So fscanf expects a pointer to the variable that’s going to receive the value that
has been scanned if it is scanned correctly. So notice how the double we don’t have an ampersand
in front of it because it’s already a pointer because it came into us as a pointer in the first
place. So we’re going to look at the buffer that the user just typed, the user just provided,
and we’re going to try to scan for a double precision floating point number. If we’re
then the system should write that double into the variable referred to by the
pointer known as the double if all that succeeded then the value should be
greater than or equal to zero if these if this command failed then the value
should be less than one so that basically means if we have a value of
less than one then we have failed and we’re going to return zero so now whether
the initial scanning sorry whether the initial input fails or the parsing of
the users input fails then we’re gonna end up returning zero for fail so then
in main you can see all we’re doing is we’re just calling on input we’re giving
it a pointer to the double that we want to populate and then we’re looking at
the result so in your you know program logic you can do something like if the
result is zero then assume we failed and if the result is one assume we succeeded
And if the result is one, assume we succeeded and, you know, respond to the user or, you
know, do something that’s appropriate for success or fail.
I think that’s everything that I need to explain.
Let’s try to run the program now.
Open a terminal.
I want to do make run and see if that works.
Maybe I’ll do a clear here.
Okay.
So I didn’t print any, you know, prompting or anything special.
so the cursor is just blinking at this point i can just enter 33.12 i guess whatever hit enter
and the program now says get it get input returned one which means success so the value that i entered
was a valid double and the value is now 33.12 so it successfully grabbed the float keep in mind
these zeros or just precision related issues are related to the printing of the double not
not necessarily related to what is inside of the double so I’m just saying
you know print along float you can you can mess with print F a little bit more
if you want to have a different value printed or more precision or less
percent precision printed let’s try to do this with some invalid input I’m just
going to do some letters and hit enter notice how the value didn’t actually
change it’s it started or actually it did change it got set to a zero because
initialized it as junk data so it got set to a zero but then more importantly get input returns
zero as its return code so when you see zero that means the command failed so you should not consider
the double as valid in any way you shouldn’t use it you should complain to the user or do whatever
and that is how to get input let’s just try something for fun double value equals 3.333
bad input i’m assuming it might but i haven’t done this in a while so we’ll see so bad input
and looks like it did not change it so whatever junk data was already in value by the time you
called on get input if get input failed then the value should probably be unchanged but again more
importantly the return value is zero meaning fail because zero is a falsy value so i’m going to get
so i’m going to get rid of that so i can go back to my solution there okay so now hopefully you are
uh you’re on your way to start getting input in c at least in the in in terms of longs uh you can
look up the formatting codes for print f if you would like to input different types of data like
so this video is just for long floats doubles uh you can you can look up all other all sorts of
five years I’ll make another video that has different types of tokens but I guess that’s
it for this video. Thanks for watching I hope you learned a little bit and had a little bit of fun.
See you in the next one.
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