how to take input int with % in c? - c

I have to take two inputs from the user with %
input example: 20% 30%
I tried this
scanf("%d%d ", &x,&y);
how can I input two values with %? I can only take two integer values.

%% matches literal % character, so this should do it:
int r = scanf("%d%%%d%%", &x,&y);
if(r != 2) {
puts("scanf error");
exit(1);
}
General note: it's more robust to read entire lines with fgets or POSIX getline, then parse them, for example with sscanf.
Also, please read scanf documentation on what %d and %% do, and what the return value actually means.

Firstly, are you really obligated to also take the %? Couldn't you instead ask the user to input integers?
If you still want to take inputs with %, I think you could treat your inputs as char*, iterate through them until you encounter the '%' char, and then delete it.
You could then use the atoi function to transform your char* into integers.

Related

Trouble reading from file

So I think this should be pretty straight forward. I actually can't even understand what could possibly be going wrong, and that's what bugs me the most.
Here's the deal: I have to read a couple of numbers from a file, but these numbers are in scientific notation. An example file would have the numbers like this:
1.00090E+00,2.90000E+00
So I thought I'd simply read it like this: get a double, ignore a character, get an int, ignore a character, get another double, ignore a character, and get a final int, so for this example, it would:
get (1.00090)
ignore (E)
get (+00)
ignore (,)
get (2.90000)
ignore (E)
get (+00)
So, I wrote these fscanf:
fscanf(arquivo, " %lf%*c%d%*c", &mantissa1, &expoente1);
x[i] = numToFloat(mantissa1, expoente1);
fscanf(arquivo, "%lf%*c%d", &mantissa2, &expoente2);
fx[i] = numToFloat(mantissa2, expoente2);
I separated them to make the understanding simpler. But then, it doesn't work. It reads the first double correctly, but then it messes up the int. And then it messes up everything on the second fscanf.
I tried all that I could think of. Put the formatted string onto the fscanf ("%lfE%d," and "%lfE%d"). To read the signal into a character variable ("%lf%*c%c%d%*c"). None of it works.
Any guesses as to what's wrong?
First, you do not need to parse the mantissa and exponent separately. scanf natively understands n.nnnE±nnn notation. This is why your format string doesn't behave as you expect; the %lf directive consumes the entire first number including the E+00 piece, then %*c consumes the comma, %d the 2, and now we're hopelessly out of sync.
Second, you should not use any of thescanf functions, ever. Instead, read a line at a time with fgets (or getline, if you have it), split it on commas with strtok, and convert numbers to binary with strtod (which also natively understands scientific notation). This will be more coding up front, but the result will be easier to read and debug, it will be easier to make it robust against malformed input, and it will have well-defined behavior on input overflow.
(Technically you do not need to use strtok, as strtod will tell you where each number ends, but it is conceptually simpler to do the splitting first.)
You might be able to adapt this example to your needs, it used the %lg format specifier:
#include <stdio.h>
int main(void){
double d, e;
int n;
printf("Enter 2 scientific floats: ");
fflush(stdout);
n = scanf("%lg %*c %lg", &d, &e);
if(n == 2) {
printf("%f\n", d);
printf("%f\n", e);
} else {
printf("Bad input\n");
}
}
Program session
Enter 2 scientific floats: 1.8e+4,4.2e-1
18000.000000
0.420000

How to read a data point from a text file in C

I have a text file full of points of the following format on different lines
LONG,LONG
i can successfully read each line and print it out, but I how can I parse the string in C such that I get each long of each point on its own?
Thanks!
if you have the line already, it's easiest to use sscanf() to do this:
long a, b;
if(sscanf(line, "%ld,%ld", &a, &b) == 2)
{
/* Successfully parsed two long integers, now store them somewhere I guess. */
}
Note that it's a good idea to check the return value of sscanf(), this protects you from wrongly accepting illegal data and getting undefined results.
You can do it in multiple steps too if you need more control, as #dasblinkenlights suggested. You can use strtol() to parse the first number from the start of the line, then if that succeeds look for the comma, and then parse the second number. It can be faster than sscanf(), but I wouldn't expect too much for something this simple.
There are many solutions to this.
One is to read the line, read the first long with strtol find the position of the comma that follows with strchr, and read the second number from there.
Another solution would be to read the line, and pass it to sscanf function with the format that accepts two comma-separated LONGs.
Use the string variant of scanf() if you say you've already got the line:
char* line;
long long1;
long long2;
sscanf(line, "%ld,%ld", &long1, &long2);
Indeed as #unwind suggests in his +1 answer, it's a very good idea to check the return value of scant(), which is the number of successfully read values.

Can I use fscanf to get only digits from text that contain chars and ints?

I want to extract digits from a file that contains characters and digits.
For example:
+ 321 chris polanco 23
I want to skip the '+' and get only the 321.
Here's the code I have so far.
while(fscanf(update, "%d", &currentIn->userid) != EOF){
currentIn->index = index;
rootIn = sort(rootIn, currentIn);
index = index + 1;
currentIn = malloc(sizeof(Index));
}
I was thinking that since I had %d that it would get the first digits that it finds but I was wrong. I'm open to better ways of doing this if you guys have any.
Instead of struggling with fscanf() (and running into format problems later), I recommend to use fgets() + sscanf() combination to process each line.
If you know the the integer you are interested in starts at 3rd position in each line of the file then you can do line+2 in sscanf() to read it. Otherwise, you can modify the sscanf() format string according to the format of your input file.
char line[MAX_LINE_LEN + 1];
While ( fgets(line, sizeof line, update) )
{
if(sscanf(line+2, "%d", &currentIn->userid) != 1)
{
/* handle failure */
}
...
}
while (fscanf(update, "%*[^0-9]%d", &currentIn->userid) == 1)
{
...
}
This skips over non-digits (that's the %*[^0-9] part) followed by an integer. The suppressed assignment isn't counted, so the == 1 ensures that you got a number.
Unfortunately, it runs into a problem if the first character in the file is a digit — as pointed out by Chris Dodd. There are multiple possible solutions to that:
ungetc('a', update); will give a non-digit to read first.
while ((fscanf(update, "%*[^0-9]"), fscanf(update, "%d", &currentIn->userid)) == 1)
Or:
while (fscanf(update, "%*[^0-9]%d", &currentIn->userid) == 1 ||
fscanf(update, "%d", &currentIn->userid) == 1)
{
...
}
Depending on which you think is more likely, you could reverse the order of these two fscanf() policies. With the scanf() family of functions, there's always a problem if the string of digits is so long that the number cannot be represented in an int; you get undefined behaviour. I don't attempt to address that.
This will pick up multiple numbers per line, one per invocation. If you want a single number per line, or otherwise want control over how each line is handled, then use fgets() or readline() to read the line, and then sscanf() to do the analysis. One advantage of this is that if you so choose, you can use careful functions like strtol() to convert digits to numbers.

Using scanf to read in certain amount of characters in C?

I am having trouble accepting input from a text file. My program is supposed to read in a string specified by the user and the length of that string is determined at runtime. It works fine when the user is running the program (manually inputting the values) but when I run my teacher's text file, it runs into an infinite loop.
For this example, it fails when I am taking in 4 characters and his input in his file is "ABCDy". "ABCD" is what I am supposed to be reading in and 'y' is supposed to be used later to know that I should restart the game. Instead when I used scanf to read in "ABCD", it also reads in the 'y'. Is there a way to get around this using scanf, assuming I won't know how long the string should be until runtime?
Normally, you'd use something like "%4c" or "%4s" to read a maximum of 4 characters (the difference is that "%4c" reads the next 4 characters, regardless, while "%4s" skips leading whitespace and stops at a whitespace if there is one).
To specify the length at run-time, however, you have to get a bit trickier since you can't use a string literal with "4" embedded in it. One alternative is to use sprintf to create the string you'll pass to scanf:
char buffer[128];
sprintf(buffer, "%%%dc", max_length);
scanf(buffer, your_string);
I should probably add: with printf you can specify the width or precision of a field dynamically by putting an asterisk (*) in the format string, and passing a variable in the appropriate position to specify the width/precision:
int width = 10;
int precision = 7;
double value = 12.345678910;
printf("%*.*f", width, precision, value);
Given that printf and scanf format strings are quite similar, one might think the same would work with scanf. Unfortunately, this is not the case--with scanf an asterisk in the conversion specification indicates a value that should be scanned, but not converted. That is to say, something that must be present in the input, but its value won't be placed in any variable.
Try
scanf("%4s", str)
You can also use fread, where you can set a read limit:
char string[5]={0};
if( fread(string,(sizeof string)-1,1,stdin) )
printf("\nfull readed: %s",string);
else
puts("error");
You might consider simply looping over calls to getc().

What is the use of the %n format specifier in C?

What is the use of the %n format specifier in C? Could anyone explain with an example?
Most of these answers explain what %n does (which is to print nothing and to write the number of characters printed thus far to an int variable), but so far no one has really given an example of what use it has. Here is one:
int n;
printf("%s: %nFoo\n", "hello", &n);
printf("%*sBar\n", n, "");
will print:
hello: Foo
Bar
with Foo and Bar aligned. (It's trivial to do that without using %n for this particular example, and in general one always could break up that first printf call:
int n = printf("%s: ", "hello");
printf("Foo\n");
printf("%*sBar\n", n, "");
Whether the slightly added convenience is worth using something esoteric like %n (and possibly introducing errors) is open to debate.)
Nothing printed. The argument must be a pointer to a signed int, where the number of characters written so far is stored.
#include <stdio.h>
int main()
{
int val;
printf("blah %n blah\n", &val);
printf("val = %d\n", val);
return 0;
}
The previous code prints:
blah blah
val = 5
I haven't really seen many practical real world uses of the %n specifier, but I remember that it was used in oldschool printf vulnerabilities with a format string attack quite a while back.
Something that went like this
void authorizeUser( char * username, char * password){
...code here setting authorized to false...
printf(username);
if ( authorized ) {
giveControl(username);
}
}
where a malicious user could take advantage of the username parameter getting passed into printf as the format string and use a combination of %d, %c or w/e to go through the call stack and then modify the variable authorized to a true value.
Yeah it's an esoteric use, but always useful to know when writing a daemon to avoid security holes? :D
From here we see that it stores the number of characters printed so far.
n The argument shall be a pointer to an integer into which is written the number of bytes written to the output so far by this call to one of the fprintf() functions. No argument is converted.
An example usage would be:
int n_chars = 0;
printf("Hello, World%n", &n_chars);
n_chars would then have a value of 12.
So far all the answers are about that %n does, but not why anyone would want it in the first place. I find it's somewhat useful with sprintf/snprintf, when you might need to later break up or modify the resulting string, since the value stored is an array index into the resulting string. This application is a lot more useful, however, with sscanf, especially since functions in the scanf family don't return the number of chars processed but the number of fields.
Another really hackish use is getting a pseudo-log10 for free at the same time while printing a number as part of another operation.
The argument associated with the %n will be treated as an int* and is filled with the number of total characters printed at that point in the printf.
The other day I found myself in a situation where %n would nicely solve my problem. Unlike my earlier answer, in this case, I cannot devise a good alternative.
I have a GUI control that displays some specified text. This control can display part of that text in bold (or in italics, or underlined, etc.), and I can specify which part by specifying starting and ending character indices.
In my case, I am generating the text to the control with snprintf, and I'd like one of the substitutions to be made bold. Finding the starting and ending indices to this substitution is non-trivial because:
The string contains multiple substitutions, and one of the substitutions is arbitrary, user-specified text. This means that doing a textual search for the substitution I care about is potentially ambiguous.
The format string might be localized, and it might use the $ POSIX extension for positional format specifiers. Therefore searching the original format string for the format specifiers themselves is non-trivial.
The localization aspect also means that I cannot easily break up the format string into multiple calls to snprintf.
Therefore the most straightforward way to find the indices around a particular substitution would be to do:
char buf[256];
int start;
int end;
snprintf(buf, sizeof buf,
"blah blah %s %f yada yada %n%s%n yakety yak",
someUserSpecifiedString,
someFloat,
&start, boldString, &end);
control->set_text(buf);
control->set_bold(start, end);
It doesn't print anything. It is used to figure out how many characters got printed before %n appeared in the format string, and output that to the provided int:
#include <stdio.h>
int main(int argc, char* argv[])
{
int resultOfNSpecifier = 0;
_set_printf_count_output(1); /* Required in visual studio */
printf("Some format string%n\n", &resultOfNSpecifier);
printf("Count of chars before the %%n: %d\n", resultOfNSpecifier);
return 0;
}
(Documentation for _set_printf_count_output)
It will store value of number of characters printed so far in that printf() function.
Example:
int a;
printf("Hello World %n \n", &a);
printf("Characters printed so far = %d",a);
The output of this program will be
Hello World
Characters printed so far = 12
Those who want to use %n Format Specifier may want to look at this:
Do Not Use the "%n" Format String Specifier
In C, use of the "%n" format specification in printf() and sprintf()
type functions can change memory values. Inappropriate
design/implementation of these formats can lead to a vulnerability
generated by changes in memory content. Many format vulnerabilities,
particularly those with specifiers other than "%n", lead to
traditional failures such as segmentation fault. The "%n" specifier
has generated more damaging vulnerabilities. The "%n" vulnerabilities
may have secondary impacts, since they can also be a significant
consumer of computing and networking resources because large
guantities of data may have to be transferred to generate the desired
pointer value for the exploit. Avoid using the "%n" format
specifier. Use other means to accomplish your purpose.
Source: link
In my opinion, %n in 1st argument of print function simply record the number of character it prints on the screen before it reach the the %n format code including white spaces and new line character.`
#include <stdio.h>
int main()
{
int i;
printf("%d %f\n%n", 100, 123.23, &i);
printf("%d'th characters printed on the screen before '%%n'", i);
}
output:
100 123.230000
15'th characters printed on the screen before '%n'(with new character).
We can assign the of i in an another way...
As we know the argument of print function:-
int printf(char *control-string, ...);
So, it returns the number the number of characters output. We can assign that return value to i.
#include <stdio.h>
int main()
{
int i;
i = printf("%d %f\n", 100, 123.23);
printf("%d'th characters printed on the screen.", i);
}
%n is C99, works not with VC++.

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