I am Just a begineer in C Programming. While solving a programming assignement I came across the need to convert an array of unsigned char to integer.
For Example:
unsigned char x[]="567";
unsigned char y[]="94";
Now I have to add the integer values in both x and y. That is:
int sum=661;
What is the simplest way to do this?
You're looking for atoi() .
You have at least two options if you use standard library. The first is atoi() function from stdlib.h and second is sscanf()function from stdio.h.
Here are examples:
atoi()
char number_string[] = "47";
int number;
number = atoi(number_string);
sscanf()
char number_string[] = "47";
int number;
int return;
return = sscanf(number_string, "%d", &number);
/* it would be good idea to check the return value here */
sscanf() gives you a better error handling. Return value of sscanf() determines how many items were succesfully read (how many variables were filled). If there was an input failure, EOF is returned.
atoi() returns 0 if no valid conversion could be performed.
Related
I'm trying to understand a line of code in a problem set provided by CS50. Herein lies the issue:
can a number both be simultaneously a string an an integer? Isn't a string traditionally considered to be a string of letters (i.e a word etc? or a paragraph)?
if a number can be both a string and an integer, what is the use of differentiating it? Particularly in the line of code below.
#include <cs50.h>
#include <stdio.h>
int get_positive_int(string prompt);
int main(void)
{
int i = get_positive_int("Positive Integer: ");
printf("%i\n", i);
}
// Prompt user for positive integer
int get_positive_int (string prompt)
{
int n;
do
{
n = get_int("%s", prompt);
}
while (n <0 || n > 8);
return n;
}
"No!" a variable can't have multiple data types. This may be possible but only with the representation context. For Example:
String a= "1";
Int b=1;
1)We can print both variables and both will give us the same output.
2)We can perform calculations on the second one but to perform calculations on the string, you may need to convert it into number format - either int, double or float.
I saw your follow up question. The difference between these two functions is clear with my second point. You have to pass a string value to the function int get_positive_int(string prompt). But in the case of int get_positive_int(int prompt), you may have to pass an integer value to this function. Both these functions will return an integer value. It depends what you are doing inside the function.
For the function int get_positive_integer(string prompt), you may get a string from the user and convert it into an integer, then return that integer value.
The answer to your question is "no". A number can have one of several C types (e.g. int, double, ...), but only one of them, and string is not a numeric type.
I'm not sure what's unclear to you about the code.
I am trying to use sscanf to convert an array of char in C to a double. My issue is best described in this short minimal working example:
#include <stdio.h>
int main (int argc, char **argv)
{
char string_one[] = "1.2e-4";
char string_two[6] = "1.2e-4";
double one, two;
sscanf(string_one, "%lf", &one);
sscanf(string_two, "%lf", &two);
printf("%e, %e\n", one, two);
return 0;
}
Compiled with GCC and running the resulting executable returns 1.200000e-04, 1.200000e-41, instead of the same number twice. The issue does not appear if the floating point number I want to convert is of the form 3e4(if there is no decimal separator).
For my program I have to define an array of char with a fixed size (as I am filling the array during the course of the program), which is why I am running into this issue.
I guess the problem might be pretty obvious - but as I am a newbie in C I am out of ideas what the problem could be. Any help appreciated!
char string_two[6] = "1.2e-4"; ... sscanf(string_two, "%lf", &two); expects to scan a string. string_two[] is not a string as it lacks a null character. Result: undefined behavior (UB).
Solution: Insure string_two[] is a string.
I'm using atoi to convert a string integer value into integer.
But first I wanted to test different cases of the function so I have used the following code
#include <stdio.h>
int main(void)
{
char *a ="01e";
char *b = "0e1";
char *c= "e01";
int e=0,f=0,g=0;
e=atoi(a);
f=atoi(b);
g=atoi(c);
printf("e= %d f= %d g=%d ",e,f,g);
return 0;
}
this code returns e= 1 f= 0 g=0
I don't get why it returns 1 for "01e"
that's because atoi is an unsafe and obsolete function to parse integers.
It parses & stops when a non-digit is encountered, even if the text is globally not a number.
If the first encountered char is not a space or a digit (or a plus/minus sign), it just returns 0
Good luck figuring out if user input is valid with those (at least scanf-type functions are able to return 0 or 1 whether the string cannot be parsed at all as an integer, even if they have the same behaviour with strings starting with integers) ...
It's safer to use functions such as strtol which checks that the whole string is a number, and are even able to tell you from which character it is invalid when parsing with the proper options set.
Example of usage:
const char *string_as_number = "01e";
char *temp;
long value = strtol(string_as_number,&temp,10); // using base 10
if (temp != string_as_number && *temp == '\0')
{
// okay, string is not empty (or not only spaces) & properly parsed till the end as an integer number: we can trust "value"
}
else
{
printf("Cannot parse string: junk chars found at %s\n",temp);
}
You are missing an opportunity: Write your own atoi. Call it Input2Integer or something other than atoi.
int Input2Integer( Str )
Note, you have a pointer to a string and you will need to establish when to start, how to calculate the result and when to end.
First: Set return value to zero.
Second: Loop over string while it is not null '\0'.
Third: return when the input character is not a valid digit.
Fourth: modify the return value based on the valid input character.
Then come back and explain why atoi works the way it does. You will learn. We will smile.
Two part question;
I'm Coming from a high level Language, so this is a question about form not function;
I've written an isnumeric() function that takes a char[] and returns 1 if the string is a number taking advantage of the isdigit() function in ctype. Similar functions are builtin to other languages and I have always used something like that to integrity check the data before converting it to a numeric type. Mostly because some languages conversion functions fail badly if you try to convert a non-number string to an integer.
But it seems like a kludge having to do all that looping to compensate for the lack of strings in C, which poses the first part of the question;
Is it acceptable practice in C to trap for a 0 return from atoi() in lieu of doing an integrity check on the data before calling atoi()? The way atoi() (and other ascii to xx functions) works seems to lend itself well to eliminating the integrity check altogether. It would certainly seem more efficient to just skip the check.
The second part of the question is;
Is there a C function or common library function for a numeric integrity check
on a string? (by string, I of course mean char[])
Is it acceptable practice in C to trap for a 0 return from atoi() in lieu of doing an integrity check on the data before calling atoi()?
Never ever trap on error unless the error indicates a programming error that can't happen if there isn't a bug in the code. Always return some sort of error result in case of an error. Look at the OpenBSD strtonum function for how you could design such an interface.
The second part of the question is; Is there a C function or common library function for a numeric integrity check on a string? (by string, I of course mean char[])
Never use atoi unless you are writing a program without error checking as atoi doesn't do any error checking. The strtol family of functions allow you to check for errors. Here is a simply example of how you could use them:
int check_is_number(const char *buf)
{
const char *endptr;
int errsave = errno, errval;
long result;
errno = 0;
result = strtol(buf, &endptr, 0);
errval = errno;
errno = errsave;
if (errval != 0)
return 0; /* an error occured */
if (buf[0] == '\0' || *endptr != '\0')
return 0; /* not a number */
return 1;
}
See the manual page linked before for how the third argument to strtol (base) affects what it does.
errno is set to ERANGE if the value is out of range for the desired type (i.e. long). In this case, the return value is LONG_MAX or LONG_MIN.
If the conversion method returns an error indication (as distinct from going bananas if an error occurs, or not providing a definitive means to check if an error has occurred) then there is actually no need to check if a string is numeric before trying to convert it.
With that in mind, using atoi() is not a particularly good function to use if you need to check for errors on conversion. Zero will be returned for zero input, as well as an error, and there is no way to check on why. A better function to use is (assuming you want to read an integral value) is strtol(). Although strtol() returns zero on integer, it also returns information that can be used to check for failure. For example;
long x;
char *end;
x = strtol(your_string, &end, 10);
if (end == your_string)
{
/* nothing was read due to invalid character or the first
character marked the end of string */
}
else if (*end != '\0`)
{
/* an integral value was read, but there is following non-numeric data */
}
Second, there are alternatives to using strtol(), albeit involving more overhead. The return values from sscanf() (and, in fact, all functions in the scanf() family) can be checked for error conditions.
There is no standard function for checking if a string is numeric, but it can be easily rolled using the above.
int IsNumeric(char *your_string)
{
/* This has undefined behaviour if your_string is not a C-style string
It also deems that a string like "123AB" is non-numeric
*/
long x;
char *end;
x = strtol(your_string, &end, 10);
return !(end == your_string || *end != '\0`);
}
No (explicit) loops in any of the above options.
Is it acceptable practice in C to trap for a 0 return from atoi() in lieu of doing an integrity check on the data before calling atoi()?
No. #FUZxxl well answers that.
Is there a C function or common library function for a numeric integrity check on a string?
In C, the conversion of a string to a number and the check to see if the conversion is valid is usually done together. The function used depends on the type of number sought. "1.23" would make sense for a floating point type, but not an integer.
// No error handle functions
int atoi(const char *nptr);
long atol(const char *nptr);
long long atoll(const char *nptr);
double atof(const char *nptr);
// Some error detection functions
if (sscanf(buffer, "%d", &some_int) == 1) ...
if (sscanf(buffer, "%lf", &some_double) == 1) ...
// Robust methods use
long strtol( const char *nptr, char ** endptr, int base);
long long strtoll( const char *nptr, char ** endptr, int base);
unsigned long strtoul( const char *nptr, char ** endptr, int base);
unsigned long long strtoull( const char *nptr, char ** endptr, int base);
intmax_t strtoimax(const char *nptr, char ** endptr, int base);
uintmax_t strtoumax(const char *nptr, char ** endptr, int base);
float strtof( const char *nptr, char ** endptr);
double strtod( const char *nptr, char ** endptr);
long double strtold( const char *nptr, char ** endptr);
These robust methods use char ** endptr to store the string location where scanning stopped. If no numeric data was found, then *endptr == nptr. So a common test could is
char *endptr;
y = strto...(buffer, ..., &endptr);
if (buffer == endptr) puts("No conversion");
if (*endptr != '\0') puts("Extra text");
If the range was exceed these functions all set the global variable errno = ERANGE; and return a minimum or maximum value for the type.
errno = 0;
double y = strtod("1.23e10000000", &endptr);
if (errno == ERANGE) puts("Range exceeded");
The integer functions allow a radix selection from base 2 to 36. If 0 is used, the leading part of the string "0x", "0X", "0", other --> base 16, 16, 8, 10.
long y = strtol(buffer, &endptr, 10);
Read the specification or help page for more details.
You probably don't need a function to check whether a string is numeric. You will most likely need to convert the string to a number so just do that. Then check if the convertion is successful.
long number;
char *end;
number = strtol(string, &end, 10);
if ((*string == '\0') || (*end != '\0'))
{
// empty string or invalid number
}
the second argument of strtol is used to indicate where the parsing ended (the first non-numeric character). That character will be \0 if we've reached the end of the string. If you want to permit other characters after the number (like ), you can use switch to check for it.
strtol works with long integers. If you need some other type, you should consult the man page: man 3 strtol. For floating-point numbers you can use strtod.
Don't trap if the program logic permits that the string is not numeric (e.g. if it comes from the user or a file).
OP later commneted:
I'm looking for a way to determine if the string contains ONLY base 10 digits or a decimal or a comma. So if the string is 100,000.01 I want a positive return from func. Any other ascii characters anywhere in the string would result in a negative return value.
If is all your interest, use;
if (buffer[strspn(buffer, "0123456789.,")] == '\0') return 0; // Success
else return -1; // Failure
I have a char array, and I want to get the first number from it.
e.g if my char array is 34 400 43 33. I want 34 as in int.
int FirstInt(char chars[])
{
return atoi(chars.substr(0, bursts.find(' ')));
}
I was thinkign something like this but it is not valid. ANy ideas?
int FirstInt(char chars[])
{
int x;
sscanf(chars, "%d", &x);
return x;
}
You don't need to tokenize the string or use sscanf if all you want is the first number:
return atoi(str);
From the man page
The atoi() function converts the initial portion of the string pointed
to by nptr to int
Which means it will stop when it finds a non-numeric character, like a white space.
Edit:
Note that it's impossible to detect errors with atoi, since it returns 0 on error in some implementations and doesn't set errno (AFAIK), so it's probably better to use strtol
See this link Converting Strings to Numbers