I want to make a function that tests for numbers in string values in C. I have no idea where to start so I did not provide any code. How can I do this?
Also, how can I make this work for hexadecimal?
<ctype.h> provides two functions, isalpha and isdigit, that may be what you're looking for. Just iterate on the characters of the string and check if isdigit() ever returns true.
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I am implementing a shell in C11, and I want to check if the input has the correct syntax before doing a system call to execute the command. One of the possible inputs that I want to guard against is a string made up of only white-space characters. What is an efficient way to check if a string contains only white spaces, tabs or any other white-space characters?
The solution must be in C11, and preferably using standard libraries. The string read from the command line using readline() from readline.h, and it is a saved in a char array (char[]). So far, the only solution that I've thought of is to loop over the array, and check each individual char with isspace(). Is there a more efficient way?
So far, the only solution that I've thought of is to loop over the array, and check each individual char with isspace().
That sounds about right!
Is there a more efficient way?
Not really. You need to check each character if you want to be sure only space is present. There could be some trick involving bitmasks to detect non-space characters in a faster way (like strlen() does to find a NUL terminator), but I would definitely not advise it.
You could make use of strspn() or strcspn() checking the returned value, but that would surely be slower since those functions are meant to work on arbitrary accept/reject strings and need to build lookup tables first, while isspace() is optimized for its purpose using a pre-built lookup table, and will most probably also get inlined by the compiler using proper optimization flags. Other than this, vectorization of the code seems like the only way to speed things up further. Compile with -O3 -march=native -ftree-vectorize (see also this post) and run some benchmarks.
"loop over the array, and check each individual char with isspace()" --> Yes go with that.
The time to do that is trivial compared to readline().
I'm going to provide an alternative solution to your problem: use strtok. It splits a string into substrings based on a specific set of ignored delimiters. With an empty string, you'd just get no tokens at all.
If you need more complicated matching than that for your shell (eg. To do quoted arguments) you're best off writing a small tokenizer/lexer. The strtok method is basically to just look for any of the delimeters you've specified, temporarily replace them with \0, returning the substring up to that point, putting the old character back, and repeating until it reaches the end of the string.
Edit:
As the busybee points out in the comment below, strtok does not put back the character that it replaces with \0. The above paragraph was worded poorly, but my intent was to explain how to implement your own simple tokenizer/lexer if you needed to, not to explain exactly how strtok works down to the smallest detail.
I was asked to write a program that operates on a given string. The commands come in the form of two letters followed by nothing, or an int(s) or string(s). The commands work on the given string (reversing it, multiplying it, replacing instances of a substring with another substring).
I'm pretty new to C and programming in general, and I have difficulty in recieving the commands themselves. How do I both make sure that the command I'm given is correct in both name and arguments? Will I need to use an array of functions (does that exist?) after I've found that the command I was given was correct?
I'd recommend that you learn how to use sscanf, which sounds perfect for what you want.
If the string is stored in the array a, you can use something like this to see if the two letters are "IA" followed by an int:
sscanf(a, "IA %d", &intVar);
If you want to check for the case of the letters "SA" followed by a string:
sscanf(a, "SA %s", &charArray);
The key here is checking the return value of sscanf, so you can know how many of the arguments were successfully assigned values from the format string. This means you can also add arguments for more strings, assuming that there's some maximum number of ints / strings that could follow the two letters.
It may sound strange that knowing a lot about iOS and having some experience in .net, I am a newcomer to C. Somewhere I got this target to find average of n numbers without using printf and scanf. I don't want the code for the program but I am seeking alternatives to the mentioned functions.
Is code with printf/scanf required here? Also do let me know if my query stands invalid.
No, neither printf nor scanf is really needed for this.
The obvious alternatives would be to read the input with something like getc or fgets and convert from characters to numbers with something like strtol.
On the output side, you'd more or less reverse that, converting from numbers to characters (e.g., with itoa which is quite common, though not actually standard), then printing out the resulting string (e.g., with fputs).
I have a file input, in which i have the following data.
1 1Apple 2Orange 10Kiwi
2 30Apple 4Orange 1Kiwi
and so on. I have to read this data from file and work on it but i dont know how to retrieve the data. I want to store 1(of 1 apple) as integer and then Apple as a string.
I thought of reading the whole 1Apple as a string. and then doing something with the stoi function.
Or I could read the whole thing character by character and then if the ascii value of that character lies b/w 48 to 57 then i will combine that as an integer and save the rest as string? Which one shall I do? Also how do I check what is the ASCII value of the char. (shall I convert the char to int and then compare, or is there any inbuilt function?)
How about using the fscanf() function if and only if your input pattern is not going to change. Otherwise you should probably use fgets() and perform checks if you want to separate the number from the string such as you suggested.
There is one easy right way to do this with standard C library facilities, one rather more difficult right way, and a whole lot of wrong ways. This is the easy right way:
Read an entire line into a char[] buffer using fgets.
Extract numbers from this line using strtol or strtoul.
It is very important to understand why the easier-looking alternatives (*scanf and atoi) should never be used. You might write less code initially, but once you start thinking about how to handle even slightly malformed input, you will discover that you should have used strtol.
The "rather more difficult right way" is to use lex and yacc. They are much more complicated but also much more powerful. You shouldn't need them for this problem.
I am using strtok().But i want to print corresponding delimiters also as we do using StringTokenizer in Java.Is there any function which provides this functionality(printing delimiters) ?
Based on OP's comments, tokenization is not what is actually desired. You want to use strstr(), not strtok(). That will tell you if the string is present, and then you can use strcpy() and strcat() as appropriate.
Please note, the "n" versions of these methods, i.e. strncpy and strncat, are safer -- less likely to crash due to buffer overrun.
i want to print corresponding delimiters also as we do using StringTokenizer in Java
Java's StringTokenizer doesn't return delimeters.
In any case, there is no such function in C. You'll have to write one (using strchr, etc.)
How about using glib?
It seems
http://library.gnome.org/devel/glib/2.26/glib-String-Utility-Functions.html#g-strsplit
is exactly what you're looking for.