Checking contents of char variable - C Programming - c

This might seem like a very simple question, but I am struggling with it. I have been writing iPhone apps with Objective C for a few months now, but decided to learn C Programming to give myself a better grounding.
In Objective-C if I had a UILabel called 'label1' which contained some text, and I wanted to run some instructions based on that text then it might be something like;
if (label1.text == #"Hello, World!")
{
NSLog(#"This statement is true");
}
else {
NSLog(#"Uh Oh, an error has occurred");
}
I have written a VERY simple C Program I have written which uses printf() to ask for some input then uses scanf() to accept some input from the user, so something like this;
int main()
{
char[3] decision;
Printf("Hi, welcome to the introduction program. Are you ready to answer some questions? (Answer yes or no)");
scanf("%s", &decision);
}
What I wanted to do is apply an if statement to say if the user entered yes then continue with more questions, else print out a line of text saying thanks.
After using the scanf() function I am capturing the users input and assigning it to the variable 'decision' so that should now equal yes or no. So I assumed I could do something like this;
if (decision == yes)
{
printf("Ok, let's continue with the questions");
}
else
{
printf("Ok, thank you for your time. Have a nice day.");
}
That brings up an error of "use of undeclared identifier yes". I have also tried;
if (decision == "yes")
Which brings up "result of comparison against a string literal is unspecified"
I have tried seeing if it works by counting the number of characters so have put;
if (decision > 3)
But get "Ordered comparison between pointer and integer 'Char and int'"
And I have also tried this to check the size of the variable, if it is greater than 2 characters it must be a yes;
if (sizeof (decision > 2))
I appreciate this is probably something simple or trivial I am overlooking but any help would be great, thanks.

Daniel Haviv's answer told you what you should do. I wanted to explain why the things you tried didn't work:
if (decision == yes)
There is no identifier 'yes', so this isn't legal.
if (decision == "yes")
Here, "yes" is a string literal which evaluates to a pointer to its first character. This compares 'decision' to a pointer for equivalence. If it were legal, it would be true if they both pointed to the same place, which is not what you want. In fact, if you do this:
if ("yes" == "yes")
The behavior is undefined. They will both point to the same place if the implementation collapses identical string literals to the same memory location, which it may or may not do. So that's definitely not what you want.
if (sizeof (decision > 2))
I assume you meant:
if( sizeof(decision) > 2 )
The 'sizeof' operator evaluates at compile time, not run time. And it's independent of what's stored. The sizeof decision is 3 because you defined it to hold three characters. So this doesn't test anything useful.
As mentioned in the other answer, C has the 'strcmp' operator to compare two strings. You could also write your own code to compare them character by character if you wanted to. C++ has much better ways to do this, including string classes.
Here's an example of how you might do that:
int StringCompare(const char *s1, const char *s2)
{ // returns 0 if the strings are equivalent, 1 if they're not
while( (*s1!=0) && (*s2!=0) )
{ // loop until either string runs out
if(*s1!=*s2) return 1; // check if they match
s1++; // skip to next character
s2++;
}
if( (*s1==0) && (*s2==0) ) // did both strings run out at the same length?
return 0;
return 1; // one is longer than the other
}

You should use strcmp:
if(strcmp(decision, "yes") == 0)
{
/* ... */
}

You should be especially careful with null-terminated string in C programming. It is not object. It is a pointer to a memory address. So you can't compare content of decision directly with a constant string "yes" which is at another address. Use strcmp() instead.
And be careful that "yes" is actually "yes\0" which will take 4 bytes and the "\0" is very important to strcmp() which will be recognized as the termination during the comparison loop.

Ok a few things:
decision needs to be an array of 4 chars in order to fit the string "yes" in it. That's because in C, the end of a string is indicated by the NUL char ('\0'). So your char array will look like: { 'y', 'e', 's', '\0' }.
Strings are compared using functions such as strcmp, which compare the contents of the string (char array), and not the location/pointer. A return value of 0 indicates that the two strings match.
With: scanf("%s", &decision);, you don't need to use the address-of operator, the label of an array is the address of the start of the array.
You use strlen to get the length of a string, which will just increment a counter until it reaches the NUL char, '\0'. You don't use sizeof to check the length of strings, it's a compile-time operation which will return the value 3 * sizeof(char) for a char[3].
scanf is unsafe to use with strings, you should alternatively use fgets(stdin...), or include a width specifier in the format string (such as "3%s") in order to prevent overflowing your buffer. Note that if you use fgets, take into account it'll store the newline char '\n' if it reads a whole line of text.

To compare you could use strcmp like this:
if(strcmp(decision, "yes") == 0) {
// decision is equal to 'yes'
}
Also you should change char decision[3] into char decision[4] so that the buffer has
room for a terminating null character.
char decision[4] = {0}; // initialize to 0

There's several issues here:
You haven't allocated enough storage for the answer:
char[3] decision;
C strings are bytes in the string followed by an ASCII NUL byte: 0x00, \0. You have only allocated enough space for ye\0 at this point. (Well, scanf(3) will give you yes\0 and place that NUL in unrelated memory. C can be cruel.) Amend that to include space for the terminating \0 and amend your scanf(3) call to prevent the buffer overflow:
char[4] decision;
/* ... */
scanf("%3s", decision);
(I've left off the &, because simply giving the name of the array is the same as giving the address of its first element. It doesn't matter, but I believe this is more idiomatic.)
C strings cannot be compared with ==. Use strcmp(3) or strncmp(3) or strcasecmp(3) or strncasecmp(3) to compare your strings:
if(strcasecmp(decision, "yes") == 0) {
/* yes */
}

C has lots of lib functions to handle this but it pays to know what you are declaring.
Declaring
char[3] decision;
is actually declaring a char array of length 3. So therefor attempting a comparison of
if(decision == "yes")
is comparing a literal against and array and therefor will not work. Since there is no defined string type in C you have to use pointers, but not directly, if you don't want to. In C strings are in fact arrays of char so you can declare them both ways eg:
char[3] decision ;
* char decision ;
Both will in point of fact work but you in the first instance the compiler will allocate the memory for you, but it will ONLY allocate 3 bytes. Now since strings in C are null terminated you need to actually allocate 4 bytes since you need room for "yes" and the null. Declaring it the second way simply declares a pointer to someplace in memory but you have no idea really where. You would then have to allocate memory to contain whatever you are going to put there since to do otherwise will more then likely cause a SEGFAULT.
To compare what you get from input you have two options, either use the strcomp() function or do it yourself by iterating through decision and comparing each individual byte against "Y" and "E" and "S" until you hit null aka \0.
There are variations on strcomp() to deal with uppercase and lowercase and they are part of the standard string.h library.

Related

Trouble with C else, if and scanf

I am new to C and I'm making my first "decent" program. I am facing trouble with my code.
char username[] = "root";
char usernametry[10];
scanf("%s",usernametry);
if (usernametry == username)
{
printf("Welcome ROOT user\n");
}
else
{
printf("Try again\n");
}
Whenever I execute my program, everything works; however, when I try to log in and type root as the username, it returns Try again. Can someone suggest why this is happening? I am very new to C so I apologize for my lack of C knowledge.
usernametry and username are arrays of type char. When you do usernametry == username, you are not comparing if two strings are equal, you are comparing the memory address of the first char in usernametry with the memory address of the first char in username. In order to compare if two strings are equal, you should use strcmp. Also, consider replacing %s in your scanf with %9s so that usernametry will never exceed 9 chars (which would have been a buffer overflow, as the 10th char needs to be '\0', the null character, which is used to terminate C-style strings).
You can't compare strings using ==, this is not a that-high-level-language. See strcmp() for comparing strings.
Incidentally, what if someone enters a name longer than 10 characters? You're drifting towards a buffer overflow bug in your code ;-)
use strcmp() functions to compare 2 strings instead of ==
use of == is wrong because in this case you compare address of usernametry and username and of course they are always differents.
if(strcmp(usernametry, username) == 0) // return 0 if equal !=0 if not equal
Use strcmp to compare two strings:
if(!strcmp(usernametry,username))
Your variables are not the same pointer.
Replace by:
if (!strcmp(usernametry, username))

C - test string equivalence without strcmp

in one of my university assignments I am restricted in the libraries I use. I am new to C and pointers and want to see if two strings (or should I say char's) are equal.
Part of me wants to loop through every char of the 'char string' and test equivalence, but then it comes back how to test equivalence (lol).
Any help is appreciated.
edit: I am seeing this:
warning: result of comparison against a string literal is
unspecified (use strncmp instead) [-Wstring-compare]
which leads to a segmentation fault. I know it has to do with this piece of code because all I added was:
if (example.name == "testName"){
printf("here!\n");
}
Part of me wants to loop through every char of the 'char string' and test equivalence
That's exactly what you need to do. Make a function mystrcmp with the signature identical to regular strcmp,
int mystrcmp ( const char * str1, const char * str2 );
and write your own implementation.
but then it comes back how to test equivalence.
When you loop character-by-character, you test equivalence of individual characters, not strings. Characters in C can be treated like numbers: you can compare them for equality using ==, check what character code is less than or greater than using < and >, and so on.
The only thing left to do now is deciding when to stop. You do that by comparing the current character of each string to zero, which is the null terminator.
Don't forget to forward-declare your mystrcmp function before using it.
A string in C is terminated with null character(0x00 or \0).You should compare both strings in a loop character by character till null char for either of the string is reached.
Loop should be broken if characters are not equal.
EDIT:
To answer your edit in question:
You should take two character pointers pointing to both strings and then copmare them like
//loop start,loop till null for any one of the string is found
if(*ptr1 != *ptr2)
{
//break loop
}
ptr1++;ptr2++;
//end loop
if((*ptr1 == *ptr2) &&(*ptr1== 0x00))
{
//strings are equal
}
Given that this is a university assignment, you should pay heed to chars just being small integers. You should also pay heed that C strings are contiguous memory buffers terminated by a binary zero (0x00).
You should also learn about pointer math. You will learn ways to shorten the code you have to write while learning something really interesting concerning the C language and how computers work. It will certainly help you if you choose a career on lower-level programming.

C strings initialization. Possible trouble

Good day everyone. I have the following c-string initialization:
*(str) = 0; where str is declared as char str[255];. The questions are:
1) is it the best way to initialize a string this way?
2) should I expect trouble with this code on 64-bit platform?
Thanks in advance.
The instruction *(str) = 0 is completely equivalent to str[0] = 0;.
The initiatlization simply place the integer 0 (which is equivalent to '\0') in the first position, making the string empty.
1) There's no "best way", the key is being consistent so if you choose the "*(str) = 0;" path, always do the same everywhere.
2) No trouble could ever come from that statement. Whatever the architecture, the compiler will take care of everything.
What this does is change the uninitialized char array str, of which each element has some undefined value, to have its first element set to the null terminator character.
As far as most string related functions go, they will traverse this string from left to right, and immediately see the null terminator.
There are surely functions that don't care about null terminators, and are given a maximum size. In these cases, you are reading undefined values, which is Badâ„¢.
Use this instead:
char str[255] = { 0 }; // zero every element
or initialize the string to something useful on first use.

C string - identical, and not matching? [duplicate]

This question already has answers here:
Using the equality operator == to compare two strings for equality in C [duplicate]
(9 answers)
Closed 9 years ago.
Got a small problem with C. Restricting myself to simple C (i.e. OS instructions), and two strings seem to not be the same. Here is my code:
char inputData[256];
int rid;
rid = read(0,inputData,256);
// Strip input
char command[rid];
int i;
for (i = 0; i<=rid-2; i++) {
command[i] = inputData[i];
}
command[rid-1] = '\0';
if (command == "exit") {
write(1,"exit",sizeof("exit"));
}
Now, if a user enters "exit" into the terminal when queried and hits enter, the if for detecting "exit" never gets run. Any ideas?
Thanks,
EDIT: I am commiting to git as I go, so the current version can be found at github.com/samheather/octo-os. It's very obviously not complete code, but it demonstrates the problem.
You can't compare strings with ==. You need to use strcmp.
if (strcmp(command, "exit") == 0) {
C strings are actually character arrays. You can think of "command" as a pointer to the first character. You want to compare every character in the string, not just the location of the first characters.
You should use strcmp to compare strings in C.
if(strcmp(command, "exit") == 0) //strcmp returns 0 if strings are equal
To quote:
A zero value indicates that both strings are equal. A value greater than zero indicates
that the first character that does not match has a greater value in str1 than in str2.
a value less than zero indicates the opposite.
As it stands right now, you're comparing the address of command with the address of the string literal "exit", which pretty much can't be the same.
You want to compare the contents, with either strcmp, or (if "only OS instructions" means no standard library functions) an equivalent you write yourself that walks through the strings and compares characters they contain.
As others said, == doesn't work with strings. The reason is that it would compare the pointers given.
In the expression
command == "exit"
command is a pointer to your array variable, while "exit" is a pointer to that string which resides in read-only data space. They can never be identical, so the comparison always is false.
That's why strcmp() is the way to go.
Use strcmp from the standard library.

Looping until a specific string is found

I have a simple question. I want to write a program in C that scans the lines of a specific file, and if the only phrase on the line is "Atoms", I want it to stop scanning and report which line it was on. This is what I have and is not compiling because apparently I'm comparing an integer to a pointer: (of course "string.h" is included.
char dm;
int test;
test = fscanf(inp,"%s", &dm);
while (test != EOF) {
if (dm=="Amit") {
printf("Found \"Atoms\" on line %d", j);
break;
}
j++;
}
the file was already opened with:
inp = fopen( .. )
And checked to make sure it opens correctly...
I would like to use a different approach though, and was wondering if it could work. Instead of scanning individual strings, could I scan entire lines as such:
// char tt[200];
//
// fgets(tt, 200, inp);
and do something like:
if (tt[] == "Atoms") break;
Thanks!
Amit
Without paying too much attention to your actual code here, the most important mistake your making is that the == operator will NOT compare two strings.
In C, a string is an array of characters, which is simply a pointer. So doing if("abcde" == some_string) will never be true unless they point to the same string!
You want to use a method like "strcmp(char *a, char *b)" which will return 0 if two strings are equal and something else if they're not. "strncmp(char *a, char *b, size_t n)" will compare the first "n" characters in a and b, and return 0 if they're equal, which is good for looking at the beginning of strings (to see if a string starts with a certain set of characters)
You also should NOT be passing a character as the pointer for %s in your fscanf! This will cause it to completely destroy your stack it tries to put many characters into ch, which only has space for a single character! As James says, you want to do something like char ch[BUFSIZE] where BUFSIZE is 1 larger than you ever expect a single line to be, then do "fscanf(inp, "%s", ch);"
Hope that helps!
please be aware that dm is a single char, while you need a char *
more: if (dm=="Amit") is wrong, change it in
if (strcmp(dm, "Amit") == 0)
In the line using fscanf, you are casting a string to the address of a char. Using the %s in fscanf should set the string to a pointer, not an address:
char *dm;
test = fscanf(inp,"%s", dm);
The * symbol declares an indirection, namely, the variable pointed to by dm. The fscanf line will declare dm as a reference to the string captured with the %s delimiter. It will point to the address of the first char in the string.
What kit said is correct too, the strcmp command should be used, not the == compare, as == will just compare the addresses of the strings.
Edit: What kit says below is correct. All pointers should be allocated memory before they are used, or else should be cast to a pre-allocated memory space. You can allocate memory like this:
dm = (char*)malloc(sizeof(char) * STRING_LENGTH);
where STRING_LENGTH is a maximum length of a possible string. This memory allocation only has to be done once.
The problem is you've declared 'dm' as a char, not a malloc'd char* or char[BUFSIZE]
http://www.cplusplus.com/reference/clibrary/cstdio/fscanf/
You'll also probably report incorrect line numbers, you'll need to scan the read-in buffer for '\n' occurences, and handle the case where your desired string lies across buffer boundaries.

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