This question already has answers here:
How do I properly compare strings in C?
(10 answers)
Closed 6 years ago.
How do I compare these two character arrays to make sure they are identical?
char test[10] = "idrinkcoke"
char test2[10] = "idrinknote"
I'm thinking of using for loop, but I read somewhere else that I couldnt do test[i] == test2[i] in C.
I would really appreciate if someone could help this. Thank you.
but I read somewhere else that I couldnt do test[i] == test2[i] in C.
That would be really painful to compare character-by-character like that. As you want to compare two character arrays (strings) here, you should use strcmp instead:
if( strcmp(test, test2) == 0)
{
printf("equal");
}
Edit:
There is no need to specify the size when you initialise the character arrays. This would be better:
char test[] = "idrinkcoke";
char test2[] = "idrinknote";
It'd also be better if you use strncmp - which is safer in general (if a character array happens to be NOT NULL-terminated).
if(strncmp(test, test2, sizeof(test)) == 0)
You can use the C library function strcmp
Like this:
if strcmp(test, test2) == 0
From the documentation on strcmp:
Compares the C string str1 to the C string str2.
This function starts comparing the first character of each string. If
they are equal to each other, it continues with the following pairs
until the characters differ or until a terminating null-character is
reached.
This function performs a binary comparison of the characters. For a
function that takes into account locale-specific rules, see strcoll.
and on the return value:
returns 0 if the contents of both strings are equal
Related
This question already has answers here:
How to find the size of an array (from a pointer pointing to the first element array)?
(17 answers)
Closed 4 years ago.
Is there any better way of getting the right length of array containing digits?
I have an array of digits: 0, 0, 1 and I try to get length of it. It obviously breaks and returns 0. I am new to C but I tried to make custom strlen function:
int custom_strlen(char *str) {
for(int i = 1; ;i++) {
if (str[i] == 0) {
return i;
}
}
return -47;
}
but it is not that efficient and in some cases returns unexpected values as well. The expected out put would be 3 in this case.
Is there any function to use?
An array of integers is not a string. C arrays do not contain length information inherently. The way strlen works is that C strings are null terminated, meaning the last character is NUL (null character), which is 0. Otherwise, there is just no way to know how long an array is.
I think you may be wanting to do an array of '0','0','1'. Can you post the array you are using?
As mentioned, C strings are null terminated.
The only choices are
Using a string terminated with some special character that you watch for (like a null) or
Keeping track of how long the string is when you create it.
FWIW, if it's not null terminated, it's not actually a string in C, it's just memory that contains chars that you happen to be interpreting as a string.
This question already has answers here:
How do I properly compare strings in C?
(10 answers)
Closed 4 years ago.
#include<stdio.h>
int main()
{
char str1[] = "ComputerProgram";
char str2[] = "ComputerProgram";
(str1==str2)? (printf("Equal")):(printf("unequal"));
return 0;
}
The answer according to me should be equal but it comes out to be unequal.
However if I use strcmp(str1,str2) == 0 answer comes out to be equal. How is it working in == case.? Also, I tried to print the ASCII values of srt1 and str2, they came out to be different. So I think that might be the reason. Now the problem is how does == work for strings?
Your arrays str1 and str2 will decay to pointers to their first elements when you compare them. That is, you compare two pointers that will never be equal.
In short, your comparison str1 == str2 is equal to &str1[0] == &str2[0].
What strcmp does differently is that is compares each character in the first string against each corresponding character in the other string, in a loop.
str1==str2 is comparing the addresses of the strings, not the strings themselves.strcmp will go to these addresses and compare all characters.
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.
I wrote this small piece of code in C to test memcmp() strncmp() strcmp() functions in C.
Here is the code that I wrote:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main() {
char *word1="apple",*word2="atoms";
if (strncmp(word1,word2,5)==0)
printf("strncmp result.\n");
if (memcmp(word1,word2,5)==0)
printf("memcmp result.\n");
if (strcmp(word1,word2)==0)
printf("strcmp result.\n");
}
Can somebody explain me the differences because I am confused with these three functions?
My main problem is that I have a file in which I tokenize its line of it,the problem is that when I tokenize the word "atoms" in the file I have to stop the process of tokenizing.
I first tried strcmp() but unfortunately when it reached to the point where the word "atoms" were placed in the file it didn't stop and it continued,but when I used either the memcmp() or the strncmp() it stopped and I was happy.
But then I thought,what if there will be a case in which there is one string in which the first 5 letters are a,t,o,m,s and these are being followed by other letters.
Unfortunately,my thoughts were right as I tested it using the above code by initializing word1 to "atomsaaaaa" and word2 to atoms and memcmp() and strncmp() in the if statements returned 0.On the other hand strcmp() it didn't. It seems that I must use strcmp().
In short:
strcmp compares null-terminated C strings
strncmp compares at most N characters of null-terminated C strings
memcmp compares binary byte buffers of N bytes
So, if you have these strings:
const char s1[] = "atoms\0\0\0\0"; // extra null bytes at end
const char s2[] = "atoms\0abc"; // embedded null byte
const char s3[] = "atomsaaa";
Then these results hold true:
strcmp(s1, s2) == 0 // strcmp stops at null terminator
strcmp(s1, s3) != 0 // Strings are different
strncmp(s1, s3, 5) == 0 // First 5 characters of strings are the same
memcmp(s1, s3, 5) == 0 // First 5 bytes are the same
strncmp(s1, s2, 8) == 0 // Strings are the same up through the null terminator
memcmp(s1, s2, 8) != 0 // First 8 bytes are different
memcmp compares a number of bytes.
strcmp and the like compare strings.
You kind of cheat in your example because you know that both strings are 5 characters long (plus the null terminator). However, what if you don't know the length of the strings, which is often the case? Well, you use strcmp because it knows how to deal with strings, memcmp does not.
memcmp is all about comparing byte sequences. If you know how long each string is then yeah, you could use memcmp to compare them, but how often is that the case? Rarely. You often need string comparison functions because, well... they know what a string is and how to compare them.
As for any other issues you are experiencing it is unclear from your question and code. Rest assured though that strcmp is better equipped in the general case for string comparisons than memcmp is.
strcmp():
It is used to compare the two string stored in two variable, It takes some time to compare them. And so it slows down the process.
strncmp():
It is very much similar to the previous one, but in this one, it compares the first n number of characters alone. This also slows down the process.
memcmp():
This function is used compare two variables using their memory. It doesn't compare them one by one, It compares four characters at one time. If your program is too concerned about speed, I recommend using memcmp().
To summarize:
strncmp() and strcmp() treat a 0 byte as the end of a string, and don't compare beyond it
to memcmp(), a 0 byte has no special meaning
strncmp and memcmp are same except the fact that former takes care of NULL terminated string.
For strcmp you'll want to be only comparing what you know are going to be strings however sometimes this is not always the case such as reading lines of binary files and there for you would want to use memcmp to compare certain lines of input that contain NUL characters but match and you may want to continue checking further lengths of input.
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.