I found several answers to this in C++ or C#, but none for C.
I need to know how to count the number of characters in a string. The goal is to determine whether there is a closed block (bounded by braces '{', '}') in a string. right now I have the following:
int closedBlock(char* value) {
int open = 0;
int i;
for (i = 0; i < strlen(value); i++) {
if (!strcmp("{", value[i])) {
open++;
} else if (!strcmp("}", value[i])) {
open--;
}
}
return !open;
}
but it crashes after on the first if check. I'm not really clear on why this does not work. I imagine it has something to do with bad pointers (that seems to always be the problem in C), but I can't figure it out. In addition to working code, and explanation of why mine is bad would be greatly helpful.
Thanks.
NOTE
I am aware that this simply check that the number of '{' is equal to the number of '}', and not truly that there is a properly closed block. I'll solve that problem after I solve this one.
strcmp will compare two null-terminated strings. It still baffles me that your compiler actually doesn't mutter about the second parameter being a char. If you want to compare a single character just use the equal-operator ==:
int closedBlock(char* value) {
int open = 0;
int length = strlen(value);
int i;
for (i = 0; i < length; i++) {
if (value[i] == '{') {
open++;
} else if (value[i] == '}') {
open--;
}
}
return !open;
}
Hint: If you work with gcc add -Wall -Wextra to your compiler call, it will often result in useful warnings.
I am aware that this simply check that the number of '{' is equal to the number of '}', and not truly that there is a properly closed block. I'll solve that problem after I solve this one.
Another hint here: when can there be an invalid block? If and only if the end-token } occurs without a preceding start-token {. You already have all tools for this, you're just missing another if-statement.
It's because you're trying to compare a string: "{" against a single character: value[i].
value (presumably) points to an array of characters, while value[i] specifies a single one of those. So you want to compare character-to-character like so:
for (i = 0; i < strlen(value); i++) {
if (value[i] == '{') {
open++;
} else if (value[i] == '}') {
open--;
}
}
Note the use of single quotes around the { and }. That tells the compiler it's a single character and not a C-string.
why you use strcmp() if you only compare one char?
you can simply use the == operator see my example:
http://ideone.com/dNCH2
best regards kenny
Related
I have a problem when I try to compare the content of two char arrays.
Sometimes
If the string is less than the actual size of the array, in it there is no sequence of '\0' but often there are unexpected characters (in the xcode debug there was the '\x01' character and in the control loop I use To check if a memory cell of the array was the same as the other, it was not the same). So to overcome this problem I decided to force the user to write the array content exactly as array size(-1): ex. array[3], the users write "hi" (without the ""). So I was wondering if there is a way to compare the two strings (I'm not talking about length but content) without fully filling the array in order to compare the content ofenter code here the two arrays through a simple cycle. (I was thinking of initializing the vector with \ 0 but I do not know if there can be problems)
thanks for the answers
this is an example of the code
}while(sent !=1);
for(contatore=0; contatore<MAXNOME; contatore++){
if(UserName[contatore] == persona.username[contatore]){
controlloUsr++;
}
}
for(contatore=0; contatore<MAXNOME; contatore++){
if(password[contatore] == persona.password[contatore]){
controlloPsw++;
}
}
if(controlloUsr == MAXNOME && controlloPsw == MAXPSW){
return 1;
} else {
return 0;
}
I'm not sure if that's what you're asking, but there is no way to browse an array of chars in C without giving the code a stop sequence, like a \0, or you will browse it forever (well, until a wild segfault appear...)
You can write your own strncmp function for this, something like :
int my_strncmp(char *str1, char *str2, size_t max, char c)
{
size_t n = 0;
while (n < max)
{
if ((str1[n] != str2[n]) || (str1[n] == c) || (str2[n] == c))
return 0;
n++;
}
return 1;
}
This will return 0 if your char c is met, or if the strings are different, or 1 if the string are the same until max char. Just be sure that one of the two conditions is met or you code will crash into a segfault / have undefined behavior, since you will browse out of your allowed memory range.
the function strcmp() and the strncmp() are made for this exact operation,.
You can read the man page for those two functions for all the details.
I want to check if a char array contains all '0's.
I tried this, but it doesn't work:
char array[8];
// ...
if (array == {'0','0','0','0','0','0','0','0'})
// do something
How do I do this?
This
array == {'0','0','0','0','0','0','0','0'}
is definitely wrong, and surely not compiling.
You can compare the values with memcmp() like this
int allZeroes = (memcmp(array, "00000000", 8) == 0);
In fact
array == {'0','0','0','0','0','0','0','0'}
is not allowed, you cannot compare arrays like this. Rather, do it in a loop:
int row_is_all_zeroes(char arr[8])
{
for (int i = 0; i < 8; i++)
{
if (arr[i] != '0')
return 0;
}
return 1;
}
If you want a more elegant solution, have a look at iharob or Sourav's answers
{'0','0','0','0','0','0','0','0'}
is called (and used as) a brace enclosed initializer list. This cannot be used for comparison anywhere.
You can make use of memcmp() to achieve this in an elegant way.
Pseudo-code
if (!memcmp(array, "00000000", 8))
{
break;
}
I need to make an C application for school and im stuck at one bit. I filled my struct with word from a file wich works fine, except for the time it just prints a random integer i guess?
My code:
char buffer[20];
int i;
for(i = 0; i < 10; i++) {
fgets(buffer,20,fp);
apcWordList[i].pcWord = strdup(buffer);
apcWordList[i].cMaxScore = 0;
apcWordList[i].tTime = time(NULL);
}
fclose(fp);
Now what i wanted to do was this(sSecretWord is the word the person guessed and score the points he got for guessing the word):
for(i = 0; i < 10; i++) {
fgets(buffer,20,fp);
if(apcWordList[i].pcWord == sSecretWord && score > apcWordList[i].cMaxScore) {
apcWordList[i].cMaxScore = score;
apcWordList[i].tTime = time(NULL);
}
}
but it crashes and I am really confused how to compare the variables and change them when needed. I hope I have explained it well enough and my English could be read well.
Assuming pcWord is a char *, you cannot compare that pointer using ==, most of the time.
There is no string data type in C, a string is never a single value.
You must use strcmp():
if(strcmp(apcWordList[i].pcWord, sSecretWorD) == 0 &&
score > apcWordList[i].cMaxScore)
The rest of your code makes little sense, I'm afraid. The loop has a typo, and there's no connection between the fgets() and the if. I don't think the fgets() should be in the second part of the code, at all.
The following expression in your if condition
apcWordList[i].pcWord == sSecretWord
compares the memory addresses they evaluate to and not the values stored at those memory locations which is probably what you want. You should use strcmp instead to compare the strings pointed to by above two variables.
strcmp(apcWordList[i].pcWord, sSecretWorD) == 0
I'm actually writing about the same program as before, but I feel like I've made significant progress since the last time. I have a new question however; I have a function designed to store the frequencies of letters contained within the message inside an array so I can do some comparison checks later. When I ran a test segment through the function by outputting all of my array entries to see what their values are, it seems to be storing some absurd numbers. Here's the function of issue:
void calcFreq ( float found[] )
{
char infname[15], alpha[27];
char ch;
float count = 0;
FILE *fin;
int i = 0;
while (i < 26) {
alpha[i] = 'A' + i++;
}
printf("Please input the name of the file you wish to scan:\n");
scanf("%s", infname);
fin = fopen ( infname, "r");
while ( !feof(fin) ) {
fscanf(fin, "%c", &ch);
if ( isalpha(ch) ) {
count += 1;
i = 0;
if ( islower(ch) ) { ch = toupper(ch); }
while ( i < 26 ) {
if ( ch == alpha[i] ) {
found[i]++;
i = 30;
}
i++;
}
}
}
fclose(fin);
i = 0;
while ( i < 26 ) {
found[i] = found[i] / count;
printf("%f\n", found[i]);
i++;
}
}
At like... found[5], I get this hugely absurd number stored in there. Is there anything you can see that I'm just overlooking? Also, some array values are 0 and I'm pretty certain that every character of the alphabet is being used at least once in the text files I'm using.
I feel like a moron - this program should be easy, but I keep overlooking simple mistakes that cost me a lot of time >.> Thank you so much for your help.
EDIT So... I set the entries to 0 of the frequency array and it seems to turn out okay - in a Linux environment. When I try to use an IDE from a Windows environment, the program does nothing and Windows crashes. What the heck?
Here are a few pointers besides the most important one of initializing found[], which was mentioned in other comments.
the alpha[] array complicates things, and you don't need it. See below for a modified file-read-loop that doesn't need the alpha[] array to count the letters in the file.
And strictly speaking, the expression you're using to initialize the alpha[] array:
alpha[i] = 'A' + i++;
has undefined behavior because you modify i as well as use it as an index in two different parts of the expression. The good news is that since you don't need alpha[] you can get rid of its initialization entirely.
The way you're checking for EOF is incorrect - it'll result in you acting on the last character in the file twice (since the fscanf() call that results in an EOF will not change the value of ch). feof() won't return true until after the read that occurs at the end of the file. Change your ch variable to an int type, and modify the loop that reads the file to something like:
// assumes that `ch` is declared as `int`
while ( (ch = fgetc(fin)) != EOF ) {
if ( isalpha(ch) ) {
count += 1;
ch = toupper(ch);
// the following line is technically non-portable,
// but works for ASCII targets.
// I assume this will work for you because the way you
// initialized the `alpha[]` array assumed that `A`..`Z`
// were consecutive.
int index = ch - 'A';
found[index] += 1;
}
}
alpha[i] = 'A' + i++;
This is undefined behavior in C. Anything can happen when you do this, including crashes. Read this link.
Generally I would advise you to replace your while loops with for loops, when the maximum number of iterations is already known. This makes the code easier to read and possibly faster as well.
Is there a reason you are using float for counter variables? That doesn't make sense.
'i = 30;' What is this supposed to mean? If your intention was to end the loop, use a break statement instead of some mysterious magic number. If your intention was something else, then your code isn't doing what you think it does.
You should include some error handling if the file was not found. fin = fopen(..) and then if(fin == NULL) handle errors. I would say this is the most likely cause of the crash.
Check the definition of found[] in the caller function. You're probably running out of bounds.
I have a function that should be passed a string with two numbers (as a regex: /-?[0-9]+ -?[0-9]+/) and return the second.
I've decided that the program should do error checking. First, it should test if the string is actually of the desired form; second, it should ensure that the first numbers (the ones that are not returned) are sequential.
Now I've been programming for a long time and this is not a difficult task. (It's made slightly more difficult by the fact that the numbers need not fit into a machine word.) But my question is about how I should do this rather than how I can. All of the solutions I've come up with are somewhat ugly.
I could use a global variable to keep the values in and compare them (or just leave the value there if it's NULL); this seems like The Wrong Thing.
I could pass one or both of the return value and the last/current line's first number by reference and modify them
I could use the return value to give a bool for there was/was not an error
etc.
So any thoughts relating to the proper way to deal with error-checking of this sort in C would be welcome.
This is related to a much more theoretical question I asked on cstheory. For reference, here is the function:
char*
scanInput(char* line)
{
int start = 0;
while (line[start] == ' ' || line[start] == '\t')
start++;
if (line[start] == '#')
return NULL; // Comment
if (line[start] == '-')
start++;
while (line[start] >= '0' && line[start] <= '9')
start++;
while (line[start] == ' ' || line[start] == '\t')
start++;
int end = start;
if (line[end] == '-')
end++;
while (line[end] >= '0' && line[end] <= '9')
end++;
if (start == end)
return NULL; // Blank line, or no numbers found
line[end] = '\0';
return line + start;
}
and it is called like so:
while(fgets(line, MAX_LINELEN, f) != NULL) {
if (strlen(line) > MAX_LINELEN - 5)
throw_error(talker, "Maximum line length exceeded; file probably not valid");
char* kept = scanInput(line);
if (kept == NULL)
continue;
BIGNUM value = strtobignum(kept);
if (++i > MAX_VECLEN) {
warning("only %d terms used; file has unread terms", MAX_VECLEN);
break;
}
// values are used here
}
The traditional solution in C is to use pass by reference (pointers) to return the values your function computes and use the return value for error handling, just like how scanf does this.
int scanInput(char **line_p int *number){
char * line = *line_p;
...
if(something bad happens){
return 1;
}
...
*linep = line + start;
*number = ...;
return 0; //success
}
int main(){
char word[100]; strcpy(word, "10 17");
char *line = word;
int number;
switch(scanInput(&line, &number)){
case 1:
default:
}
}
Extra points:
It might be a good idea to use some enum to give a meaning to the error codes.
If you can use C++ (or similar) exceptions are often the best solution for error handling, since you don't have to fill your code with ifs anymore
Global variables are generaly evil. If you are tempted to use them, consider instead encapsulating the state you need in a struct and passing a pointer to it around. Treat it as a "this" pointer, in the OO sense.
Ultimately, you are going to need to isolate and convert both big numbers in each line. To check that the first number on the line is the one that follows the previous, you will have to keep a record of the last such number found. So, you will probably need a structure such as:
BIGNUM old_value = 0; // See notes below
while (fgets(line, sizeof(line), f) != 0)
{
BIGNUM value1;
BIGNUM value2;
if (ScanDoubleBigNum(line, &value1, &value2) != 0)
...handle line format error...
if (old_value == 0 || are_consecutive(old_value, value1))
{
// OK - valid information found
// Release old_value
old_value = value1;
process(value2);
// Release value2
}
else
...handle non-consecutive error...
}
The are_consecutive() function determines whether its second argument is one greater than its first. The process() function does whatever you need to do with the second value. The ScanDoubleBigNum() function is related to your ScanInput() but it reads two values. The actual code will call another function (call it ScanBigNum()) containing about half of ScanInput() (since that contains essentially the same code twice), plus the conversion that currently occurs in your loop. The code in ScanDoubleBigNum() will call ScanBigNum() twice. Note that ScanBigNum() will need to identify where the scan finishes so that the second call can continue where the first stopped.
I'm taking the liberty of assuming that a BIGNUM is an allocated structure identified by a pointer, so the initialization BIGNUM old_value = 0; is a way of indicating there is no value yet. There is presumably a function to release a BIGNUM. If this is incorrect, then you need to adapt the proposed code to accommodate the actual behaviour of the BIGNUM type. (Is this based on OpenSSL or SSLeay code?)