I am currently trying to handle string in C and I am having trouble placing the split values of a string into an array. Bellow is the code I have created in an attempt to achieve this.
#include <stdio.h>
#include <string.h>
int main(){
char str[]="titanic.txt";
char parts[2][5];
char *name = strtok(str, ".");
for (int i = 0; i < 2; i++){
parts[i][5] = name;
char name = strtok(NULL, ".");
}
printf("%c\n", str[0]);
return 0;
}
The output I would be expecting from this would hopefully look something like this.
char part[2][10]{
{'t', 'i', 't', 'a', 'n', 'i', 'c'},
{'t', 'x', 't'}
};
alternatively, I have tried something like this using string copy as such.
#include <stdio.h>
#include <string.h>
int main(){
char str[]="titanic.txt";
char parts[2][10];
char *name = strtok(str, ".");
for (int i = 0; i < 2; i++){
strcpy(parts[i], name);
name = strtok(NULL, ".");
}
printf("%s\n", parts[1]);
return 0;
}
Which, did what I want it to, but I would like to try and achieve this without string copy because I feel it will help me understand strings, characters, and arrays better. I do not want to reinvent the wheel I just want a deeper understanding.
The parts array should be an array of pointers, not a 2-dimensional array of characters:
char *parts[2];
Then you assign:
parts[i] = name;
If you want to copy from the input string to parts, you need to declare the 2nd dimension large enough for the largest possible string:
char parts[2][10];
and then you should use strcpy() rather than assignment to copy it.
strcpy(parts[i], name);
Notice that you don't give a second index to parts when doing this, that's only used to access specific characters of the string, not the string as a whole.
In case separating a file name from its extension is actually relevant to whatever you're trying to accomplish, I'll assume you're using Windows and point you to the documentation for the recommended Windows API library for this, <pathcch.h>.
With regard to your actual question, your first code doesn't work because:
You're assigning to a single character in the line parts[i][5] = name;. You're also overflowing since parts[i] has the type char [5], which only has positions 0 to 4.
You redeclare name as a single char rather than a char * pointer, which is the correct type, and was declared as such (correctly) outside the loop scope. Your new variable char name overwrites the existing char *name variable.
You cannot get around using strcpy-family functions to assign to a character array for this (and most other) use cases. The only time the syntax char foo[] = "Hello World"; is valid is immediately on declaration. Also, string literals are stored in read-only memory.
Following your coding style, this is the solution:
#include <stdio.h>
#include <string.h>
int main()
{
char str[] = "titanic.txt";
char delim[] = ".";
char parts[2][10];
char *ptr = strtok(str, delim);
int i = 0;
for (i = 0; i < 2; i++){
if(ptr != NULL)
{
snprintf(parts[i], sizeof(parts[0]) ,"%s", ptr);
ptr = strtok(NULL, delim);
}
}
printf("'%s'\n", parts[0]);
printf("'%s'\n", parts[1]);
return 0;
}
Related
I am working on a initials project where you enter a name and it prints the initials. When I try and combine the strings it returns Segmentation fault instead of the initials.
#include <stdio.h>
#include <stdlib.h>
#include <cs50.h>
#include <string.h>
#include <ctype.h>
int main(void) {
printf("Name: ");
string name = GetString();
printf("\n");
int length = strlen(name);
string initials = "";
int arraylength = length + 1;
char string[arraylength];
string[0] = toupper(name[0]);
int count = 1;
for(int l = 1;l<=length;l++) {
if(name[l] == ' ') {
l++;
count++;
string[l] = toupper(name[l]);
}
}
count++;
string[count] = '\0';
for(int c = 0;c<=count;c++) {
strcat(initials, &string[c]);
}
printf("%s\n", initials);
}
That's why a string type would cause confusion, you make a pointer to a single char. And you then pass it to strcat() that's simply wrong.
A string, as expected by strlen() or strcat() or all str* functions, is not simple a char pointer which is what the type string in your code is.
In c a c-string is actually a sequence of bytes with the last byte being '\0', and it's not optional. You create a pointer to a single char and that is not the same as a string that I just described.
Any of the str* functions will try to find the '\0' but since you passed the address of a stack variable which is not an array, the behavior is undefined when any of these functions try to increment and dereference the passed pointer.
When you do understand how strings work in c you would see that using strcat() for the concatenation of many chunks of a large string together is not very efficient, you would also know that you just need to append a single char and that you can by simply using index notation, like
char string[8];
string[0] = 'E';
string[1] = 'x';
string[2] = 'a';
string[3] = 'm';
string[4] = 'p';
string[5] = 'l';
string[6] = 'e';
string[7] = '\0'; // The very necessary terminator
I am trying to use strtok() to parse a string deliminated by spaces. From what I can tell, I am using it correctly, but it won't run on ideone.com. Is there anything wrong with the following code? I just get Runtime error time: 0 memory: 2288 signal:11
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
static void test(char *command)
{
char* output = strtok(command, " ");
printf("%s\n", output);
}
int main(void) {
test("set_rate 200");
return 0;
}
Here it is on ideone.com if you want to try it.
Always consult the man pages first.
strtok(3) says:
Be cautious when using these functions. If you do use them, note that:
These functions modify their first argument.
These functions cannot be used on constant strings.
strtok will modify the string passed as the first parameter, so you can't pass a string literal, change your main like this:
int main(void) {
char str[] = "set_rate 200";
test(str);
return 0;
}
str here is an char array, but not a string literal.
As strtok modifies the string, it requires that the string is not in read-only memory. So when you pass a string literal to your test function, it crashes.
This is better:
char s[] = "set_rate 200";
test(s);
When you pass hard coded string that was stored in read-only memory.
strtok() does not work with string which is read-only memory.
You need to use string variable rather than string literal .
you can first store string in some variable and then you can pass it to the function.
char[]="set_rate 200";
test(str);
See example:
char *str = malloc(20);
char *tok = NULL;
int len = 0;
strcpy(str, "This is a string");
len = strlen(str);
printf("string before strtok(): %s\n", str);
tok = strtok(str, " ");
while (tok) {
printf("Token: %s\n", tok);
tok = strtok(NULL, " ");
}
Edit
From #Yu Hao comment i am adding this
char *str = "set_rate 200";
test(str); // This won't work. here str is pointer to the string literal.
What I am trying to do is to break the user input in parts with whitespace as a delimiter, copy the parts into the array (tokenAr) and compare the tokenAr[0] (the first part) if it is equal to sHistory. if they are equal, check the value of tokenAr[1] if it is "1", "2" etc, to execute the corresponding command that is entered in the history array. This is what i have tried to far and it crashes. I am using TCC on Windows x64.
EDIT: I forgot to mention that I began learning C, just two days ago.
EDIT2: I run the program in a debugger and it has raised an Acces Violation(Segmentation Fault) in line if(strcmp(tokenArPtr[0],sHistory)==0)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char *argv[])
{
int i=1; int j=1; int k=0;
char history[100][100] = {0};
char sKey[] = "exit";
char sInput[100];
char sHistory[]="history";
do
{
//gather user input
printf ("hshell> ");
fgets (sInput, 100, stdin);
strcpy(history[i],sInput);
i++;
//END_gather user input
//Tokenizing
char delims[] = " ";
char *tokenArPtr[5];
char *result = NULL;
result = strtok(sInput, delims);
tokenArPtr[0] = result;
while (result!=NULL)
{
puts(result);
result= strtok(NULL, delims);
tokenArPtr[k+1] = result;
puts(tokenArPtr[k]);
puts("=====");
k++;
}
k=0;
/*
//END_Tokenizing
if(strcmp(tokenArPtr[0],sHistory)==0)
{
for(j=1;j<i;j++)
{
printf("%d. %s \n",j,history[j]);
}
}
else if (strcmp (sKey,tokenArPtr[0]) != 0)
{
printf("\nCommand not found \n");
}*/
}while (strcmp (sKey,sInput) != 0);
return 0;
}
EDIT 3: I used the result variable instead of the tokenArPtr directly, but when debugging, I noticed that the values of the array are not being updated.
Which type does strtok return? char *. What is the type of tokenAr[k]? char. What type does strcmp expect as input? char * and char *. What is the type of tokenAr[0]? char.
See a problem? You should. The * is pretty significant.
Assuming tokenAr is declared like char *tokenAr[2];, how many char * values can tokenAr store? What happens when k exceeds 2? You need to ensure you don't overflow your tokenAr array.
history is uninitialised. Using an uninitialised variable is undefined behaviour. I suggest initialising it, like this: char history[100][100] = { 0 };
Which book are you reading?
While tokenizing, the loop will never end because the test is on the variable "result" that will never change... So you're finally going to a buffer overflow with "tokenAr"... Modify your code to test "tokenAr".
Edit: And tokenAR should be an array... (I don't know how it can compile...)
There are many problems... First of all you should include string.h which will show you some errors in compilation.
I believe that the main problem is here:
char tokenAr[2];
result = strtok(sInput, delims);
while (result!=NULL)
{
tokenAr[k] = strtok(NULL, delims);
k++;
}
tokenAr should be an array of pointers, not chars. And are you sure that k will never exceed 2? An assertion would help debugging.
Well, I declared a global array of chars like this char * strarr[];
in a method I am tokenising a line and try to put everything into that array like this
*line = strtok(s, " ");
while (line != NULL) {
*line = strtok(NULL, " ");
}
seems like this is not working.. How can I fix it?
Thanks
Any number of things could be going wrong with the code you haven't shown us, such as undefined behaviour by strtoking a string constatnt, or getting your parameters wrong when calling the function.
But the most likely problem from the code we can see is the use of *line instead of line, assuming that line is of type char *.
Use the following code as a baseline:
#include <stdio.h>
#include <string.h>
int main (void) {
char str[] = "My name is paxdiablo";
// Start tokenising words.
char *line = strtok (str, " ");
while (line != NULL) {
// Print current token and get next word.
printf ("[%s]\n", line);
line = strtok(NULL, " ");
}
return 0;
}
This outputs:
[My]
[name]
[is]
[paxdiablo]
and should be easily modifiable into something you can use.
Be aware that, if you're trying to save the character pointers returned from strtok (which would make sense for using *line), they are transitory and will not be what you expect after you're done. That's because modifications are made in-place within the source string. You can do it with something like:
#include <stdio.h>
#include <string.h>
int main (void) {
char *word[4]; // The array of words.
size_t i; // General counter.
size_t nextword = 0; // For preventing array overflow.
char str[] = "My name is paxdiablo";
// Start tokenising.
char *line = strtok (str, " ");
while (line != NULL) {
// If array not full, duplicate string to array and advance index.
if (nextword < sizeof(word) / sizeof(*word))
word[nextword++] = strdup (line);
// Get next word.
line = strtok(NULL, " ");
}
// Print out all stored words.
for (i = 0; i < nextword; i++)
printf ("[%s]\n", word[i]);
return 0;
}
Note the specific size of the word array in that code above. The use of char * strarr[] in your code, along with the message tentative array definition assumed to have one element is almost certainly where the problem lies.
If your implementation doesn't come with a strdup, you can get a reasonably-priced one here :-)
/* strchr example */
#include <stdio.h>
#include <string.h>
int main ()
{
char str[] = "This is a sample string";
char * pch;
printf ("Looking for the 's' character in \"%s\"...\n",str);
pch=strchr(str,'s');
while (pch!=NULL)
{
printf ("found at %d\n",pch-str+1);
pch=strchr(pch+1,'s');
}
return 0;
}
How would I index the str so that I would replace every 's' with 'r'.
Thanks.
You don't need to index the string. You have a pointer to the character you want to change, so assign via the pointer:
*pch = 'r';
In general, though, you index using []:
ptrdiff_t idx = pch - str;
assert(str[idx] == 's');
You can use the following function:
char *chngChar (char *str, char oldChar, char newChar) {
char *strPtr = str;
while ((strPtr = strchr (strPtr, oldChar)) != NULL)
*strPtr++ = newChar;
return str;
}
It simply runs through the string looking for the specific character and replaces it with the new character. Each time through (as with yours), it starts with the address one beyond the previous character so as to not recheck characters that have already been checked.
It also returns the address of the string, a trick often used so that you can use the return value as well, such as with:
printf ("%s\n", chngChar (myName, 'p', 'P'));
void reeplachar(char *buff, char old, char neo){
char *ptr;
for(;;){
ptr = strchr(buff, old);
if(ptr==NULL) break;
buff[(int)(ptr-buff)]=neo;
}
return;
}
Usage:
reeplachar(str,'s','r');
Provided that your program does really search the positions without fault (I didn't check), your question would be how do I change the contents of an object to which my pointer pch is already pointing?