Please help me understand what I am doing wrong with my double pointer arithmetic. I know I clearly doing something incorrect but what? Look at the line where I marked, "bad pointer". My intuition told me that this should work, but I guess not. Programmed in C.
/*
*This function searches for prefixes within the string array.
*#param stringArray array containing strings
*#param searchPrefix a string or characters to search for at beginning of string
*#return void
*/
void prefixSearch(char* stringArray[SIZE], char* searchPrefix){
int count = strlength(searchPrefix);//size of the prefix search
while(count > 0){
if(**stringArray == *searchPrefix){
printf("%c match %c\n", **stringArray, *searchPrefix);
**stringArray++;//want to move to next character, instead get bad pointer.
*searchPrefix++;//moves to next char
}else{
stringArray++;//no match, go to next string
}
count--;//decrement
}
}
The * and ++ operators have the same precedence, and right-to-left associativity, so the line that's not working parses like this:
*(*(stringArray++));
I don't actually understand your code, but that can't be what you intended since the dereference operators have no effect. Presumably you wanted this instead:
(**stringArray)++;
You have to move to next character so
use
(*stringArray)++ instead of **stringArray++
And
searchPrefix++ instead of *searchPrefix++
Related
I was trying to remove spaces of a string after scanning it. The program has compiled perfectly but it is not showing any output and the output screen just keeps getting shut down after scanning the string. Is there a logical error or is there some problem with my compiler(I am using a devc++ compiler btw).
Any kind of help would be appreciated
int main()
{
char str1[100];
scanf("%s",&str1);
int len = strlen(str1);
int m;
for (m=0;m<=len;){
if (&str1[m]==" "){
m++;
}
else {
printf("%c",&str1[m]);
}
m++;
}
return 0;
}
Edit : sorry for the error of m=1, I was just checking in my compiler whether that works or not and just happened to paste that code
Your code contains a lot of issues, and the behaviour you describe is very likely not because of a bug in the compiler :-)
Some of the issues:
Use scanf("%s",str1) instead of scanf("%s",&str1). Since str1 is defined as a character array, it automatically decays to a pointer to char as required.
Note that scanf("%s",str1) will never read in any white space because "%s" is defined as skipping leading white spaces and stops reading in when detecting the first white space.
In for (m=1;m<=len;) together with str1[m], note that an array in C uses zero-based indizes, i.e. str1[0]..str1[len-1], such that m <= len exceeds array bounds. Use for (m=0;m<len;) instead.
Expression &str1[m]==" " is correct from a type perspective, but semantically a nonsense. You are comparing the memory address of the mth character with the memory address of a string literal " ". Use str1[m]==' ' instead and note the single quotes denoting a character value rather than double quotes denoting a string literal.
Statement printf("%c",&str1[m]) passes the memory address of a character to printf rather than the expected character value itself. Use printf("%c",str1[m]) instead.
Hope I found everything. Correct these things, turn on compiler warnings, and try to get ahead. In case you face further troubles, don't hesitate to ask again.
Hope it helps a bit and good luck in experiencing C language :-)
There are many issues:
You cannot read a string with spaces using scanf("%s") , use fgets instead (see comments).
scanf("%s", &str1) is wrong anyway, it should be scanf("%s", str1);, str1 being already the address of of the string
for (m = 0; m <= len;) is wrong, it should be for (m = 0; m < len;), because otherwise the last character you will check is the NUL string terminator.
if (&str1[m]==" ") is wrong, you should write if (str1[m]==' '), " " does not denote a space character but a string literal, you need ' ' instead..
printf("%c", &str1[m]); is wrong, you want to print a char so you need str1[m] (without the &).
You should remove both m++ and put that into the for statement: for (m = 1; m < len; m++), that makes the code clearer.
And possibly a few more problems.
And BTW your attempt doesn't remove the spaces from the string, it merely displays the string skipping spaces.
There are a number of smaller errors here that are adding up.
First, check the bounds on your for loop. You're iterating from index 1 to index strlen(str1), inclusive. That's a reasonable thing to try, but remember that in C, string indices start from 0 and go up to strlen(str1), inclusive. How might you adjust the loop to handle this?
Second, take a look at this line:
if (&str1[m] == " ") {
Here, you're attempting to check whether the given character is a space character. However, this doesn't do what you think it does. The left-hand side of this expression, &str1[m], is the memory address of the mth character of the string str1. That should make you pause for a second, since if you want to compare the contents of memory at a given location, you shouldn't be looking at the address of that memory location. In fact, the true meaning of this line of code is "if the address of character m in the array is equal to the address of a string literal containing the empty string, then ...," which isn't what you want.
I suspect you may have started off by writing out this line first:
if (str1[m] == " ") {
This is closer to what you want, but still not quite right. Here, the left-hand side of the expression correctly says "the mth character of str1," and its type is char. The right-hand side, however, is a string literal. In C, there's a difference between a character (a single glyph) and a string (a sequence of characters), so this comparison isn't allowed. To fix this, change the line to read
if (str1[m] == ' ') {
Here, by using single quotes rather than double quotes, the right-hand side is treated as "the space character" rather than "a string containing a space." The types now match.
There are some other details about this code that need some cleanup. For instance, look at how you're printing out each character. Is that the right way to use printf with a character? Think about the if statement we discussed above and see if you can tinker with your code. Similarly, look at how you're reading a string. And there may be even more little issues here and there, but most of them are variations on these existing themes.
But I hope this helps get you in the right direction!
For loop should start from 0 and less than length (not less or equal)
String compare is wrong. Should be char compare to ' ' , no &
Finding apace should not do anything, non space outputs. You ++m twice.
& on %c output is address not value
From memory, scanf stops on whitespace anyway so needs fgets
int main()
{
char str1[100];
scanf("%s",str1);
int len = strlen(str1);
int m;
for (m=0;m<len;++m){
if (str1[m]!=' '){
printf("%c",str1[m]);
}
}
return 0;
}
There are few mistakes in your logic.
scanf terminates a string when it encounter any space or new line.
Read more about it here. So use fgets as said by others.
& in C represents address. Since array is implemented as pointers in C, its not advised to use & while getting string from stdin. But scanf can be used like scanf("%s",str1) or scanf("%s",&str[1])
While incrementing your index put m++ inside else condition.
Array indexing in C starts from 0 not 1.
So after these changes code will becames something like
int main()
{
char str1[100];
fgets(str1, sizeof str1 , stdin);
int len = strlen(str1);
int m=0;
while(m < len){
if (str1[m] == ' '){
m++;
}
else {
printf("%c",str1[m]);
m++;
}
}
return 0;
}
I am applying printf and/or other functions to a certain string of characters, read from a file. I want to skip the first 5 characters under certain conditions. Now I thought to be clever by, if the conditions apply, increasing the string pointer by 5:
if (strlen(nav_code) == 10 ) {nav_code = 5+nav_code;}
but the compiler refuses this:
error: assignment to expression with array type
What have I misunderstood? How to make my idea work - or is it a bad idea anyway?
It's probably becuase nav_code is not a pointer but a character array like char nav_code[50]. Try the following:
char nav_code[50];
char *nav_code_ptr = nav_code;
if (strlen(nav_code_ptr) == 10 ) {nav_code_ptr += 5;}
// forth on, use nav_code_ptr instead of nav_code
I am applying printf and/or other functions to a certain string of characters, read from a file. I want to skip the first 5 characters under certain conditions.
If printf is all what you need, then sure you can skip the first 5 characters.
Given nav_code is string (either char array or char pointer), then:
printf( "%s", nav_code + 5 ); // skip the first 5 characters
Of course you need to make sure your string has more than 5 characters, otherwise it's flat out illegal as out-of-bound access.
In your code, nav_code is an array and arrays cannot be assigned.
Instead, use a pointer, initialize that with the address of the first element of the array, make pointer arithmetic on that pointer and store the updated result back to the pointer.
I'm trying to increment a pointer to a string by hand using the dereference operator like I've seen it work in the while loop I have bellow. It works for the first character, but when I try to increment the pointer by hand like it is in the while loop, it doesn't work and I get back garbage. Could someone please tell me what I'm not understanding? This code works the way I expect it.
char *pointToString = "some string or something";
printf("%c\n", *pointToString); // this returns the letter 's'
while(*pointToString != '\0'){
printf("%c", *pointToString);
*pointToString++;
}
This works and prints out 's' and then the string "some string or something", but why can't I access the second element of the string if I increment it by hand? When I do this, I get back garbage.
printf("%c\n", *pointToString+1);
This returns something other than the letter 'o'. I also tried this, but still got garbage:
printf("%c\n", *pointToString+sizeof(char));
I'm not sure if that's correct but I figured that I'm incrementing a pointer like I did in the while loop:
*pointToString++;
So why couldn't I do that by itself?
You cannot access the next element using the expression
printf("%c\n", *pointToString+1);
because the unary operator * has higher precedence than binary +. First, the current character is accessed; then, 1 is added to it. Proper use of parentheses will fix this problem:
printf("%c\n", *(pointToString+1));
Reference: Operator precedence table in C.
From my understanding, this should be your fix:
*(pointToString++); // Iterates first, and then has the pointer point to the correct value
This is what's happening in your one line:
*pointToString++; // Points to the value first and then iterates.
Hope this helps.
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.
Hey guys, i'm working on a program that gets a postfix expression and calculates it..
I have two functions:
Converts infix to postfix
Calculate the postfix
When I try small expressions, like 1+1 or (1+1)*1, it works fine
but when i use all the operands I get something nasty,
Here is the example:
2*2/2+1-1
gets something like:
222/*11-+T_CHECKÖÐ7?█Ã
If you see, the expression is right until the 'T'
I believe it's some parameter mistake, so i'll put the header and return values here
1st)
char* convert(char *infix);
char *post = (char *)malloc(sizeof(char)*tamP);
return post;
2nd)
int evaluate(char *postfix)
while (*postfix != '\0')
return result;
Caller)
char* post = convert(infix);
result = evaluate(post);
Thanks
That kind of weird string looks more like a buffer overflow error. You are likely overwriting the null-terminator, so when the string is printed (or later used), it keeps going until it finds one, examining random program memory until it gets there.
Check that all of your string manipulations are correct.
It is possible that you are not adding the '\0' character at the end of 'post' (after the last sensible character) in the convert(char*) function. That's one reason I can think of.
Try setting the complete string to '\0' before you do anything with it:
memset(post, 0, tamP);
should do.