I'm trying to figure out why this doesn't work:
#include <stdio.h>
int main ()
{
char *orig = "Hey you guys.";
char *str;
str = &orig;
while(*str++) {
if (*str == 'y')
*str = '#';
}
puts(orig);
return 0;
}
// OUTPUT => "Hey you guys."
// Not "he# #ou gu#s." as expected.
By assigning str = &orig, I thought that str would share the same memory address as orig.
What am I missing?
(1) for sharing memory you want to do str = orig, since str is already a pointer type.
(2) orig is defined as a string literal, a constant - so you cannot modify the value "Hey you guys.", even not when accessing it via str, it will result in a run time error.
EDIT: Issue #3: In your while loop, you first increase the pointer, and only then checks if it is 'y' and modify. By doing so - you will miss the first element. "yasdf" will become "yasdf" and not "#asdf", as you expect. [well I think that what you expect anyway...]
To achieve what you are after, you can follow this: [using strcpy and a buffer, to avoid writing on constant memory]
#include <stdio.h>
#include <string.h>
int main ()
{
char *orig = "Hey you guys.";
char buff[14]; //the length of orig + 1 byte for '\0'
char *str = buff; //since str and buff are already pointers
strcpy(str,orig);
while(*str) {
if (*str == 'y')
*str = '#';
str++;
}
puts(buff);
return 0;
}
Two things:
&orig is the address of the pointer. Perhaps you want str = orig. Your compiler should have warned you about a pointer type mismatch here; if it didn't, then turn up the warning level until it does.
Modifying a constant literal string won't always work. Use char orig[] = "Hey you guys.", which copies the literal string into an array called orig that you can safely modify.
you miss the first element
you cannot change a string literal
Modified code:
#include <stdio.h>
int main ()
{
char orig[] = "Hey you guys.";
char *str;
str = orig;
int i;
while(*str){
if (*str == 'y')
*str = '#';
*str++;
}
puts(str);
puts(orig);
return 0;
}
Related
I have written following c code to update the char array rb but it is printing garbage value
#include <stdio.h>
void update(char* buff){
char word[] = "HII_O";
buff = word;
return;
}
int main(){
char rb[6];
update(rb);
rb[5] = '\0';
printf("[%s]\n",rb);
return 0;
}
The restriction is we can't use any other library. So how to solve this
Within the function update the parameter buff is a local variable of the function that will not be alive after exiting the function.
You can imagine the function call the following way
update( rb );
//...
void update( /*char* buff*/){
char *buff = rb;
char word[] = "HII_O";
buff = word;
return;
}
As you see the original array was not changed.
That is at first the pointer buff was initialized by the address of the first element of the source array rb.
char *buff = rb;
and then this pointer was reassigned with the address of the first element of the local character array word
buff = word;
What you need is to copy characters of the string literal "HII_O" into the source array rb using standard string function strcpy or strncpy.
For example
#include <string.h>
#include <stdio.h>
void update(char* buff){
strcpy( buff, "HII_O" );
}
int main(){
char rb[6];
update(rb);
printf("[%s]\n",rb);
return 0;
}
As you cannot use any library function, just do de copying cell by cell, change
void update( /*char* buff*/){
char *buff = rb;
char word[] = "HII_O";
buff = word;
return;
}
(you cannot assign arrays as a whole in C)
into:
void update(char *buff) {
char *word = "HII_O";
int index;
/* copy characters, one by one, until character is '\0' */
for (index = 0; word[index] != '\0'; index = index + 1) {
buff[index] = word[index];
}
/* index ended pointing to the next character, so we can
* do the next assignment. */
buff[index] = '\0'; /* ...and copy also the '\0' */
}
buff is a local variable to the function. It is initialized to point to the first element of the rb array in main but changes to buff will not change the rb array. So
buff = word;
makes buff point to the string literal "HII_O" but there is no change to the rb array.
The normal solution would be
void update(char* buff){
strcpy(buff, "HII_O");
}
However, you write ...
The restriction is we can't use any other library.
Well, in order to set a fixed value like your code does, you don't need any library function.
You don't any need other variables, string literals, etc.
Just simple character assignments like:
void update(char* buff){
buff[0] = 'H';
buff[1] = 'I';
buff[2] = 'I';
buff[3] = '_';
buff[4] = 'O';
buff[5] = '\0';
}
int main(){
char rb[6];
update(rb);
printf("[%s]\n",rb);
return 0;
}
this program it suppose to print Hello World but guess what exited, segmentation fault why is that happening ?
#include <stdio.h>
#include <string.h>
char f(char *a, char *b)
{
int i , m, n;
m = strlen(a);
n = strlen(b);
for (i = 0; i<=n; i++)
{
a[m+i] = b[i];
}
}
int main() {
char*str1 = "hello ";
char*str2 = "world!";
str1=f(str1, str2);
printf("%s", str1);
return 0;
}
You are not allowed to modify string literals. Use arrays with enough elements instead for strings to be modified.
Also assigning the return value of f to str1 is a bad idea because no return statement is executed in the function f and using its return value invokes undefined behavior. The return type should be changed to void if you are not going to return anything.
#include <stdio.h>
#include <string.h>
void f(char *a, char *b)
{
int i , m, n;
m = strlen(a);
n = strlen(b);
for (i = 0; i<=n; i++)
{
a[m+i] = b[i];
}
}
int main() {
char str1[16] = "hello ";
char*str2 = "world!";
f(str1, str2);
printf("%s", str1);
return 0;
}
First of all, this:
char*str1 = "hello ";
is a pointer to constant data, which means that you can't change the string "hello "
This is a constant pointer to variable data:
char str1[] = "hello ";
Which means that str1 always points to the same address in memory, but you can modify the content of that chunk of memory.
However str1 will have a fixed size of 7 characters (don't forget to count \0), so you can't append another string to it.
You could define a size #define SIZE 20 large enough to store both strings and declare
char str1[SIZE] = "hello ";
Or you could declare str1 as a VLA (variable length array) after having declared the string to append:
char*str2 = "world!";
char str1[strlen("hello ")+strlen(str2)+1] = "hello ";
Where the +1 is for \0.
Is it important that you copy characters one by one?
Because if it's not you can just copy one string to another like this.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main() {
char str1[] = "hello ";
char str2[] = "world!";
char *result = malloc(strlen(str1) + strlen(str2) + 1);
strcpy(result, str1);
strcat(result, str2);
printf("%s", result);
return 0;
}
First you are not allowed to change a constant string, that is undefined behaviour.
Secondly your f function has no return statement and thus returns random data, making the str1 variable in main point to random memory. Using it then also has undefined behaviour.
To fix it you should allocate new memory and concatenate the string into that
char* f(const char *s1, const char *s2)
{
char *s = malloc(strlen(s1) + strlen(s2) +1);
if (s) {
strcpy(s, s1);
strcat(s, s2);
}
return s;
}
The extra one byte allocated is for the terminating zero.
Both arguments are const as there is no reason to modify them, which allows both arguments to be literal strings.
For starters you may not change string literals (in this case the string literal pointed to by the pointer str1).
char*str1 = "hello ";
char*str2 = "world!";
Any attempt to change a string literal results in undefined behavior.
You need to allocate a character array large enough to store the result string with the appended string literal pointed to by the pointer str2.
Secondly there is already the standard C function strcat that performs the required task. If you have to write such a function yourself then it seems you should not use any string function as for example strlen.
And the return type char of your function does not make a sense. And moreover actually your function returns nothing.
So this assignment
str1=f(str1, str2);
results in undefined behavior.
The function and the program in whole can be written the following way without using standard string functions.
#include <stdio.h>
char * f( char *s1, const char *s2 )
{
char *p = s1;
while ( *p ) ++p;
while ( ( *p++ = *s2++ ) );
return s1;
}
int main(void)
{
char s1[14] = "Hello ";
char *s2 = "World!";
puts( f( s1, s2 ) );
return 0;
}
The program output is
Hello World!
Pay attention to that the second function parameter shall have the qualifier const because the pointed string is not changed within the function. And the function return type should be char * that is the function should return the result string.
I'm working on a project where I have to replace some char in a string.
I do not understand one of the errors I see.
#include <stdio.h>
#include <string.h>
#include <ctype.h>
void replaceLetters(char *text, char original, char new_char);
{
for (int counter = 0; text[counter] != '\0'; counter++)
{
if (text[counter] == original)//Error occurs here
{
text[counter] = new_char;
}
printf("%c", chr[counter]);
}
return 0;
}
int main()
{
char *text = "HallO";
char original = 'O';
char new_char = 'N';
replaceLetters(text, original, new_char);
return 0;
}
At the if statement the following error occurs: thread 1 exc_bad_access (code=1 address=0x0).
What does this mean, and how can I address it?
In c, string literals like "HallO" are stored in global read-only memory. If you want to modify the string, you will need to keep it in a buffer on the stack.
char text[6] = "HallO";
"What does this mean, and how can I address it?"
It is an access violation. The string you have defined
char *text = "HallO";
is referred to in C as a string literal, and is created in an area of read-only memory, resulting in an access violation.
This can be easily addressed by creating the original variable such that it is editable. eg:
char text[6] = "HallO"; //okay
char text[] = "HallO"; //better, let the compiler do the computation
char text[100] = "HallO"; //useful if you know changes to string will require more room
I have the following program:
#include <stdio.h>
#define MAXLEN 100
char *my_strcat(char *strp1,char *strp2) {
char str[MAXLEN], *strp;
strp = str;
while (*strp1 != '\0') {
*strp++ = *strp1++;
}
while (*strp2 != '\0') {
*strp++ = *strp2++;
}
*strp = '\0';
strp = str;
return strp;
}
void test_strcat(void) {
char *strp1, *strp2, *strp3, str1[MAXLEN], str2[MAXLEN];
printf("Testing strcat! Give two strings:\n");
gets_s(str1, sizeof(str1));
gets_s(str2, sizeof(str2));
strp1 = str1;
strp2 = str2;
strp3 = my_strcat(strp1, strp2);
printf("Concatenated string: %s", strp3);
}
int main(void) {
test_strcat();
}
The function char *mystrcat is supposed to concatenate two strings, and I test it with
test_strcat. The program runs without errors but instead of printing the concatenated string a smiley symbol is printed. I have gone through the program with debugging and it
appears that the result sent back by my_strcat is the correct string. However, when
going into the last line where strp3 is supposed to be printed it appears red in the
debugging tool, implying that its value is about to change. After the printf call, strp3
no longer points to the concatenated string. Anyone knows what could be causing this error?
Here is the problem:
char str[MAXLEN], *strp;
strp = str; // str is a local variable
...
return strp; // <<== WRONG!!!
Since str is a local variable that disappears as soon as you return, the value pointed to by strp becomes invalid the instance the caller gets the control back.
Use malloc instead of allocating memory in the automatic storage area (i.e. on the stack) will fix this problem:
char *str = malloc(strlen(strp1)+strlen(strp2)+1);
char *strp = str;
I suggest you 2 ways as following.
first,
char *my_strcat(char *strp1,char *strp2) {
static char str[MAXLEN * 2]; /* from char str[MAXLEN] */
second,
char *my_strcat(char *strp1,char *strp2) {
char *str = malloc(strlen(strp1) + strlen(strp2) + 1);
because in my_strcat function, you allocated the str as auto variable.
When my_strcat function is finish, str will be freed.
so I was practicing writing c code with pointers using the K&R. For one problem with strcat function, I couldn't find out what was wrong with my code, which according to Visual Studio, returned the destination string unchanged after the strcat function. Any suggestion is appreciated!
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int strcat(char* s, char* t);
int main(void)
{
char *s="hello ", *t="world";
strcat(s,t);
printf("%s",s);
return 0;
}
int strcat(char* s,char* t)
{
int i;
i=strlen(s)+strlen(t);
s=(char*) malloc(i);
while(*s!='\0')
s++;
while('\0'!=(*s++=*t++))
;
return 0;
}
I'm pretty sure that strcat returns a char* in the real implementation (holding the original value of the first string).
strcat is not supposed to alter the first parameter's address, so you shouldn't call malloc.
Point #2 means that you need to declare char *s as char s[20] in main (where 20 is some arbitrary number big enough to hold the whole string).
If you really want to alter the value of the an input parameter you will need to pass the address of the value - so it would need to be strcat(char **s, ...) in the function declaration/definition, and called with strcat(&s, ...) in main.
1) defining string in this way
char *s="hello "
means that you are defined a literal string. a literal string is saved into read only memory so you can not edit it
you have to define your string as a char array in order to be able to edit it
char s[100] = "hello ";
2) when you define your function in this way
int strcat(char* s,char* t)
you can not change the address of s into the function strcat(). So assigning memory with malloc() into the function will not change the s address when leaving the function
3) change your function strcat to
int strcat(char** s,char* t)
{
int i;
char *u, *v;
i=strlen(*s)+strlen(t);
v = *s;
u=(char*) malloc(i+1);
while(*v!='\0')
*u++ = *v++;
while('\0'!=(*u++=*t++));
*s = u;
return 0;
}
and you call it in the main with:
char *s="hello ", *t="world";
strcat(&s,t);
In
strcat(char* s, char* t)
the 's' is send by value. The value of 's' at call time is copied into the stack then strcat() is call. At the return of strcat the modified version is discard from the stack. So the calling value of 's' is never changed (and you create a memory leak).
Beward, in C every memory cell can be change, even parameters or instructions sections; some changes can be very hard to understand.
Since you are trying to do like the real strcat it's said that the first parameter
The string s1 must have sufficient space to hold the result.
so you don't need to use malloc
char *strcat(char* s, const char* t);
int main(void)
{
char s[15] = {0}; //
char *t = "world"; //const char * so you can't change it
strcpy(s, "Hello ");
strcat(s,t);
printf("%s\n",s);
return (0);
}
char *strcat(char* s, const char* t)
{
int i = 0;
while (s[i] != '\0')
i++;
while (*t != '\0')
s[i++] = *t++;
s[i] = '\0'; //useless because already initialized with 0
return (s);
}
#include<stdio.h>
#include<string.h>
#define LIMIT 100
void strcatt(char*,char*);
main()
{
int i=0;
char s[LIMIT];
char t[LIMIT];
strcpy(s,"hello");
strcpy(t,"world");
strcatt(s,t);
printf("%s",s);
getch();
}
void strcatt(char *s,char *t)
{
while(*s!='\0')
{
s++;
}
*s=' ';
++s;
while(*t!='\0')
{
*s=*t;
s++;
t++;
}
*s=*t;
}
Dear user,
you don't have to complicate things that much. The simpliest code for strcat, using pointers:
void strcat(char *s, char *t) {
while(*s++); /*This will point after the '\0' */
--s; /*So we decrement the pointer to point to '\0' */
while(*s++ = *t++); /*This will copy the '\0' from *t also */
}
Although, this won't give you report about the concatenation's success.
Look at this main() part for the rest of the answer:
int main() {
char s[60] = "Hello ";
char *t = "world!";
strcat(s, t);
printf("%s\n", s);
return 0;
}
The s[60] part is very important, because you can't concatenate an another string to it's end if it doesn't have enough space for that.