Thread: strcpy()

The string literal "aaa" comes with the null character, so yes, a will be initialised to be terminated with the null character. That said, it is simpler here to write:

Code:

char a[4] = "aaa";

or:

Code:

char a[] = "aaa";

Thanks for replies. Here is my last code
Edit: Not my last code anymore I changed it and made another post below. Didn't deleted or edited this in case you wonder my process.

Code:

int main() {
    char str1[100];
    char str2[100];
    char str3[100];


    printf("Enter a sentence: ");
    fgets(str1, 100, stdin);
    printf("Enter a string you want to change: ");
    fgets(str2, 100, stdin);
    printf("New string: ");
    fgets(str3, 100, stdin);
    puts(str1);
    *(strchr(str1, '\n')) = '\0';
    *(strchr(str2, '\n')) = '\0';
    *(strchr(str3, '\n')) = '\0';


    int i;
    for(i = 0; str2[i] != '\0'; i++){}
    char *p = strstr(str1, str2);
    char tmp = *(p+i);
    strncpy(p, str3, 50);
    *(p+i) = tmp;
    puts(str1);
    return 0;
}

I want to explain what I want to do by an example. User will enter 3 strings and 1 string will be printed.

For example entered these 3 strings:

1st: aaa bbb ccc.
2nd: aaa
3rd: ddd

printed string: ddd bbb ccc.

With the code I gave above I get

ddd

That's because strncopy adds '\0' but as you can see I tried to change it in code. First I found out how many characters are there in the 2nd string. Then created a char named tmp and I saved the *(p+i) there.
At this example 2nd string is aaa so i is 3. *(p+i) points out the blank at the 1st string after aaa because it will be changed into '\0' so I want to save what is there at tmp and then put it back after strncpy. Since what I get printed out is only "ddd" it obviously doesn't work.

Edit: I solved the problem by changing "strncpy(p, str3, 50);" into "strncpy(p, str3, i);" but still it bothers me why my way didn't work. Also this is not the end of my problems I explained what I wanted to do further more below.

--------------------------------

This is only the first step. My objective is searching 2nd string in the 1st string (no need for checking if there is a 2nd string in the 1st string) then change it(them) with the 3rd string. In this example there is only one 2nd string in the 1st string which is aaa.

My 2nd step is changing aaa in this string "aaa bbb aaa." into ddd so outcome should be "ddd bbb ddd."

Last step is really bothering me. At this step I want 2nd and 3rd strings to have different sizes.

For example:

aaa bbb aaa.
aaa
x

will print: x bbb x.

A little help would be appreciated

Didn't want to mess up earlier post. This is my final code and I solved the 2nd step too.

Code:

int main() {    
    char str1[100];
    char str2[100];
    char str3[100];


    printf("Enter a sentence: ");
    fgets(str1, 100, stdin);
    printf("Enter a string you want to change: ");
    fgets(str2, 100, stdin);
    printf("New string: ");
    fgets(str3, 100, stdin);
    *(strchr(str1, '\n')) = '\0';
    *(strchr(str2, '\n')) = '\0';
    *(strchr(str3, '\n')) = '\0';


    int i;
    for(i = 0; str2[i] != '\0'; i++){}
    char *p = strstr(str1, str2);
    strncpy(p, str3, i);
    while( p != NULL ) {
        p = strstr(p+1, str2);
        if( p != NULL )
            strncpy(p, str3, i);
    }
    puts(str1);
    return 0;
}

Example:

aaa bbb aaa.aaa.aaa.
aaa
xxx

prints: xxx bbb xxx.xxx.xxx.

Now we came to the last part. I explained this before this post. I want 2nd and 3rd string to have different number of characters.

Example:

aaa bbb aaa.aaa.aaa.
aaa
R

prints: R bbb R.R.R.

I cannot think a solution for this since it will change other characters' index as well as addresses. Need a hand here

This is wrong because fgets will not store the newline character if it was not read:

Code:

*(strchr(str1, '\n')) = '\0';

Rather, you should write:

Code:

char *p = strchr(str1, '\n');
if (p)
{
    *p = '\0';
}

alternatively:

Code:

str1[strcspn(str1, "\n")] = '\0';

This:

Code:

for(i = 0; str2[i] != '\0'; i++){}

should be replaced by a call to strlen:

Code:

i = strlen(str2);

This is not safe since strstr could return a null pointer:

Code:

char *p = strstr(str1, str2);
strncpy(p, str3, i);

That is, you should check the value of p before proceeding with strncpy.

Originally Posted by Vsky

I want 2nd and 3rd string to have different number of characters.
(...)
I cannot think a solution for this since it will change other characters' index as well as addresses.

You could start with a basic solution of shifting the remaining portion of the source string each time you do a replacement.

Originally Posted by Vsky

This code works perfectly fine with me I don't know what looks wrong to you. fgets always stores the newline as far as I see.

Compile and run this program twice:

Code:

#include <stdio.h>
#include <string.h>

int main(void)
{
    char line[7];
    if (fgets(line, sizeof(line), stdin))
    {
        char *p = strchr(line, '\n');
        if (p)
        {
            printf("Newline found\n");
        }
        else
        {
            printf("Newline not found\n");
        }
    }
    return 0;
}

On the first run of the program, enter "hello" as input. On the second run of the program, enter "hello!" as input. Observe the differing output.

Originally Posted by Vsky

Could you be more specific

Imagine you want to replace "aaa" with "bb" in "aaa ccc aaa ddd". You would start with:
aaa ccc aaa ddd
replace the first "aaa" with "bb":
bba ccc aaa ddd
Shift left by 1 to overwrite the "a" that was not replaced:
bb ccc aaa ddd
Then replace the next "aaa" with "bb":
bb ccc bba ddd
Shift left by 1 to overwrite the "a" that was not replaced:
bb ccc bb ddd
and you're done.