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cstringstrcat
Refael
Refael

Reputation: 7353

strcat paste second string at beginning of first string

i use strcat() to connect two strings like: 

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

    int main(int argc, char *args[])
    {
       char *str1; // "456"
       char *str2; // "123"

       strcat(str1,str2);
       printf("%s",str1);
    }

i get: 

456123

but i need the second string on beginning of first string like: 

123456

how can i do it ?

Upvotes: 5

Views: 37015

Answers (6)

Hexagon
Hexagon

Reputation: 36

If you only want to add a single char :

char *add_char_to_beginning(char c, char **str)
{
    char *tmp = strdup(*str);
    *str = (char *) realloc(*str, sizeof(char) * (strlen(*str) + 2));
    *(str[0]) = c;
    strcpy(*str + 1, tmp);
    free(tmp);

    return *str;
}

Upvotes: 0

Hexagon
Hexagon

Reputation: 36

If you segfault on this because you have two non-allocated strings with different sizes, maybe it'll solve the probleme. I first calculated the size difference to allocate the first string with the bigger size (in which [padding] will be concatenated at the begining). Then added the code of @Rohan.

if (len1 < len2) {
    char *tmp = strdup(num1);
    num1 = (char *) malloc(sizeof(char) * (strlen(num1) + diff));
    strcpy(num1, padding);
    strcat(num1, tmp);
    printf("%s\n", num1);
    free(tmp);
}

Upvotes: 0

Sornii
Sornii

Reputation: 421

try to use this with static sized arrays, works for me in my project.

void strbeg(char* strReceive, char* strInsert)
{
    int strInsertLength = strlen(strInsert);
    int strReceiveLength = strlen(strReceive);

    for(int i = strReceiveLength; i >= 0; i--)
    {
        strReceive[i + strInsertLength] = strReceive[i];
    }

    for(int i = 0; i < strInsertLength; i++)
    {
        strReceive[i] = strInsert[i];
    }
}

Upvotes: 1

alk
alk

Reputation: 70981

Verbatim from man strcat:

char *strcat(char *dest, const char *src);

DESCRIPTION

The strcat() function appends the src string to the dest string, overwriting the null byte ('\0') at the end of dest, and then adds a terminating null byte. The strings may not overlap, and the dest string must have enough space for the result.

As the programmer you need to make sure that the pointer char * dest references enough valid memory to hold the addtional chars that will be copied from where char* src points to.

To succesfully prefix str1 with str2 declare them as follows:

char str2[3 + 1] = "123"; // three for "123" plus 1 for the terminating 0
char str1[2 + 3 + 1] = "46"; // 2 for "46" plus 3 for "123" plus 1 for the terminating zero

to actually concatenate the two char arrays do so:

strcat(str1, str2);

Upvotes: 0

Rohan
Rohan

Reputation: 53386

Do strcat(str2,str1);, switch the parameters. But you will get resultant string in str2, which you can set to str1 if you really want to use str1 further in your program.

However, you need to take care appropriately for memory space available in str2.

If you want to change str1 then, do this

char *tmp = strdup(str1);

strcpy(str1, str2); //Put str2 or anyother string that you want at the begining
strcat(str1, tmp);  //concatenate previous str1

...
free(tmp); //free the memory

Upvotes: 7

unxnut
unxnut

Reputation: 8839

You need to use strcpy instead of strcat.

Upvotes: 0

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