Attach a String to another String in C WITHOUT any spaces

Question

this is my first post in this forum so please be patient.

I need to make a short programm, where the user can enter 2 strings which should be attached afterwards. I already got this code below (I am not allowed to use other "includes").

What I need to know is: How can I deny any spaces which the user will enter?

Example: 1. String "Hello " | 2. String "World"

Result should be "HelloWorld" instead of "Hello World".

#include <stdio.h>

void main()
{
    char eingabe1[100];
    char eingabe2[100];
    int i = 0;
    int j = 0;
    printf("Gib zwei Wörter ein, die aneinander angehängt werden sollen\n");
    printf("1. Zeichenkette: ");
    gets(eingabe1);
    printf("\n");
    printf("2. Zeichenkette: ");
    gets(eingabe2);
    printf("\n");

    while (eingabe1[i] != '\0')
    {
        i++;
    }

    while (eingabe2[j] != '\0')
    {
        eingabe1[i++] = eingabe2[j++];
    }
    eingabe1[i] = '\0';
    printf("Nach Verketten: ");
    puts(eingabe1);

}

Answer 1

You have to filter out the spaces as you copy your strings.

You have two string indices, i for the first string and and j for the second string. You could make better use of these indices if you used i for the reading position (of both strings subsequently; you can "reuse" loop counters in independent loops) and j for the writing position.

Here's how. Note that the code attempts to prevent buffer overflow by only adding characters if there is space in the string. This check needs only to be done when copying the second string, because j <= i when you process the first string.

#include <stdio.h>

int main()
{
    char str1[100] = "The quick brown fox jumps over ";
    char str2[100] = "my big sphinx of quartz";

    int i = 0;
    int j = 0;

    while (str1[i] != '\0') {
        if (str1[i] != ' ') str1[j++] = str1[i];
        i++;
    }

    i = 0;
    while (str2[i] != '\0') {
        if (str2[i] != ' ' && j + 1 < sizeof(str1)) str1[j++] = str2[i];
        i++;
    }

    str1[j] = '\0';

    printf("'%s'\n", str1);

    return 0;
}

Answer 2

In addition to avoiding spaces between your two words, you also have to avoid the newline ('\n') character placed in the input buffer by the user pressing Enter. You can do that with a simple test after you have read the line with fgets() NOT gets(). gets() is no longer part of the standard C library and should not be used due to insecurity reasons. Plus fgets provides simple length control over the number of characters a user may enter at any time.

Below, you run into trouble when you read eingabe1. After the read, eingabe1 contains a '\n' character at its end. (as it would using any of the line-oriented input functions (e.g. getline(), fgets(), etc) To handle the newline, you can simply compare its length minus '1' after you loop over the string to find the nul character. e.g.:

if (eingabe1[i-1] == '\n') i--;     /* remove trailing '\n', update i    */

By simply reducing the index 'i', this will guarantee that the concatenation with eingabe2 will not have any spaces or newline characters between the words.

Putting the pieces together, and using fgets in place of the insecure gets, after #define MAX 100'ing a constant to prevent hardcoding your array indexes, you could come up with something similar to:

#include <stdio.h>

#define MAX 100

int main (void)
{
    char eingabe1[MAX] = {0};
    char eingabe2[MAX] = {0};
    int i = 0;
    int j = 0;

    printf("Gib zwei Wörter ein, die aneinander angehängt werden sollen\n");
    printf("1. Zeichenkette: ");
    /* do NOT use gets - it is no longer part of the C library */
    fgets(eingabe1, MAX, stdin);

    putchar ('\n');
    printf("2. Zeichenkette: ");
    /* do NOT use gets - it is no longer part of the C library */
    fgets(eingabe2, MAX, stdin);
    putchar ('\n');

    while (eingabe1[i]) i++;  /* set i (index) to terminating nul  */
    if (i > 0) {
        if (eingabe1[i-1] == '\n') i--;    /* remove trailing '\n' */
        while (i && eingabe1[i-1] == ' ')  /* remove trailing  ' ' */
            i--;
    }

    while (eingabe2[j]) {     /* concatenate string - no spaces    */
        eingabe1[i++] = eingabe2[j++];
    }
    eingabe1[i] = 0;         /* nul-terminate eingabe1 */

    printf("Nach Verketten: %s\n", eingabe1);

    return 0;
}

Output

$ ./bin/strcatsimple
Gib zwei Wörter ein, die aneinander angehängt werden sollen
1. Zeichenkette: Lars

2. Zeichenkette: Kenitsche

Nach Verketten: LarsKenitsche

Let me know if you have any further questions. I have highlighted the changes with comments above.

Answer 3

/**
    return: the new len of the string;
*/
int removeChar(char* string, char c) {

    int i, j;
    int len = strlen(string)+1;             // +1 to include '\0'

    for(i = 0, j = 0 ; i < len ; i++){
        if( string[i] == c )
            continue;                       // avoid incrementing j and copying c
        string[ j ] = string[ i ];          // shift characters
        j++;
    }

    return j-1;   // do not count '\0';
}

int main(){

    char str1[] =       "sky is flat ";
    char str2[100] =    "earth is small ";

    strcat( str2, str1 );
    printf("with spaces:\n\t'%s'\n", str2) ;
    removeChar(str2, ' ');

    printf("without spaces:\n\t'%s'\n", str2 );

}


/**
    BONUS: this will remove many characters at once, eg "\n \r\t"
    return: the new len of the string;
*/
int removeChars(char* string, char *chars) {

    int i, j;
    int len = strlen(string); 

    for(i = 0, j = 0 ; i < len ; i++){
        if( strchr(chars,string[i]) )
            continue;                       // avoid incrementing j and copying c
        string[ j ] = string[ i ];          // shift characters
        j++;
    }
    string[ j ]=0;
    return j;   
}

Answer 4

Thank you everyone for all the answers. I got the solution now.

I read some advices from you and will try to remember for the future.

See the code below: (Excuse me for the strange names for the variables, I use german words)

A few notices: I am not allowed to use library functions I am not allowed to use fgets for some reasons as a trainee

#include <stdio.h>

void main()
{
    char eingabe1[100];
    char eingabe2[100];
    int i = 0;
    int j = 0;
    printf("gib zwei wörter ein, die aneinander angehängt werden sollen\n");
    printf("1. zeichenkette: ");
    gets(eingabe1);
    printf("\n");
    printf("2. zeichenkette: ");
    gets(eingabe2);
    printf("\n");

    //Attach Strings
    while (eingabe1[i] != '\0')
    {
        i++;
    }
    while (eingabe2[j] != '\0')
    {
        eingabe1[i++] = eingabe2[j++];
    }

    //Remove Space

    eingabe1[i] = '\0';
    i = 0;
    j = 0;
    while (eingabe1[i] != '\0')
    {
        if (eingabe1[i] != 32)
        {
            eingabe2[j++] = eingabe1[i];
        }
        i++;
    }
    eingabe2[j] = '\0';
    printf("Nach verketten: ");
    puts(eingabe2);

}

Answer 5

Sounds like homework to me.

I just wanted to mention that you probably shouldn't use sizeof() on strings these days because there may be multibyte characters in there. Use strlen() instead. The only time sizeof() would be appropriate is if you're going to malloc() a certain number of bytes to store it.

I write little loops fairly often to do low level text stuff one character at a time, just be aware that strings in C usually have a 0 byte at the end. You have to expect to encounter one and be sure you put one on the output. Space is 0x20 or decimal 32 or ' ', it's just another character.