Thread: strcat problems

Hey guys, I'm trying to get into C and C++ from PHP and Java. Obviously I may be biased towards the two that I find easier. However, I have run into several problems that are ruining my experience with this language.

First is the terrible implementation of strings. Especially in C.

QQ aside, I wanted to know how to make something like this work;

Code:

char szX = "1";
char szY[12] = "gugugaga"
strcat(szY, szX)
//I want "gugugaga1"

the problem here is that the compiler wont accept anything other than a constant char value.

The code above is part of the while clause and as such, the value of x changes every time the loop runs.

Another question would be

Does the C compiler automatically place a "/0" at the end of y[12]?

Assuming that I am new to this language and the nuances of pointers and such like have not been completely mastered. How can I join the two strings?

TL;DR: How do I get the strcat to join to variable char strings? How do I do it at all?

Code:

char szX[] = "1";    // "1" == {'1', '\0'}

I suggest you should go grab a good tutorial on C.
Because you know java and php doesn't mean you would be able to just straightaway code in C.

If a value is supposed to change on each iteration of while-loop you can do:

Code:

char szX[2];
int counter = 0;
while ( condition )
{
    szX[0] = itoa( counter );
    strcat( szY, szX );
    counter++;
}

// Or more involved...
char date[12];
int year, month, day;
year = 2010;
month = 1;
day = 1;
while ( month <= 12 )
{
    while ( day <= 30 )
    {
        sprintf( date, "%02d-%02d-%4d\n", day, month, year );
        printf( "%s", date );
        day++;
    }
    month++;
    day = 1;
}

No go. The compiler basically want a constant string at the second argument of strcat

strcat( szY, "1");

works

strcat( szY, szX ) doesn't work.

Is it that I have a bad compiler, or is it the quirky language?

It isn't a quirk or your compiler. It would not surprise me if strcat() arguments were restricted, which forces them to be different pointers.

Originally Posted by thundercraker

Hey guys, I'm trying to get into C and C++ from PHP and Java. Obviously I may be biased towards the two that I find easier. However, I have run into several problems that are ruining my experience with this language.

First is the terrible implementation of strings. Especially in C.

First off, C doesn't have strings, at all. None, zero, zip, nada

Many of the C library functions treat arrays of characters as strings, but that is purely a construct of the libraries.

QQ aside, I wanted to know how to make something like this work;

Code:

char szX = "1";
char szY[12] = "gugugaga"
strcat(szY, szX)
//I want "gugugaga1"

the problem here is that the compiler wont accept anything other than a constant char value.

To add variable content to char arrays you need library functions, not the equals sign.

If you want to add x on the the back end of y you first need to make sure that x has a sufficient buffer to hold both strings... if X is 10 chars and Y is 7, you need 17 characters for the text and one for the trailing null. The null is essential as it tells library functions when to stop reading.

Code:

char szX[50] = {"This is "};
char szY[] = {"a test"};
strcat (szX,szY); 
puts (szX);

Or similar.

The C compiler will not allocate the memory for you. As you get further into the language you will discover this is actually an advantage because it lets you control how, where and when a string buffer is created.

The code above is part of the while clause and as such, the value of x changes every time the loop runs.

If that's the case you need to use library functions, not initializers to get the task done... Look into functions such as strcpy(), memcpy() and wcscpy() for more on this.

Another question would be

Does the C compiler automatically place a "/0" at the end of y[12]?

No, the compiler does not. The various string handling functions in the libraries do.

Assuming that I am new to this language and the nuances of pointers and such like have not been completely mastered. How can I join the two strings?

TL;DR: How do I get the strcat to join to variable char strings? How do I do it at all?

Actually it's very easy... my example (poor as it is) should point you in the right direction. But you would be a lot further ahead if you stopped thinking "strings" and started thinking "character arrays" ...

That is my quandary atm

Originally Posted by CommonTater

Another question would be

Does the C compiler automatically place a "/0" at the end of y[12]?

No, the compiler does not. The various string handling functions in the libraries do.

Yes, it does when you initialize a char array to a string literal, and provided that there is enough space in the array.

Code:

char str_a[5] = "foobar";    // {'f', 'o', 'o', 'b', 'a'} and a warning from a decent compiler
char str_b[] = "foobar";      // {'f', 'o', 'o', 'b', 'a', 'r', '\0'};

Originally Posted by thundercraker

How do i make the above work?

You use code tags and sprintf().

Ah I think my problem is with the fact that I am using a changing value for the second argument, like

Code:

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

int main(void)
{
    char first[12] = "hello ";
    char second[] = "world";
    for(int i; i<6; i++) 
       strcat(first, second[i]);
    printf("%s\n", first);
    return 0;
}

trying this gives me "invalid conversion from char to const char"

Okay. What do you think does expression second[i] evaluate to?