How to Append an Int to a String in C++

How to concatenate string and int in C?

Strings are hard work in C.

#include <stdio.h>

int main()
{
   int i;
   char buf[12];

   for (i = 0; i < 100; i++) {
      snprintf(buf, 12, "pre_%d_suff", i); // puts string into buffer
      printf("%s\n", buf); // outputs so you can see it
   }
}

The 12 is enough bytes to store the text "pre_", the text "_suff", a string of up to two characters ("99") and the NULL terminator that goes on the end of C string buffers.

This will tell you how to use snprintf, but I suggest a good C book!

concatenate two integers into a char * in C

You can just do it with one sprintf:

char str[100];
sprintf(str, "%d:%d", a, b);

How to convert an int to string in C?

EDIT: As pointed out in the comment, itoa() is not a standard, so better use sprintf() approach suggested in the rivaling answer!

You can use itoa() function to convert your integer value to a string.

Here is an example:

int num = 321;
char snum[5];

// convert 123 to string [buf]
itoa(num, snum, 10);

// print our string
printf("%s\n", snum);

If you want to output your structure into a file there is no need to convert any value beforehand. You can just use the printf format specification to indicate how to output your values and use any of the operators from printf family to output your data.

How to add/append character to a string?

You probably want something like this:

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

char box[100] = "";

int main()
{
  FILE* fp = fopen("somefilename", "r");
  int c;

  for (c = getc(fp); c != EOF; c = getc(fp)) {    
    char tempC[2];
    tempC[0] = c;
    tempC[1] = 0;
    strncat(box, tempC, 1);
  }
}

This compiles but it is terrible code:

there is no check if fopen succeeds
Using strncat here is very inefficient. As an exercise change this without using strncat or other str... functions.
usage of a global variable without need

Concatenate int to string using C Preprocessor

Classical C preprocessor question....

#define STR_HELPER(x) #x
#define STR(x) STR_HELPER(x)

#define MAJOR_VER 2
#define MINOR_VER 6
#define MY_FILE "/home/user/.myapp" STR(MAJOR_VER) STR(MINOR_VER)

The extra level of indirection will allow the preprocessor to expand the macros before they are converted to strings.

How to add integer in a string?

Do something like

#include <string>

// ...

std::string s( "February " );

s += std::to_string( day );

Here is a demonstrative program

#include <iostream>
#include <string>

int main()
{
    std::string s( "February " );

    int day = 20;

    s += std::to_string( day );

    std::cout << s << '\n';
}

Its output is

February 20

Another approach is to use a string stream.

Here is one more demonstrative program.

#include <iostream>
#include <sstream>
#include <string>

int main()
{
    std::ostringstream oss;
    int day = 20;

    oss << "February " << day;

    std::string s = oss.str();

    std::cout << s << '\n';
}

Its output is the same as shown above.