Strings

C is slightly awkward for dealing with character strings, especially compared to languages like Python.

Declaring strings

Constant character strings are written inside double-quotation marks (line 5 below).

Strings are actually, under the hood, arrays of characters. Single character variables are declared using single-quotation marks (line 7 below).

String variables can be declared as indicated on line 8 below. The double square brackets signify an array.

#include <stdio.h>

int main(void) {

  printf("Hello world\n");

  char c = 'p';
  char s[] = "paul";

  printf("c=%c and s=%s\n", c, s);

  return 0;
}

Output:

Hello world
c=p and s=paul

null-termination

An important thing to remember about strings is that they are always null-terminated. This means that the last element of the character array is a "null" character, abbreviated \0. When you declare a string as in line 8 above, you don't have to put the null termination character in yourself, the compiler does it for you. We can use the sizeof() function to inspect our character string above to see how long it actually is:

#include <stdio.h>

int main(void) {

  char s[] = "paul";

  printf("s is %ld elements long\n", sizeof(s));

  return 0;
}

Output:

s is 5 elements long

So, to summarize, in C, strings are simply null-terminated arrays of characters.

Modifying strings

Importantly, once a string is declared to be a given length, you cannot just make it longer or shorter by reassigning a new constant to the variable. Well, you can sort of make it shorter, by writing a new shorter string in the old string array, and terminating the new (shorter) string with a null character. Then essentially you will have a short string sitting in a long array, but that's ok, since we know where the end is (the null termination character).

String handling routines in the C standard library

The standard C library, (which you can load into your program by including the statement #include <string.h> at the top of your program), contains many useful routines for manipulating these null-terminated strings.

I suggest you consult a reference source (or Wikipedia, e.g. C String Handling) for a list (it's relatively long) of all the functions that exist for manipulating null-terminated strings in C. There are functions for copying strings, concatenating strings, getting the length of strings, comparing strings, etc.

An example: Concatenating two strings

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

int main(void) {

  char s1[] = "paul";
  char s2[] = "gribble";
  char s3[256];
  printf("s3=%s, strlen(s3)=%ld\n", s3, strlen(s3));

  strcat(s3, s1);
  printf("s3=%s, strlen(s3)=%ld\n", s3, strlen(s3));

  strcat(s3, " ");  
  printf("s3=%s, strlen(s3)=%ld\n", s3, strlen(s3));

  strcat(s3, s2);
  printf("s3=%s, strlen(s3)=%ld\n", s3, strlen(s3));

  return 0;
}

Output:

s3=, strlen(s3)=0
s3=paul, strlen(s3)=4
s3=paul , strlen(s3)=5
s3=paul gribble, strlen(s3)=12

Comparing two strings

Importantly, you cannot simply use the == operator to test whether two strings are equal. Remember, strings are arrays of characters. You have to use a special string handling function to test equality of two strings, since it has to do a "deep" comparison, comparing each element against each other. Here's how you would do it:

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

int main(void) {

  char s1[] = "paul";
  char s2[] = "paul";
  char s3[] = "peter";
  char s4[] = "dave";

  printf("strcmp(s1,s2)? %d\n", strcmp(s1,s2));
  printf("strcmp(s1,s3)? %d\n", strcmp(s1,s3));
  printf("strcmp(s1,s4)? %d\n", strcmp(s1,s4));

  return 0;
}

Output:

strcmp(s1,s2)?0
strcmp(s1,s3)?-4
strcmp(s1,s4)?12

Note that the strcmp(s1,s2) function returns  0 if s1 and s2 are equal, a positive value if s1 is (lexicographically) less than s2, and a negative value if s1 is greater than s2.

Converting strings to and from numeric types

Strings to numbers

There are several functions to convert strings to numeric types like integers and floating-point numbers. You will need to #include <stdlib.h> at the top of your program.

• double atof(s) converts the string pointed to by s into a floating-point number (a double), returning the result
• int atoi(s) converts string s into an integer

There are a host of others, again I suggest consulting a reference source for a comprehensive list.

Numbers to strings

The common way of converting a numeric type like an integer or a floating-point number into a string, is to use the sprintf() function. It it used much like the printf() function we have seen before, but instead of printing something to the screen, sprintf() "prints" something to a character string. Here's how to use it:

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

int main(void) {

  char s1[256];
  char s2[256];
  int i1 = 12;
  double d1 = 3.141;

  sprintf(s1, "%d", i1);
  sprintf(s2, "%.3f", d1);

  printf("s1 = %s\n", s1);
  printf("s2 = %s\n", s2);

  return 0;
}

Output:

s1 =12
s2 =3.141

Note how on lines 6 and 7 when s1 and s2 are declared, I declare them as character arrays large enough to hold 256 characters. If you try to sprintf() to a string that is not big enough to hold what you're trying to put into it, then you will end up writing values beyond the end of the string, and onto who knows what, in memory. If you are dealing with strings that you know will be relatively short (things like filenames, subject names, dates, etc) then probably the easiest way of doing things is to use preallocated strings that are long enough to hold any reasonable value (e.g. 256 characters long). After all we have enough RAM in our computers these days not to have to worry too much about 256 bytes here and there.

Numbers to string II (slightly esoteric)

There is, however, a way to do this without having to hard-code the size of the string to be written to, although it's a little bit roundabout. However it does illustrate several principles of C so let's have a look at it.

First we will use the snprintf() function in a roundabout way to determine the number of bytes that the numeric to string conversion will result in. Then we will use malloc() to allocate a new string (character array) of that length. Finally we will use sprintf() to write to that character array. The first step ensures that we have a character array (a string) that is just the right length to recieve the converted numeric: not too small, and not too big.

Here is some sample code that demonstrates this, first for an integer conversion, and then for a floating-point conversion:

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

int main(void) {

  int size;

  int x = 8765309;
  size = snprintf(NULL, 0, "%d", x);
  char *xc = malloc(size + 1);
  sprintf(xc, "%d", x);

  double y = 876.5309;
  size = snprintf(NULL, 0, "%.4f", y);
  char *yc = malloc(size + 1);
  sprintf(yc, "%.4f", y);

  printf("xc = %s\n", xc);
  printf("yc = %s\n", yc);

  free(xc);
  free(yc);

  return 0;
}

Output:

xc =8765309
yc =876.5309

Note on lines 10 and 15, where we use snprintf(), we are passing NULL as the first argument. The snprintf() function is like sprintf(), but it takes as its second argument, the maximum number of bytes to write out to the destination string. Thus snprintf() can be thought of as a "safe" version of sprintf() in that you know for sure that you will never write out more than the maximum number of bytes you ask for. Thus you can avoid over-writing past the end of your destination string buffer. The snprintf() function will return as its return value, the number of bytes that would have been written had the second argument been sufficiently large (not counting the termination \0 character).

So here we are passing NULL as the first argument, and 0 as the second. So as a result, snprintf() won't actually write any characters anywhere, it will simply return the number of characters that would have been written. Then on lines 11 and 16, we can use malloc() to dynamically allocate character arrays of exactly the required length.

Arrays of Strings

We have seen that strings are just arrays of characters, terminated by a null character. We have also seen that the variables that hold strings (like arrays of other types, e.g. int or  double, are actually pointers to the head of the array. We can use an array of pointers, where each pointer is a pointer to the head of a character array (in other words a string), to store an array of strings. Here is an example:

#include <stdio.h>

int main(int argc, char *argv[])
{
  char *provinces[] = { "British Columbia", "Alberta", "Saskatchewan", 
                        "Manitoba", "Ontario", "Quebec", "New Brunswick",
                        "Nova Scotia", "Prince Edward Island", "Newfoundland",
                        "Yukon", "Northwest Territories", "Nunavut" };
  int i;
  for (i=0; i<13; i++) {
    printf("provinces[%d] = %s\n", i, provinces[i]);
  }
  return 0;
}

Output:

provinces[0]=BritishColumbia
provinces[1]=Alberta
provinces[2]=Saskatchewan
provinces[3]=Manitoba
provinces[4]=Ontario
provinces[5]=Quebec
provinces[6]=NewBrunswick
provinces[7]=NovaScotia
provinces[8]=PrinceEdwardIsland
provinces[9]=Newfoundland
provinces[10]=Yukon
provinces[11]=NorthwestTerritories
provinces[12]=Nunavut

Exercise

• Alter the program above that prints out the provinces, so that it prints out each province using all upper case letters. Hint: Ascii Table. Another hint:

#include <stdio.h>

int main(int argc, char *argv[])
{
  char c = 'a';
  printf("%c - 32 = %c\n", c, c-32);
  return 0;
}

Output

a -32= A

Solution

// gcc -Wall -o go 9_1.c

#include <stdio.h>

int main(int argc, char *argv[])
{
  char *provinces[] = { "British Columbia", "Alberta", "Saskatchewan", 
                        "Manitoba", "Ontario", "Quebec", "New Brunswick",
                        "Nova Scotia", "Prince Edward Island", "Newfoundland",
                        "Yukon", "Northwest Territories", "Nunavut" };
  int i, j;
  char tmpchar;
  for (i=0; i<13; i++) {
    printf("provinces[%d] = %c", i, provinces[i][0]);
    j=1;
    while (provinces[i][j] != 0) {
      tmpchar = provinces[i][j];
      if ((provinces[i][j] >= 'a') && (provinces[i][j] <= 'z')) {
	tmpchar = provinces[i][j] - 32;
      }
      printf("%c", tmpchar);
      j++;
    }
    printf("\n");
  }
  return 0;
}

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Source: Paul Gribble, http://web.archive.org/web/20160503160158/http://gribblelab.org/CBootcamp/9_Strings.html
This work is licensed under a Creative Commons Attribution 4.0 License.