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Defining strings

Strings in C are actually arrays of characters. Although using pointers in C is an advanced subject, fully explained later on, we will use pointers to a character array to define simple strings, in the following manner:

char * name = "John Smith";

This method creates a string which we can only use for reading. If we wish to define a string which can be manipulated, we will need to define it as a local character array:

char name[] = "John Smith";

This notation is different because it allocates an array variable so we can manipulate it. The empty brackets notation [] tells the compiler to calculate the size of the array automatically. This is in fact the same as allocating it explicitly, adding one to the length of the string:

char name[] = "John Smith";
/* is the same as */
char name[11] = "John Smith";

The reason that we need to add one, although the string John Smith is exactly 10 characters long, is for the string termination: a special character (equal to 0) which indicates the end of the string. The end of the string is marked because the program does not know the length of the string - only the compiler knows it according to the code.

String formatting with printf

We can use the printf command to format a string together with other strings, in the following manner:

char * name = "John Smith";
int age = 27;

/* prints out 'John Smith is 27 years old.' */
printf("%s is %d years old.\n", name, age);

Notice that when printing strings, we must add a newline (\n) character so that our next printf statement will print in a new line.

String Length

The function ‘strlen’ returns the length of the string which has to be passed as an argument:

char * name = "Nikhil";
printf("%d\n",strlen(name));        

String comparison

The function strncmp compares between two strings, returning the number 0 if they are equal, or a different number if they are different. The arguments are the two strings to be compared, and the maximum comparison length. There is also an unsafe version of this function called strcmp, but it is not recommended to use it. For example:

char * name = "John";

if (strncmp(name, "John", 4) == 0) {
    printf("Hello, John!\n");
} else {
    printf("You are not John. Go away.\n");
}

String Concatenation

The function ‘strncat’ appends first n characters of src string to the destination string where n is min(n,length(src)); The arguments passed are destination string, source string, and n - maximum number of characters to be appended. For Example:

char dest[20]="Hello";
char src[20]="World";
strncat(dest,src,3);
printf("%s\n",dest);
strncat(dest,src,20);
printf("%s\n",dest);

Exercise

Define the string first_name with the value John using the pointer notation, and define the string last_name with the value Doe using the local array notation.

Tutorial Code

#include <stdio.h>
#include <string.h>
int main() {
  /* define first_name */
  /* define last_name */
  char name[100];

  last_name[0] = 'B';
  sprintf(name, "%s %s", first_name, last_name);
  if (strncmp(name, "John Boe", 100) == 0) {
      printf("Done!\n");
  }
  name[0]='\0';
  strncat(name,first_name,4);
  strncat(name,last_name,20);
  printf("%s\n",name);
  return 0;
}

Expected Output

Done!
JohnBoe

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