function fgets: §7.7 p 134 of The C Programming Language

Function fgets

Problem: Write a program gets a string from the user and prints it out vertically. The user is the person running the program. It might be you. It might be someone who has bought your program. Example: if the user has entered the string dog, then the program will print:

d
o
g

User Input

char Arrays and Function fgets

We will get to this program in a moment. First let's talk about getting a string from the user. There are two parts to getting this accomplished.

First we need to set up a place for the user's string to be stored. The declaration

   char str[100];

sets aside 100 storage locations to store 100 character codes. It's a char array of size 100. In addition it sets up str as symbol for the address of the first storage location. We call str a symbolic constant. It is a constant not a variable, since the purpose of str is to be the address of the beginning of the array. If we could change str as though it were a variable it would no longer correspond to that beginning address.

Second: fgets. The function fgets takes three arguments. The first argument is an address of the start of a storage region. The second argument is one more than the maximum number of character codes to store.

   fgets(str, 100, stdin);

The third argument specifies the file to be used for input. The name stdin (declared in stdio.h) specifies input from keyboard buffer, that is, stuff the user has typed. If the user has already typed something, fgets uses that, otherwise fgets stops and waits for the user to type a bunch of keys and press the enter key.

If the number of characters is small enough,-- in this case less than 100,-- fgets will store all the codes including that of the newline character. If not less than 100 it will store the first 99. In any case at the very end fgets stores a zero to mark the end of the string.

Now let's write the program that gets a string and prints it out vertically. We now know how to do the first part, getting the string.

#include<stdio.h>
int main() {
  char str[100]; 
  fgets(str, 100, stdin);

We can print out the first two characters of the string this way:

#include<stdio.h>
int main() {
  char str[100]; 
  fgets(str, 100, stdin);
  int i=0;
  while (i<2) {
    printf("%c\n", str[i]);
    i += 1;
  }

Here variable i is used as an index for str. If i has a zero in it, then str[i] is the first char code in the array and if i has a 1 in it, then str[i] is the second char code in the array. The conversion %c in the printf converts the code to the corresponding character. The newline sets up the next print to be down one line on the left. So this program works if the string has length 2. But its length could be larger or smaller. If the user's input is dog we have three charaters to print. On the other hand if the the input is the plural of dog, then we have four characters to print. We cannot know the length of the string ahead of time. What we do know is that at the end of the string there is a zero.

#include<stdio.h>
int main() {
  char str[100]; 
  fgets(str, 100, stdin);
  int i = 0;
  while (str[i] != 0) {
    printf("%c\n", str[i]);
    i += 1;
  }
}

The loop above repeats

while the code at index i is non-zero:
   print the char at index i and move to a newline (\n) and
   make index i one larger

So all of the user's string prints provided it has less than 100 characters.

Another version. This is based on the idea that C/C++ treats any non-zero value as true.

#include<stdio.h>
int main() {
  char str[100]; 
  fgets(str, 100, stdin);
  int i = 0;
  while (str[i]) {
    printf("%c\n", str[i]);
    i += 1;
  }
}

Finally we have a version using pointers. Pointer pc has initial address str. Thus *pc is the first code stored in the array. The assignment pc += 1 increases pc so it is now the address of the next item in the array. Each code *pc is printed as a character until 0 is reached.

#include<stdio.h>
int main() {
  char str[100]; 
  fgets(str, 100, stdin);
  char *pc = str;
  while (*pc) {
    printf("%c\n", *pc);
    pc += 1;
  }
}

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