Reading strings with fgets

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These little programs will show you how to read input from the console and save it into character arrys. We are also going to use a few string functions such as getc, getchar, putc, putchar, strcpy, strcat and strlen.

Contents

Understanding Standard I/O

stdin
  • The standard input for reading
stdout
  • The standard output for writing
stderr
  • The standard error for writing error messages.

Get one character from user

Write a program that gets one character from standard input and then prints it on the standard output. 

#include <stdio.h>
 
char input;
main ()
{
        printf("Enter in one characher: ");
        input = getc(stdin);
        printf("The character you entered is %c\n", input);
        return (0);
}

Using the getchar() function

Write a program that uses both getc and getchar. Get two characters from the standard input and then display both characters on the standard output using printf. Save the characters in variables. 

#include <stdio.h>
 
char input;
char input2;
main ()
{
        printf("Enter in two charachers: ");
        input = getc(stdin);
        input2 = getchar();
        printf("The character you entered is %c\n", input);
        printf("The second character you entered is %c\n", input2);
        return (0);
}

Output

Enter in two charachers: QA
The character you entered is Q
The second character you entered is A

Printing output with putc

Write a program that gets a character from the standard input and then displays the character on the standard output using putc. 

#include <stdio.h>
 
char input1;
main()
{
        printf("Please enter a char: ");
        input1 = getchar();
        printf("you entered :");
        putc(input1, stdout);
        return (0);
}

Printing output with putchar

Write a program that reads a character from the standard input and then displays the character on the standard output using putchar. 

#include <stdio.h>
 
char input1;
main()
{
        printf("Please enter a char: ");
        input1 = getchar();
        printf("you entered :");
        putchar(input1);
        return (0);
}

fgets example

Write a program that takes a string as an input and then displays the length of the string and displays the string on the standard output. 

#include <stdio.h>
#include <string.h>
 
char line[100];  /* character arrys that contains 100 characters */
main ()
{
 printf("Enter some text: ");
 fgets(line, sizeof(line), stdin);  /* fgets takes 3 parameters */
 printf("the length of line is %d\n", strlen(line));
 printf("you entered: \n%s\n",line);
 return(0);
}

fgets with file

Another example that reads from a file. 

#include <stdio.h>
 
int main()
{
   FILE * pFile;
   char mystring [100];
 
   pFile = fopen ("myfile.txt" , "r");
   if (pFile == NULL) perror ("Error opening file");
   else {
     fgets (mystring , 100 , pFile);
     puts (mystring);
     fclose (pFile);
   }
   return 0;
}

Parameters for fgets

str
  • Pointer to an array of chars where the string read is stored.
num
  • Maximum number of characters to be read (including the final null-character). Usually, the length of the array passed as str is used.
stream
  • Pointer to a FILE object that identifies the stream where characters are read from.

To read from the standard input, stdin can be used for this parameter.

Output aligning with printf

Write a program that shows how to left or right align numbers using printf. 

#include <stdio.h>
 
int num1, num2, num3, num4, num5;
main()
{
        num1= 1;
        num2= 12;
        num3= 123;
        num4= 1234;
        num5= 12345;
 
        printf("%8d  %-8d\n", num1, num1);
        printf("%8d  %-8d\n", num2, num2);
        printf("%8d  %-8d\n", num3, num3);
        printf("%8d  %-8d\n", num4, num4);
        printf("%8d  %-8d\n", num5, num5);
        return (0);
}

Output

       1  1
      12  12
     123  123
    1234  1234
   12345  12345

Playing with Strings

Let's write a program that asks for the first and last name and then displays it on the standard output. 

#include <stdio.h>
#include <string.h>
 
char first[100];
char last[100];
char full[200];
 
main ()
{
 printf("Please enter your first name: ");
 fgets(first, sizeof(first), stdin);
 printf("Please enter your last name: ");
 fgets(last, sizeof(first), stdin);
 strcpy(full, first);
 strcat(full, " ");
 strcat(full, last);
 printf("Your full name is %s.\n",full);
 return (0);
}

Output

Please enter your first name: qais
Please enter your last name: chaudry
Your full name is qais
 chaudry
.

As you can see from the above out that we have a problem. The name is not on the same line and the full stop is also from down. So lets fix this issue. The reason we are getting a new line is because when we finish typing the first name we hit enter and that is also stored in the string. We need to get rid of that and replace it with NULL. We can do this by (first)-1='\0'. The statement will look like 

first[strlen(first)-1]='\0';

fgets but without new-line

We need to do this for both, first name and last name so our code will look like this: 

#include <stdio.h>
#include <string.h>
 
char first[100];
char last[100];
char full[200];
 
main ()
{
 printf("Please enter your first name: ");
 fgets(first, sizeof(first), stdin);
 printf("Please enter your last name: ");
 fgets(last, sizeof(first), stdin);
 first[strlen(first)-1]='\0';
 strcpy(full, first);
 strcat(full, " ");
 last[strlen(last)-1]='\0';
 strcat(full, last);
 printf("Your full name is %s.\n",full);
 return (0);
}

Output on one line

Please enter your first name: qais
Please enter your last name: chaudry
Your full name is qais chaudry.

Multiple Dimensional Arrays

Arrays can be more than one dimension. It is declared

  • int matrix[2][3]; /*a typical matrix*/

C allows the programmer to use as many dimensions as needed such as four_dimension[10][12][9][5];

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