C - User Input

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Need for User Input in C

Every computer application accepts certain data from the user, performs a predefined process on the same to produce the output. There are no keywords in C that can read user inputs. The standard library that is bundled with the C compiler includes stdio.h header file, whose library function scanf() is most commonly used to accept user input from the standard input stream. In addition, the stdio.h library also provides other functions for accepting input.

To understand the need for user input, consider the following C program −

#include <stdio.h>
int main()
{
   int price, qty, ttl;

   price = 100;
   qty = 5;
   ttl = price*qty;

   printf("Total : %d", ttl);
   return 0;
}

Output

Total : 500

The above program calculates the total purchase amount by multiplying the price and quantity of an item purchased by a customer. For another transaction with different values of price and quantity, you need to edit the program, put the values, then compile and run again. To do this every time, is a tedious activity. Instead, there must be a provision to assign the values to variable after the program is run. The scanf() function reads the user input during the runtime, and assigns the value to a variable.

C User Input Function: The scanf()

The C language recognizes the standard input stream as stdin and is represented by the standard input device such as a keyboard. C always reads the data from the input stream in the form of characters. The scanf() function converts the input to desired data type with appropriate format specifiers.

Syntax

int scanf(const char *format, &var1, &var2, . . .);

The first argument to scanf() function is a format string. It indicates the data type of the variable in which the user input is to be parsed. It is followed by one or more pointers to the variables. The variable names prefixed by & gives the address of the variable.

User Input Format Specifiers

Following format specifiers are used in the format string −

Going back to the previous example, we shall use the scanf() function to accept the value for price and qty, instead of assigning them any fixed value.

Example: User Inputs in C

#include <stdio.h>
int main()
{
   int price, qty, ttl;

   printf("Enter price and quantity: ");
   scanf("%d %d", &price, &qty);

   ttl = price*qty;

   printf("Total : %d", ttl);
   return 0;
}

Output

When the above program is run, C waits for the user to enter the values.

Enter price and quantity:

When the values are entered and you press Enter, the program proceeds to the subsequent steps.

Enter price and quantity: 100 5
Total : 500

What is more important is that, for another set of values, you don't need to edit and compile again. Just run, C again waits for the user input. In this way, the program can be run any number of times with different inputs.

Enter price and quantity: 150 10
Total : 1500

Integer Input

The %d format specifier has been defined for signed integer. The following program reads the user input and stores it in the int variable num.

Example: Integer Input in C

#include <stdio.h>
int main()
{
   int num;
   printf("Enter an integer: ");
   scanf("%d", &num);
   printf("You entered an integer : %d", num);
   return 0;
}

Output

Enter an integer: 234
You entered an integer : 234

If you enter a non-numeric value, the output is 0.

The scanf() function can read values to one or more variables. While inputting the values they must be separated by any white space, tab or enter.

Example: Multiple Integer Inputs in C

#include <stdio.h>
int main()
{
   int num1, num2;
   printf("Enter two integers: ");
   scanf("%d %d", &num1, &num2);
   printf("You entered two integers : %d and %d", num1, num2);
   return 0;
}

Output

Enter two integers: 45 57
You entered two integers : 45 and 57

or

Enter two integers: 45
57

Float Input

For floating point input, you need to use %f format specifier.

Example: Float Input in C

#include <stdio.h>
int main()
{
   float num1;
   printf("Enter a number: ");
   scanf("%f", &num1);
   printf("You entered a floating-point number: %f", num1);
   return 0;
}

Output

Enter a number: 34.56
You entered a floating-point number: 34.560001

The scanf() function may read inputs for different types of variables. In the following program, user input is stored in an integer and a float variable −

Example: Integer and Float Inputs in C

#include <stdio.h>
int main()
{
   int num1;
   float num2;
   printf("Enter two numbers: ");
   scanf("%d %f", &num1, &num2);
   printf("You entered an integer: %d a floating-point number: %6.2f", num1, num2);
   return 0;
}

Output

Enter two numbers: 65 34.5678
You entered an integer: 65 a floating-point number:  34.57

Character Input

The %c format specifier reads a single character from the keyboard. However, we must give a blank space before %c in the format string. This is because the %c conversion specifier won't automatically skip any leading whitespace, so if there's a stray newline in the input stream (from a previous entry, for example) the scanf() call will consume it immediately.

scanf(" %c", &c);

The blank in the format string tells scanf to skip leading whitespace, and the first non-whitespace character will be read with the %c conversion specifier.

Example: Character Input in C

#include <stdio.h>
int main()
{
   char ch;
   printf("enter a single character: ");
   scanf(" %c", &ch);
   printf("You entered character : %c", ch);
   return 0;
}

Output

enter a single character: x
You entered character : x

Example: Multiple Character Inputs in C

The following program reads two characters separated by a space in two char variables.

#include <stdio.h>
int main()
{
   char ch1, ch2;
   printf("enter a two characters: ");
   scanf("%c %c", &ch1, &ch2);
   printf("You entered characters : %c and %c", ch1, ch2);
   return 0;
}

Output

enter a two characters: x y
You entered characters : x and y

The stdio.h header file also provides getchar() function. Unlike scanf(), getchar() doesn't have a format string. Also, it reads a single key stroke without the enter key. The following program reads a single key into a char variable −

Example: Character Input Using gets()

#include <stdio.h>
int main()
{
   char ch;
   printf("enter a character: ");
   ch = getchar();
   puts("You entered: ");
   putchar(ch);
   printf("\nYou entered character : %c", ch);
   return 0;
}

Output

enter a character: W
You entered:
W
You entered character : W

You can also use the unformatted putchar() function to print a single character.

Example: Reading Character Sequence

The program below reads a series of characters till the user presses Ente key

#include <stdio.h>
int main()
{
   char ch;
   char word[10];
   int i=0;
   printf("enter characters. End with pressing enter: ");
   while (1){
      ch = getchar();
      word[i] = ch;
      if (ch=='\n')
         break;
      i++;
   }
   printf("\nYou entered word : %s", word);
   return 0;
}

Output

enter characters. End with pressing enter: Hello

You entered word : Hello

String Input

There is also a %s format specifier that reads a series of characters into a char array.

Example: String Input Using scanf()

The following program accepts the string input from the keyboard −

#include <stdio.h>
int main()
{
   char name[20];
   printf("Enter your name: ");
   scanf("%s", name);
   printf("You entered the name : %s", name);
   return 0;
}

Output

Enter your name: Ravikant
You entered the name : Ravikant

However, as mentioned earlier, C uses the white space (blank space) as the delimiter character. Hence, if you try to input a string that contains a blank space, only the characters before the space are stored as the value.

Enter your name: Ravikant Soni
You entered the name : Ravikant

The gets() function overcomes this limitation. It is an unformatted string input function. All the characters till you press Enter are stored in the variable.

Example: String Input Using gets()

#include <stdio.h>
int main()
{
   char name[20];
   printf("Enter your name: ");
   gets(name);
   printf("You entered the name : %s", name);
   return 0;
}

Output

Enter your name: Ravikant Soni
You entered the name : Ravikant Soni

User input is an important aspect of an application in C programming. In this chapter, we learned the usage of formatted and unformatted console input functions, scanf(), getchar(), and gets() with different examples.