More on Format Specifiers

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Minimum Field Width

Suppose you want the program to display output that occupies a minimum number of spaces on the screen. You can achieve this by adding an integer value after the percent sign of a format specifier.

For example, if you want to display an integer using a minimum of 8 spaces, you'd write %8d in your printf statement.

This example gives a demonstration:

#include <stdio.h>

int main() {
  int x = 123;
	
  printf("Printing 123 using %%0d displays %0d\n", x);
  printf("Printing 123 using %%1d displays %1d\n", x);
  printf("Printing 123 using %%2d displays %2d\n", x);
  printf("Printing 123 using %%3d displays %3d\n", x);
  printf("Printing 123 using %%4d displays %4d\n", x);
  printf("Printing 123 using %%5d displays %5d\n", x);
  printf("Printing 123 using %%6d displays %6d\n", x);
  printf("Printing 123 using %%7d displays %7d\n", x);
  printf("Printing 123 using %%8d displays %8d\n", x);
  printf("Printing 123 using %%9d displays %9d\n", x);
	
  return 0;
}

Output:

Notice that in the first 4 cases, 123 is displayed in the same way as when you normally use %d. Why? Simple - the number of spaces on the screen that 123 can be displayed is greater than or equal to 3.

But also, if you write %09d, the program will display zeros before the number itself. In the above example, it will display:

Printing 123 using %09d displays 000000123

An advantage of using this, is that you can count the minimum field of the number!

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Making It Look Neater

The output from the example above doesn't look very neat does it?! That's because the numbers are aligned to the right of the minimum field. In other words, 1,2 and 3 are the digits in the furthest 3 spaces of the minimum field.

To align your output on the left, you insert a minus sign before the number in the format specifier. But if you do this to the previous example, all the output lines will be the same.

A better example:

#include <stdio.h>

int main() {
  int x = 12;
  int y = 123;
  int z = 12345;
  
  printf("Printing 12 using %%9d \t\t displays %9d\n", x);
  printf("Printing 12 using %%09d \t\t displays %09d\n", x);
  printf("Printing 12 using %%-9d \t\t displays %-9d\n", x);
  printf("Printing 12 using %%-09d \t displays %-09d\n", x);
  
  printf("Printing 123 using %%9d \t\t displays %9d\n", y);
  printf("Printing 123 using %%09d \t displays %09d\n", y);
  printf("Printing 123 using %%-9d \t displays %-9d\n", y);
  printf("Printing 123 using %%-09d \t displays %-09d\n", y);
  
  printf("Printing 12345 using %%9d \t displays %9d\n", z);
  printf("Printing 12345 using %%09d \t displays %09d\n", z);
  printf("Printing 12345 using %%-9d \t displays %-9d\n", z);
  printf("Printing 12345 using %%-09d \t displays %-09d\n", z);
  
  return 0;
}

Output:

\t acts like a standard tab. Notice how it begins with a backslash, \ - just like the newline character.

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More Precision

You can gain more control with the displaying of integers by placing a dot, followed by an integer, after the minimum field specifier. The dot and this integer is known as a PRECISION SPECIFIER.

The integer you add specifies the maximum field width when displaying an integer or string.

If you're using %f, the format specifier for floating point numbers, you can control the number of decimal places that is displayed (which is 6 by default). How? By using the precision specifier. This time, the number after the dot is the number of decimal places. The number before the dot is still the minimum field width.

This example should help clarify things:

#include <stdio.h>

int main() {
  float x = 3.141592;
  
  printf("Printing 3.141592 using %%f \t displays %f\n", x);
  printf("Printing 3.141592 using %%1.1f \t displays %1.1f\n", x);
  printf("Printing 3.141592 using %%1.2f \t displays %1.2f\n", x);
  printf("Printing 3.141592 using %%3.3f \t displays %3.3f\n", x);
  printf("Printing 3.141592 using %%4.4f \t displays %4.4f\n", x);
  printf("Printing 3.141592 using %%4.5f \t displays %4.5f\n", x);
  printf("Printing 3.141592 using %%09.3f   displays %09.3f\n", x);
  printf("Printing 3.141592 using %%-09.3f  displays %-09.3f\n", x);
  printf("Printing 3.141592 using %%9.3f    displays %9.3f\n", x);
  printf("Printing 3.141592 using %%-9.3f   displays %-9.3f\n", x);
  
  return 0;
}

Output:

You may have noticed that if you use a negative value for the minimum width specifier, the output will not be affected by a zero after the minus sign.

Also, in the case for decimal numbers, the decimal point occupies a character space on the screen.

Prev: printf and scanf
Next: Comparision and Logical Operators

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