Integer division in awk

Question

I want to divide two numbers in awk, using integer division, i.e truncating the result. For example

k = 3 / 2
print k

should print 1

According to the manual,

Division; because all numbers in awk are floating-point numbers, the result is not rounded to an integer

Is there any workaround to get an integer value?

The reason is that I want to get the middle element of an array with integer indexes [0 to num-1].

Solution 1

Use the int function to get the integer part of the result, truncated toward 0. This produces the nearest integer to the result, located between the result and 0. For example, int(3/2) is 1, int(-3/2) is -1.

Source: The AWK Manual - Numeric Functions

Solution 2

In simple cases, you can safely use int() which truncates towards zero¹:

awk 'BEGIN { print int(3 / 2) }'    # prints 1
gawk 'BEGIN { print int(-3 / 2) }'  # prints -1; not guaranteed in POSIX awk

Keep in mind that awk always uses double-precision floating point numbers² and floating-point arithmetic³, though. The only way you can get integers and integer arithmetic is to use external tools, e.g. the standard expr utility:

awk 'BEGIN { "expr 3 / 2" | getline result; print result; }'    # prints 1

This is really awkward, long, slow, … but safe and portable.

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^{1 In POSIX awk, truncation to zero is guaranteed only for positive arguments: int(x) — Return the argument truncated to an integer. Truncation shall be toward 0 when x>0. GNU awk (gawk) uses truncation toward zero even for negative numbers: int(x) — Return the nearest integer to x, located between x and zero and truncated toward zero. For example, int(3) is 3, int(3.9) is 3, int(-3.9) is -3, and int(-3) is -3 as well.
2 Numeric expressions are specified as double-precision floats in Expressions in awk in POSIX.
3 All arithmetic shall follow the semantics of floating-point arithmetic as specified by the ISO C standard (see Concepts Derived from the ISO C Standard). — POSIX awk: Arithmetic functions}

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If you choose to use floats, you should know about their quirks and be ready to spot them and avoid related bugs. Several scary examples:

• Unrepresentable numbers:

  awk 'BEGIN { x = 0.875; y = 0.425; printf("%0.17g, %0.17g\n", x, y) }'
  # prints 0.875, 0.42499999999999999
  

• Round-off errors accumulation:

  awk 'BEGIN{s=0; for(i=1;i<=100000;i++)s+=0.3; printf("%.10f, %d\n",s,int(s))}'
  # prints 29999.9999999506, 29999
  

• Round-off errors ruin comparisons:

  awk 'BEGIN { print (0.1 + 12.2 == 12.3) }'    # prints 0
  

• Precision decreases with magnitude, causing infinite loops:

  awk 'BEGIN { for (i=10^16; i<10^16+5; i++) printf("%d\n", i) }'
  # prints 10000000000000000 infinitely many times
  

Read more on how floats work:

1. Stack Overflow tags floating-point wiki

2. Wikipedia article Floating point

3. GNU awk arbitrary precision arithmetic – contains both info on the specific implementation and general knowledge

Solution 3

Safe and quick awk integer division can be done with:

q=(n-n%d)/d+(n<0)