Bc reduces floating point number

Question

How to truncate a floating point number using bc

For example, if I do

 echo '4.2-1.3' | bc 

which outputs 2.9 , how do I get it to truncate / use the floor to get 2

Answer 1

Use the / operator.

 echo '(4.2-1.3) / 1' | bc 

Answer 2

Splitting by 1 works fine if scale is 0 (for example, if you start bc with bc and don't change scale ), but fail if scale positive (for example, if you start bc with bc -l or increase scale ). (See Decryption below.) For a general solution, use the trunc function, as shown below:
define trunc(x) { auto s; s=scale; scale=0; x=x/1; scale=s; return x }

A transcript that illustrates how dividing by 1 itself fails in the case of bc -l , but how the trunc function works fine when truncated to zero:

 > bc -l bc 1.06.95 [etc...] for (x=-4; x<4; x+=l(2)) { print x,"\t",x/1,"\n"} -4 -4.00000000000000000000 -3.30685281944005469059 -3.30685281944005469059 -2.61370563888010938118 -2.61370563888010938118 -1.92055845832016407177 -1.92055845832016407177 -1.22741127776021876236 -1.22741127776021876236 -.53426409720027345295 -.53426409720027345295 .15888308335967185646 .15888308335967185646 .85203026391961716587 .85203026391961716587 1.54517744447956247528 1.54517744447956247528 2.23832462503950778469 2.23832462503950778469 2.93147180559945309410 2.93147180559945309410 3.62461898615939840351 3.62461898615939840351 define trunc(x) { auto s; s=scale; scale=0; x=x/1; scale=s; return x } for (x=-4; x<4; x+=l(2)) { print x,"\t",trunc(x),"\n"} -4 -4 -3.30685281944005469059 -3 -2.61370563888010938118 -2 -1.92055845832016407177 -1 -1.22741127776021876236 -1 -.53426409720027345295 0 .15888308335967185646 0 .85203026391961716587 0 1.54517744447956247528 1 2.23832462503950778469 2 2.93147180559945309410 2 3.62461898615939840351 3