Difficulty with mod operator [duplicate]

Question

I would be grateful if somebody could help me with this question.

A C program contains the following declarations and initial assignments.

int i = 8, j = 5; float x = 0.005, y = - 0.01; char c = 'c', d = 'd';

Determine the value of the following expression using values assigned to the variables for the expression:

(i - 3 * j) % ( c + 2 *d)/ (x - y )

I tried this manually first:

( i- 3 * j) % ( c + 2 *d ) / ( x - y) ( 8 - 3*5) % ( 99 + 2 * 100 ) / ( 0.005 - (-0.01) ) ( -7 ) % ( 299 ) / ( 0.015 )

Keeping precedence and associativity in mind, I used the mod operator first:

( 292 ) / ( 0.015 )

Which gave the answer 19466.66.

This does not match with the answer given in the book or when I used this in codeblocks, both of which gave the answer as - 466.6667

The codeblocks program is as below

#include <stdio.h> #include <stdlib.h> int main() { int i = 8,j = 5, c = 'c',d = 'd'; float x = 0.005, y = -0.01, a = 0.0; // a is the float variable to which the value is assigned a = (i-3*j)%(c+2*d)/(x-y); printf("%f\n",a); return 0; }

Try not to use line numbers in your code. We'll just have to delete that, tediously, if we want to run it and test. — tadman
– tadman, Commented Mar 31, 2021 at 5:38
This is an exercise in using operators and ASCII values given in the book. Thanks for the edit. — watercolour man
– watercolour man, Commented Mar 31, 2021 at 5:48
You have an arithmetic error: 0.005 - 0.01 == -0.005, not -0.015. — Nate Eldredge
– Nate Eldredge, Commented Mar 31, 2021 at 5:52
Also your code has y = -0.01 but the problem has y = 0.01; which is it supposed to be? Finally (-7) % 299 is not 292 but -7. — Nate Eldredge
– Nate Eldredge, Commented Mar 31, 2021 at 5:58

tadman · Accepted Answer · 2021-03-31 05:55:57Z

The crux of this is the integer part before division:

(i-3*j)%(c+2*d)

Where that evaluates to:

(8-3*5) % (99 + 2 * 100) (8-15) % (99 + 200) -7 % 299

So now it depends on what definition of modulo is being used by C, as there are several it could be. C interprets this as -7 while other languages always map to the 0 to 298 range.

The rest is just simple division.

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