"Break" a number

Question

Origins

tl;dw (too lazy, didn't write): thought of it right before I fell asleep

Challenge

Breaking a number is defined as the following steps:

• Take a number or a parseable data type as input in any allowed way.
• Duplicate the first digit (64 -> 664)
• Subtract 4 from the last digit (664 -> 660)
• If the subtraction gets a negative last digit, move the negative sign (-) to the front of the number (73 -> 773 -> -771)
• If the original number was negative, and the last digit becomes negative, remove all negative signs entirely. (-44441 -> 444443)
• Output the number in any allowed way.

Here are a few test cases:

64 -> 660
73 -> -771
thisisnotanumber -> (handle the situation)
432188 -> 4432184
-83213 -> 883211 (two negative signs turn into a positive)
0 -> -4 (0 -> 00 -> 0-4 -> -04 -> -4)

Clarifications

Standard loopholes are disallowed.

By "parseable data type", I mean any data type (String, Float, etc) that can be converted into the number type your program/function uses (Integer, Number, etc).

Your answer can be a function or a program. In both cases, provide an explanation of the code.

If it is a function, give a usage of it.

Test cases (or online interpreter) would be a nice plus to your answer.

Preceding 0s (-04) aren't allowed.

Behavior should be exactly replicated from the test cases above.

Answer 1

Batch, 105 bytes

@set/an=%1,n*=s=n^>^>31^|1
@set n=%n:~0,1%%n%
@cmd/cset/ad=n%%10,e=d-4,t=e^^^>^^^>31^^^|1,(n-d+e*t)*s*t

Alternatively the first line can be @set/as=%1^>^>31^|1,n=%1*s, also for 105 bytes. Explanation (without quoting metacharacters):

set /a n = %1               Get parameter
set /a s = n >> 31 | 1      Get sign of parameter
set /a n *= s               Get absolute value of parameter
set n=%n:~0,1%%n%           Duplicate the first digit
cmd /c                      Cause the final result to be printed
set /a d = n % 10           Get last digit
set /a e = d - 4            Calculate new last digit
set /a t = e >> 31 | 1      Get sign of new last digit
set /a e *= t               Get absolute value of new last digit
set /a (n - d + e) * s * t  Replace last digit and correct sign of result

Answer 2

Ruby, 88 87 + 3 (-n flag) = 90 bytes

Regex approach that reads lines from STDIN. Probably better solved with Perl if it's just regex like this?

Returns a nonsense NoMethodError if the input is not a number, which costs 14 bytes. If there's no need to worry about invalid inputs like that (the spec implied it needed handling) then the first line of the code can be removed.

-2 bytes from @Dada

+(~/^-?\d+$/)
sub(/\d/){$&*2}
sub(/.$/){$&.to_i-4}
p sub(/(-)?(.+)-/){$1?$2:?-+$2}.to_i

Answer 3

Pyth, 31 bytes

*i++hJja0QTPJa0K-eJ4T^_1x<K0<Q0

Naive implementation of the question.

Test suite.

Answer 4

Python, 79 78 Bytes

lambda x:"+-"[(int(x)<0)!=(int(x[-1])-4<0)]+x[1]+x[1:-1]+str(int(x[-1])-4)[-1]

anonymous lambda function, does all of the four parts, then adds them together. Expects input as a string with explicit sign e.g. "+64", output is of the same form. Throws ValueError if the string cannot be converted to an int.

Answer 5

C, 191 bytes

f(char*n){if(!isdigit(*n)&!isdigit(n[1]))return 0;char*m;strcpy(m+1,n);*m=m[1];if(*n==45)m[1]=m[2];int a=strlen(n);if(m[a]<52)m[a]=100-m[a],m--,*m=45;else m[a]-=4;return atoi(m[1]^45?m:m+2);}

Ungolfed with explanations:

f(char*n){
    if(!isdigit(*n)&!isdigit(n[1])) /* if first two chars aren't digits
                                     * if it was only the first one, negative numbers wouldn't pass
                                     * if it was only the second one, 0 wouldn't pass */
        return 0;        // handling
    char*m;
    strcpy(m+1,n);        // copy leaving space for doubling the number
    *m=m[1];              // double the first digit or the minus sign
    if(*n==45)            // if negative
        m[1]=m[2];        // double the first digit
    int a=strlen(n);      /* length of the nunber
                           * it doesn't let me save 2 bytes, i dunno why. */
    if(m[a]<52)           // if the last digit is less than 4
        m[a]=100-m[a],    // subtract 4
        m--,              // leave space for minus sign
        *m=45;            // put minus sign
    else
        m[a]-=4;          // subtract 4 from the last digit
    return atoi(m[1]^45?m:m+2); /* if there are two minus signs, skip them,
                                 * turn the string into an integer, and return */
}