Data commit

Task/Gotchas/00-META.yaml

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+---
+from: http://rosettacode.org/wiki/Gotchas

Task/Gotchas/00-TASK.txt

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+;Definition
+In programming, a [[wp:Gotcha_(programming)|'''gotcha''']] is a valid construct in a system, program or programming language that works as documented but is counter-intuitive and almost invites mistakes because it is both easy to invoke and unexpected or unreasonable in its outcome.
+
+;Task
+Give an example or examples of common gotchas in your programming language and what, if anything, can be done to defend against it or them without using special tools.
+<br><br>

Task/Gotchas/6502-Assembly/gotchas-1.6502

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+LDA 'J'  ;load the 8-bit value stored at memory address 0x004A into the accumulator.
+OR 3     ;bitwise OR the accumulator with the 8-bit value stored at memory address 0x0003
+
+
+LDA #'7' ;load the ASCII code for the numeral 7 (which is 0x37) into the accumulator.

Task/Gotchas/6502-Assembly/gotchas-10.6502

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+LDA #$20
+STA $30FF
+LDA #$40
+STA $3100
+JMP ($30FF) ;rather than take the high byte from $3100, the high byte is taken from $3000 instead.

Task/Gotchas/6502-Assembly/gotchas-2.6502

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+byte $45  ;this is the literal constant value $45, not "the value stored at memory address 0x0045"

Task/Gotchas/6502-Assembly/gotchas-3.6502

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+INC $2005 ;the intent was to scroll the NES's screen to the right one pixel. That's not gonna happen.
+;What actually happens? Who knows! (I made this mistake once long ago.)

Task/Gotchas/6502-Assembly/gotchas-4.6502

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+LDA #$20
+CMP #$19
+BCS foo  ;this branch is always taken, since #$20 >= #$19. If this were any other CPU this branch would never be taken!

Task/Gotchas/6502-Assembly/gotchas-5.6502

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+LDA #8
+SEC
+SBC #4 ;eight minus four

Task/Gotchas/6502-Assembly/gotchas-6.6502

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+CLC
+ADC ___ ;your operand/addressing mode of choice goes here

Task/Gotchas/6502-Assembly/gotchas-7.6502

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+LDX #$FF
+LDA $80,X ;loads from address $007F, not $017F

Task/Gotchas/6502-Assembly/gotchas-8.6502

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+LDA $2080,X ;loads from the correct address regardless of the value of X.

Task/Gotchas/6502-Assembly/gotchas-9.6502

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+LDA #$20
+STA $3000
+LDA #$40
+STA $3001
+JMP ($3000) ;evaluates to JMP $4020

Task/Gotchas/68000-Assembly/gotchas-1.68000

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+MOVE.L $12345678,D0  ;move the 32-bit value stored at memory address $12345678 into D0
+MOVE.L #$12345678,D0 ;load the D0 register with the constant value $12345678

Task/Gotchas/68000-Assembly/gotchas-2.68000

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+DC.B $45  ;this is the literal constant value $45, not "the value stored at memory address 0x0045"

Task/Gotchas/68000-Assembly/gotchas-3.68000

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+MOVEA.L #$A04000,A0  ;load the address $A04000 into A0
+MOVE.L A0,D0         ;move the quantity $A04000 into D0
+MOVE.B (A0),D0       ;get the 8-bit value stored at memory address $A04000, and store it into the lowest byte of D0.
+MOVE.W (4,A0),D1     ;get the 16-bit value stored at memory address $A04004, and store it into the low word of D1.

Task/Gotchas/68000-Assembly/gotchas-4.68000

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+LEA $A04000,A0  ;effectively MOVEA.L #$A04000,A0
+LEA (4,A0),A0   ;effectively ADDA.L #4,A0

Task/Gotchas/68000-Assembly/gotchas-5.68000

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+MOVE.L #$12345678,D0 ;set the entire register to a known value for demonstration purposes.
+
+MOVE.W #$7FFF,D0 ;D0 = $12347FFF
+ADD.W #1,D0      ;D0 = $12348000
+TRAPV ;the above operation set the overflow flag, so this instruction will call the signed overflow handler.
+;Even though the entire register didn't overflow, the portion we were operating on did, so that counts.

Task/Gotchas/68000-Assembly/gotchas-6.68000

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+MOVE.B #16-1,D0 ;loop 16 times
+forloop:
+; loop body goes here
+DBRA D0,forloop

Task/Gotchas/68000-Assembly/gotchas-7.68000

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+MOVEA.W #$8000,A0 ;MOVEA.L #$FF8000,A0
+MOVEA.W #$7FFF,A0 ;MOVEA.L #$007FFF,A0

Task/Gotchas/68000-Assembly/gotchas-8.68000

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+MOVE.W #$FF,D0   ;MOVE.W #$00FF,D0
+MOVE.L #$8000,D2 ;MOVE.L #$00008000,D2

Task/Gotchas/68000-Assembly/gotchas-9.68000

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+MOVE.W #$FF,D0
+EXT.W D0 ;D0 = $????FFFF
+
+MOVE.L #$8000,D1
+EXT.L D1 ;D1 = $FFFF8000

Task/Gotchas/C/gotchas-1.c

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+if(a=b){} //assigns to "a" the value of "b". Then, if "a" is nonzero, the code in the curly braces is run.
+
+if(a==b){} //runs the code in the curly braces if and only if the value of "a" equals the value of "b".

Task/Gotchas/C/gotchas-10.c

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+int main()
+{
+int x = 3;
+int y = 5;
+int z = 7;
+printf("%d %d %d %x %x",x,y,z);  //on an Intel cpu the first %x reveals %%ebp and the second reveals the return address.)
+}

Task/Gotchas/C/gotchas-2.c

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+int foo[4] = {4,8,12,16};

Task/Gotchas/C/gotchas-3.c

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+int foo[4] = {4,8,12,16};
+int x = foo[0];  //x = 4
+int y = foo[3];  //y = 16
+int z = foo[4];  //z = ?????????

Task/Gotchas/C/gotchas-4.c

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+int foo()
+{
+char bar[20];
+return sizeof(bar);
+}

Task/Gotchas/C/gotchas-5.c

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+int foo()
+{
+#define size_of_bar 20  //the sizeof operator is the same as doing this essentially.
+
+char bar[size_of_bar];
+return size_of_bar;
+}

Task/Gotchas/C/gotchas-6.c

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+int gotcha(char bar[])
+{
+return sizeof(bar);
+}

Task/Gotchas/C/gotchas-7.c

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+myArray[40];
+int x = gotcha(myArray);

Task/Gotchas/C/gotchas-8.c

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+myArray[40];
+int x = gotcha(&myArray[0]);

Task/Gotchas/C/gotchas-9.c

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+int foo(char buf[],int length){}
+
+int main()
+{
+char myArray[30];
+int j = foo(myArray,sizeof(myArray)); //passes 30 as the length parameter.
+}

Task/Gotchas/J/gotchas-1.j

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+   ex1=: 1 2 3 4 5
+   ex2=: '1 2 3 4 5'
+   ex1
+1 2 3 4 5
+   ex2
+1 2 3 4 5
+   10+ex1
+11 12 13 14 15
+   10+ex2
+|domain error

Task/Gotchas/J/gotchas-2.j

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+   1 2 3 + 4 5 6
+5 7 9
+   +/ 1 2 3 + 4 5 6
+21
+   1 2 3 +/@:+ 4 5 6
+21
+   1 2 3 +/@+ 4 5 6
+5 7 9

Task/Gotchas/J/gotchas-3.j

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+   10 100 1000 +/ .*1 2 3  NB. vector multiplication
+3210
+   10 100 1000 +/.*1 2 3   NB. complex conjugate of signum of right argument grouped by the left argument
+1
+1
+1

Task/Gotchas/J/gotchas-4.j

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+   A=: 2 3 5 7 11    NB. legal
+   B=: 999           NB. legal
+   B 0} A            NB. functional array update (does not change A)
+   NB. the use of a single brace to denote indexing might also confuse people
+999 3 5 7 11
+   0} A              NB. legal
+2
+   999 0} A          NB. not legal
+|rank error
+   (999)0} A         NB. what was intended
+999 3 5 7 11

Task/Gotchas/J/gotchas-5.j

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+   0x100   NB. 0*e^100 where e is the natural logarithm base 2.71828...
+0
+   0xff
+|ill-formed number
+   16b100 16bff
+256 255
+   8ad90   NB. 8 on the complex plane at an angle of 90 degrees from the real axis
+0j8
+   -100   NB. two tokens
+_100
+   _100+10   NB. three tokens
+_90
+   -100+10   NB. four tokens
+_110

Task/Gotchas/J/gotchas-6.j

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+   1+2*3   NB. processed right-to-left
+7
+   2*3+1   NB. processed right-to-left
+8

Task/Gotchas/MIPS-Assembly/gotchas-1.mips

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+move $t0,$zero   ;load 0 into $t0
+beqz $t0,myLabel ;branch if $t0 equals 0
+addiu $t0,1      ;add 1 to $t0. This happens during the branch, even though the program counter never reaches this instruction.

Task/Gotchas/MIPS-Assembly/gotchas-2.mips

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+la $a0,0xDEADBEEF
+lw $t0,($a0) ;load the 32-bit value at memory address 0xDEADBEEF
+addiu $t0,5  ;5 is actually added BEFORE the register has gotten its new value from the memory load above. It will be clobbered.

Task/Gotchas/Perl/gotchas.pl

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+sub array1 { return @{ [ 1, 2, 3 ] } }
+sub  list1 { return qw{ 1 2 3 }      }
+
+# both print '3', but why exactly?
+say scalar array1();
+say scalar  list1();
+
+sub array2 { return @{ [ 3, 2, 1 ] } }
+sub  list2 { return qw{ 3 2 1 }      }
+
+say scalar array2(); # prints '3', number of elements in array
+say scalar  list2(); # prints '1', last item in list

Task/Gotchas/Phix/gotchas.phix

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+(phixonline)-->
+ <span style="color: #008080;">forward</span> <span style="color: #008080;">procedure</span> <span style="color: #000000;">p</span><span style="color: #0000FF;">()</span>
+ <span style="color: #000000;">p</span><span style="color: #0000FF;">()</span>
+ <span style="color: #008080;">function</span> <span style="color: #000000;">f</span><span style="color: #0000FF;">(</span><span style="color: #004080;">object</span> <span style="color: #000000;">o</span><span style="color: #0000FF;">)</span> <span style="color: #008080;">return</span> <span style="color: #000000;">o</span> <span style="color: #008080;">end</span> <span style="color: #008080;">function</span>
+ <span style="color: #008080;">constant</span> <span style="color: #000000;">hello</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">f</span><span style="color: #0000FF;">(</span><span style="color: #008000;">"hello"</span><span style="color: #0000FF;">)</span>
+ <span style="color: #008080;">procedure</span> <span style="color: #000000;">p</span><span style="color: #0000FF;">()</span>
+     <span style="color: #0000FF;">?</span><span style="color: #000000;">hello</span>  <span style="color: #000080;font-style:italic;">-- fatal error: hello has not been assigned a value</span>
+ <span style="color: #008080;">end</span> <span style="color: #008080;">procedure</span>
+<!--

Task/Gotchas/Raku/gotchas-1.raku

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+say 123 ~ 456; # join two integers together
+say "12" + "5.7"; # add two numeric strings together
+say .sqrt for <4 6 8>; # take the square root of several allomorphic numerics

Task/Gotchas/Raku/gotchas-2.raku

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+say my $bag = <a b a c a b d>.Bag;
+say $bag{'a'}; # a count?
+say $bag< a >; # another way

Task/Gotchas/Raku/gotchas-3.raku

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+say my $bag = (1, '1', '1', <1 1 1>).Bag;
+say $bag{ 1 }; # how many 1s?
+say $bag{'1'}; # wait, how many?
+say $bag< 1 >; # WAT
+dd $bag; # The different numeric types LOOK the same, but are different types behind the scenes

Task/Gotchas/Raku/gotchas-4.raku

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+.say for (1,2,3), (4,5,6);

Task/Gotchas/Raku/gotchas-5.raku

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+.say for (1,2,3);

Task/Gotchas/Raku/gotchas-6.raku

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+.say for (1,2,3),;

Task/Gotchas/Raku/gotchas-7.raku

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+.say for flat (1,2,3), (4,5,6);

Task/Gotchas/Wren/gotchas.wren

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+class Rectangle {
+    construct new(width, height) {
+        // Create two fields.
+        _width = width
+        _height = height
+    }
+
+    area {
+       // Here we mis-spell _width.
+       return _widht * _height
+    }
+
+    isSquare {
+        // We inadvertently use '=' rather than '=='.
+        // This sets _width to _height and will always return true
+        // because any number (even 0) is considered 'truthy' in Wren.
+        if (_width = _height) return true
+        return false
+    }
+
+    diagonal {
+        // We use 'sqrt' instead of the Math.sqrt method.
+        // The compiler thinks this is an instance method of Rectangle
+        // which will be defined later.
+        return sqrt(_width * _width + _height * _height)
+    }
+}
+
+var rect = Rectangle.new(80, 100)
+System.print(rect.isSquare) // returns true which it isn't!
+System.print(rect.area)     // runtime error: Null does not implement *(_)
+System.print(rect.diagonal) // runtime error (if previous line commented out)
+                            // Rectangle does not implement 'sqrt(_)'

Task/Gotchas/X86-Assembly/gotchas.x86

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+label:
+;loop body goes here
+DEC ECX
+JNZ label

Task/Gotchas/Z80-Assembly/gotchas-1.z80

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+LD HL,(&C000)  ;load the word at address &C000 into HL
+LD A,(HL)      ;treating the value in HL as a memory address, load the byte at that address into A.
+EX (SP),HL     ;exchange HL with the top two bytes of the stack.
+
+JP (HL)        ;set the program counter equal to HL. Nothing is loaded from memory pointed to by HL.

Task/Gotchas/Z80-Assembly/gotchas-2.z80

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+ld a,&46
+ld bc,&0734
+out (C),a ;write &46 to port &0734 if the ports are 16-bit. Otherwise, it writes to port &34.

Task/Gotchas/Z80-Assembly/gotchas-3.z80

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+EI  ;RETI doesn't enable interrupts on this Z80.
+RET