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POWhy odd * even does not return exact value in C?
 

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  1. POWhy odd * even does not return exact value in C?
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    <p>After finishing my project (The Calculator using PIC18 with Keypad 4x4, LCD and 4 LEDs), I faced wierd issue in the multiplication operation</p> <p>When multiply odd (> 1) * even (> 0) the result is not exactly correct, for example, 3 * 4 = 11.99999</p> <p>====================</p> <p>Here I would try to descripe the situation:</p> <p>The user press a button of the keypad, then the program takes the char from the keypad and stores it into char[] (A char at a time) so finally we have two char[] Operand[13] and Operand[13]. After Entering Operand1 and Operand2 the program converts it to floating number using atof() function then apply some operation ('+', '-', '*' or '/') and stores the result in int Result. Finally, the program convert this int to char[] using FloatToStr() function and display it on the LCD.</p> <p>=====================</p> <p>Here is my code: (Sorry, it's kinda long but I want you help me solving this issue)</p> <pre><code>unsigned char Key[4][4] = {{'1','2','3','4'}, // Keypad 4x4 Matrix {'5','6','7','8'}, {'9','0','.','E'}, {'+','-','*','/'}}; unsigned char Operand_String[2][13], Operation_String, Result_String[15]; // Where we store the operand, operation &amp; Result as chars unsigned char Row, Column; // Row &amp; Column determine the location of Key in the Keypad matrix that the user pressed unsigned char State; // 1: Operand1 - 2: Operand2 - 3: Operation - 4: Result unsigned int EntryCount; // Counter for counts entries on LCD unsigned int Operand1_length, Operand2_length; // Lengths of Operand1 &amp; Operand2 bit Direction; // 1: right - 0: left bit Operand1_Sign, Operand2_Sign; // 1: negative - 0: positive signed float Operand1, Operand2, Result; // Fields where we store Operand1, Operand2 and Result typedef unsigned int boolean; // ** DEFINING ** #define false 0 // ** BOOLEAN ** #define true (!false) // ** TYPE ** boolean ButtonIsPressed = false; // Flag that indicates whether a button is pressed or no button is pressed boolean FloatingPointIncluded = false; // Flag that indicates if there is a floating point in the LCD or not boolean Operand1_signed = false; // Flag that indicates if Operand1 has a sign or not boolean Operand2_signed = false; // Flag that indicates if Operand2 has a sign or not // ***** Lcd pinout settings ***** sbit LCD_RS at RD4_bit; sbit LCD_EN at RD5_bit; sbit LCD_D7 at RD3_bit; sbit LCD_D6 at RD2_bit; sbit LCD_D5 at RD1_bit; sbit LCD_D4 at RD0_bit; // ***** Pin Direction ***** sbit LCD_RS_Direction at TRISD4_bit; sbit LCD_EN_Direction at TRISD5_bit; sbit LCD_D7_Direction at TRISD3_bit; sbit LCD_D6_Direction at TRISD2_bit; sbit LCD_D5_Direction at TRISD1_bit; sbit LCD_D4_Direction at TRISD0_bit; // ***** End LCD module connections ***** // ***** Method that determines no. of Row and Column of keypad matrix where the user presses the button of that location on the keypad ***** void scan_key() { unsigned char portValue[4][4] = {{0b11101110, 0b11101101, 0b11101011, 0b11100111}, {0b11011110, 0b11011101, 0b11011011, 0b11010111}, {0b10111110, 0b10111101, 0b10111011, 0b10110111}, {0b01111110, 0b01111101, 0b01111011, 0b01110111}}; unsigned char temp[4] = {0B11111110, 0B11111101, 0B11111011, 0B11110111}; unsigned int loop_col = 1; unsigned int loop_row = 1; for (loop_col = 1; loop_col &lt; 5; loop_col++) { PORTB = temp[loop_col - 1]; for (loop_row = 1; loop_row &lt; 5; loop_row++) { if ( PORTB == portValue[loop_row - 1][loop_col - 1]) { Row = loop_row; Column = loop_col; return; } } PORTB = 0B11110000; } } // ***** Interrupt Service Routine (ISR) ***** void interrupt() org 0x08 { if (INTCON.TMR0IF) // Timer0 Interrupt { scan_key(); //Delay_ms(40); INTCON.RBIE = 1; INTCON.TMR0IF = 0; } else if (INTCON.RBIF) // PORTB Interrupt { INTCON.TMR0IF = 1; INTCON.TMR0IE = 1; INTCON.RBIE = 0; INTCON.RBIF = 0; ButtonIsPressed = true; } } // ***** Method that calculates the result of the arithmatic Operation ***** float CalculateResult() { Operand1 = atof(Operand_String[0]); Operand2 = atof(Operand_String[1]); if(Operand1_sign == 1) Operand1 = - Operand1; if(Operand2_sign == 1) Operand2 = - Operand2; switch(Operation_String) { case '+': Result = Operand1 + Operand2; break; case '-': Result = Operand1 - Operand2; break; case '*': Result = Operand1 * Operand2; break; case '/': Result = Operand1 / Operand2; break; } return Result; } // ***** Method that makes LEDs blink ***** void LEDsBlink(int iteration) { char PORTA_Temp; int i; PORTA_Temp = PORTA; if(iteration &lt; 0) { PORTA = ~PORTA; Delay_ms(200); } else { for(i = 0; i &lt; iteration; i++) { PORTA = 0x0F; Delay_ms(50); PORTA = 0x00; Delay_ms(50); } PORTA = PORTA_Temp; } } // ***** Method that resets the variables ***** void Reset_Values() { EntryCount = 0; State = 1; Row = 0; Column = 0; Direction = 0; Operand1_Sign = 0; Operand2_Sign = 0; Operand1 = 0; Operand2 = 0; Result = 0; memset(Operand_String, 0, 2 * 13); memset(Result_String, 0, 15); ButtonIsPressed = false; Operand1_signed = false; Operand2_signed = false; PORTA = 0x0F; // Turn on the 4 LEDs } void main() { // ***** Initializations of PIC18F4550 ***** TRISA = 0; // Configure the 4 LEDs as output PORTA = 0x0F; // Turn on the 4 LEDs TRISB = 0xF0; // Configure RB0 ~ RB3 as outputs &amp; RB4 ~ RB7 as inputs PORTB = 0xF0; // Assign 0xF0 OSCCON = 0x63; // 4 MHz - Internal oscillator INTCON2.B7 = 0; // PORTB pull-ups are enabled by individual port latch values INTCON.RBIF = 0; // Reset the interrupt flag INTCON.RBIE = 1; // RB Change interrupt ON INTCON.GIE = 1; // Global interrupts ON ADCON1 = 0b00001111; // Digital inputs // ***** Initializations of LCD ***** Lcd_Init(); // Initialize LCD Lcd_Cmd(_LCD_CLEAR); // Clear LCD Lcd_Cmd(_LCD_CURSOR_OFF); // Cursor off Lcd_Out(1, 2, "Fouad Al-Malki"); // ** WELCOME ** Lcd_Out(2, 4, "CALCULATOR"); // ** MESSAGE ** Delay_ms(2000); // delay for 2 seconds Lcd_Cmd(_LCD_CLEAR); // Clear DCD Lcd_Out(1, 4, "Operand1: "); // Write "Operand1: " at first row Lcd_Cmd(_LCD_SECOND_ROW); // Make current position at second row Lcd_Cmd(_LCD_BLINK_CURSOR_ON); // Cursor on // ***** Reset all values ***** Reset_Values(); while(1) { // ***** Control of LCD ***** if(ButtonIsPressed) { if(State == 1) { if(Key[row-1][column-1] != 'E') { if((EntryCount &lt;= 13 &amp;&amp; Key[row-1][column-1] != '+' &amp;&amp; Key[row-1][column-1] != '-' &amp;&amp; Key[row-1][column-1] != '/' &amp;&amp; Key[row-1][column-1] != '*')) { if((Key[row-1][column-1] == '.' &amp;&amp; !FloatingPointIncluded &amp;&amp; EntryCount &gt; 0) || Key[row-1][column-1] != '.') { Lcd_Chr_Cp(Key[row-1][column-1]); Operand_String[0][EntryCount] = Key[row-1][column-1]; EntryCount++; if(Key[row-1][column-1] == '.') FloatingPointIncluded = true; } else LEDsBlink(3); } else if(!Operand1_signed &amp;&amp; EntryCount == 0 &amp;&amp; (Key[row-1][column-1] == '+' || Key[row-1][column-1] == '-')) { if(Key[row-1][column-1] == '+') Operand1_Sign = 0; else if(Key[row-1][column-1] == '-') Operand1_Sign = 1; Lcd_Chr_Cp(Key[row-1][column-1]); Operand1_signed = true; } else LEDsBlink(3); } else if(Key[row-1][column-1] == 'E' &amp;&amp; EntryCount != 0) { State = 2; FloatingPointIncluded = false; Operand1_length = EntryCount; EntryCount = 0; Row = 0; Column = 0; Lcd_Cmd(_LCD_CLEAR); // Clear display Lcd_Out(1, 4, "Operand2: "); Lcd_Cmd(_LCD_SECOND_ROW); Lcd_Cmd(_LCD_BLINK_CURSOR_ON); // Cursor on if(PORTA.B0 == 1) { PORTA = 0x08; } } else LEDsBlink(3); } else if(State == 2) { if(Key[row-1][column-1] != 'E') { if((EntryCount &lt;= 13 &amp;&amp; Key[row-1][column-1] != '+' &amp;&amp; Key[row-1][column-1] != '-' &amp;&amp; Key[row-1][column-1] != '/' &amp;&amp; Key[row-1][column-1] != '*')) { if((Key[row-1][column-1] == '.' &amp;&amp; !FloatingPointIncluded &amp;&amp; EntryCount &gt; 0) || Key[row-1][column-1] != '.') { Lcd_Chr_Cp(Key[row-1][column-1]); Operand_String[1][EntryCount] = Key[row-1][column-1]; EntryCount++; if(Key[row-1][column-1] == '.') FloatingPointIncluded = true; } else LEDsBlink(3); } else if(!Operand2_signed &amp;&amp; EntryCount == 0 &amp;&amp; (Key[row-1][column-1] == '+' || Key[row-1][column-1] == '-')) { if(Key[row-1][column-1] == '+') Operand2_Sign = 0; else if(Key[row-1][column-1] == '-') Operand2_Sign = 1; Lcd_Chr_Cp(Key[row-1][column-1]); Operand2_signed = true; } else LEDsBlink(3); } else if(Key[row-1][column-1] == 'E' &amp;&amp; EntryCount != 0) { State = 3; FloatingPointIncluded = false; Operand2_length = EntryCount; EntryCount = 0; Row = 0; Column = 0; Lcd_Cmd(_LCD_CLEAR); // Clear display Lcd_Out(1, 4, "Operation: "); Lcd_Cmd(_LCD_SECOND_ROW); Lcd_Cmd(_LCD_BLINK_CURSOR_ON); // Cursor on if(PORTA.B0 == 1) { PORTA = PORTA &lt;&lt; 1; Direction = 0; } } else LEDsBlink(3); } else if(State == 3) { if(Key[row-1][column-1] != 'E') { if(EntryCount == 0 &amp;&amp; (Key[row-1][column-1] == '+' || Key[row-1][column-1] == '-' || Key[row-1][column-1] == '/' || Key[row-1][column-1] == '*')) { Lcd_Chr_Cp(Key[row-1][column-1]); Operation_String = Key[row-1][column-1]; EntryCount++; } else LEDsBlink(3); } else if(Key[row-1][column-1] == 'E' &amp;&amp; EntryCount != 0) { State = 4; FloatingPointIncluded = false; EntryCount = 0; Row = 0; Column = 0; Lcd_Cmd(_LCD_CLEAR); // Clear display Lcd_Out(1, 3, "The Result: "); Lcd_Cmd(_LCD_SECOND_ROW); Lcd_Cmd(_LCD_CURSOR_OFF); // Cursor off Result = CalculateResult(); //printOut(Result, "/*rn"); FloatToStr(Result, Result_String); Lcd_Out(2,1,Result_String); PORTA = 0x0F; } else LEDsBlink(3); } else if(State == 4) { if(Key[row-1][column-1] == 'E') { Reset_Values(); Lcd_Cmd(_LCD_CLEAR); // Clear display Lcd_Out(1, 4, "Operand1: "); Lcd_Cmd(_LCD_SECOND_ROW); Lcd_Cmd(_LCD_BLINK_CURSOR_ON); // Cursor on PORTA == 0x0F; } } ButtonIsPressed = false; } // ***** Control of LEDs ***** else { if(State == 1) { if(PORTA == 0x0F) PORTA = 0x01; Delay_ms(300); if(PORTA &lt; 0x08) PORTA = PORTA &lt;&lt; 1; if(PORTA == 0x08) { Delay_ms(300); PORTA = 0x01; } } if(State == 2) { Delay_ms(300); PORTA = PORTA &gt;&gt; 1; if(STATUS.C == 1) { PORTA = 0x08; STATUS.C = 0; } } if(State == 3) { Delay_ms(300); if (Direction == 0) { PORTA = PORTA &gt;&gt; 1; if (PORTA.B0 == 1) Direction = 1; } else { PORTA = PORTA &lt;&lt; 1; if (PORTA.B3 == 1) Direction = 0; } } if(State == 4) { LEDsBlink(-1); } } } } </code></pre>
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      1. POWhy odd * even does not return exact value in C?
      1. USClaudiu
      1. VTFavorite
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      1. POWhy odd * even does not return exact value in C?
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      1. VTDownMod
    3. VO
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      1. VTUpMod
    1. COPlease reduce the code sample to a [minimal test case](http://sscce.org/) so we don't have to wade through extraneous code.
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      1. POWhy odd * even does not return exact value in C?
      1. USoutis
    2. COWhere in your code do you see that as a result? floating point multiplication should work fine for the example you quote above. You should whittle down your code to the smallest possible that reproduces what you see.
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      1. POWhy odd * even does not return exact value in C?
      1. USJim Buck
    3. COThis looks *almost* like C, but it isn't; C doesn't have bits, sbits, "at X" declaration modifiers, binary numeric literals, or global variables that you can use without declaring them at all.
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      1. POWhy odd * even does not return exact value in C?
      1. USzwol
 

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