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Nested if-statements

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• Observation
  • The if-statement:

    if ( C ) { s1; s2; ... }

    give rise to an assembler program with the following structure:

    instructions to perform a compare specified by the condition C branch of FALSE of the condition C to label IfEnd instructions to perform s1 instructions to perform s2 .... IfEnd:

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  • The if-else-statement:

    if ( C ) { s1; s2; ... } else { t1; t2; ... }

    give rise to an assembler program with the following structure:

    instructions to perform a compare specified by the condition C branch of FALSE of the condition C to label Else instructions to perform s1 instructions to perform s2 .... bra IfEnd Else: instructions to perform t1 instructions to perform t2 .... IfEnd:

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  • Note:

    The statements (s1, s2, ... t1, t2, ...) inside the then/else parts can be "translated" into assembler code independently from the if-statement Use this property to handle: Nesting of an if/if-else statement inside another if/if-else statment

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• Nesting of if-statements
  • Example:

    // x = max of a and b when c > 0 // otherwise, x = min of a and b int x, a, b, c; if ( c > 0 ) { // Find maximum if ( a > b ) { x = a; } else { x = b; } } else { // Find minimum if ( a < b ) { x = a; } else { x = b; } }

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• Modular approach
  • Method:

    Work from the outside inwards Ignore the inner section (body of the then and else) of the if-statement at first After coding the outer if-statement, write the code of the statements inside the then part and else part

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• Coding the outer if-statement
  • Example with the then (and else) body stripped:

    if ( c > 0 ) { "then" body stripped } else { "else" body stripped }

  • Corresponding program flow chart:

    +----------------+ FALSE (branch) | test c > 0 | --------+ +----------------+ | | True | V | +-----------------+ | | code for body | | | of "then" part | | +-----------------+ | | | +------------------------------+ (jump over "else" part) | | | | If1_Else: +<-----------------+ | | | V | +-----------------+ | | code for body | | | of "else" part | | +-----------------+ | | | | | If1_End: +<-----------------------------+ | V

  • Corresponding assembler code:

    move.l c,d0 cmp.l #0,d0 Tests: c ?? 0 ble If1_Else Branch when !(c < 0) <=> c <= 0 ..... code for body of "then" part ..... bra If1_End If1_Else: ..... code for body of "else" part ..... If1_End:

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• Next: code the body of the "then" (and "else") part (individually)
  • Body of the "then" part (ignore the outer if-statement !):

    if ( a > b ) { x = a; } else { x = b; }

  • Corresponding program flow chart:

    +----------------+ FALSE (branch) | test a > b | --------+ +----------------+ | | True | V | +-----------------+ | | x = a | | +-----------------+ | | | +------------------------------+ (jump over "else" part) | | | | If2_Else: +<-----------------+ | | | V | +-----------------+ | | x = b | | +-----------------+ | | | | | If2_End: +<-----------------------------+ | V

  • Corresponding assembler code:

    move.l a,d0 cmp.l b,d0 Tests: a ?? b ble If2_Else Branch when !(a > b) <=> a <= b move.l a,d0 move.l d0,x bra If2_End If2_Else: move.l b,d0 move.l d0,x If2_End:

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• Continued....: code the body of the "else" part (individually)
  • Body of the "else" part (ignore the outer if-statement !):

    if ( a < b ) { x = a; } else { x = b; }

  • Corresponding program flow chart:

    +----------------+ FALSE (branch) | test a < b | --------+ +----------------+ | | True | V | +-----------------+ | | x = a | | +-----------------+ | | | +------------------------------+ (jump over "else" part) | | | | If3_Else: +<-----------------+ | | | V | +-----------------+ | | x = b | | +-----------------+ | | | | | If3_End: +<-----------------------------+ | V

  • Corresponding assembler code:

    move.l a,d0 cmp.l b,d0 Tests: a ?? b bge If3_Else Branch when !(a < b ) <=> a >= b move.l a,d0 move.l d0,x bra If3_End If3_Else: move.l b,d0 move.l d0,x If3_End:

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• Finally, put the "then" and "else" parts into their places
  • Originally:

    move.l c,d0 cmp.l #0,d0 Tests: c ?? 0 ble If1_Else Branch when c <= 0 ..... code for body of "then" part ..... bra If1_End If1_Else: ..... code for body of "else" part ..... If1_End:

  • After inserting the code for the "then" body:

    move.l c,d0 cmp.l #0,d0 Tests: c ?? 0 ble If1_Else Branch when c <= 0 ----------------------------------------------------------------------- move.l a,d0 cmp.l b,d0 Tests: a ?? b ble If2_Else Branch when a <= b move.l a,d0 move.l d0,x bra If2_End If2_Else: move.l b,d0 move.l d0,x If2_End: ----------------------------------------------------------------------- bra If1_End If1_Else: ..... code for body of "else" part ..... If1_End:

  • Finally, after inserting the code for the "else" body:

    move.l c,d0 cmp.l #0,d0 Tests: c ?? 0 ble If1_Else Branch when c <= 0 ----------------------------------------------------------------------- move.l a,d0 cmp.l b,d0 Tests: a ?? b ble If2_Else Branch when a <= b move.l a,d0 move.l d0,x bra If2_End If2_Else: move.l b,d0 move.l d0,x If2_End: ----------------------------------------------------------------------- bra If1_End If1_Else: ---------------------------------------------------------------------- move.l a,d0 cmp.l b,d0 Tests: a ?? b bge If3_Else Branch when a >= b move.l a,d0 move.l d0,x bra If3_End If3_Else: move.l b,d0 move.l d0,x If3_End: ---------------------------------------------------------------------- If1_End:

  • Note:

    you may have multiple labels marking the same location E.g.: If1_End and If3_End mark the same location in the program.... That's OK, it will not compromise the correctness of the code (Besides, compilers do that all the time)