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120 lines
4.6 KiB
Markdown
120 lines
4.6 KiB
Markdown
# While Loops
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## Overview
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We have already [covered](../if/README.md) the `if` statement. A `while`
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loop is exactly the same with the addition of at least one branch and a
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label. It really is that simple.
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To illustrate this, here is a flow chart of an `if` statement (on the
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left) compared to a `while` loop (on the right).
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The closing brace in an `if` statement is indicated by the red arrow
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head. This isn't a branch, the code flow simply falls through to the
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statement beyond the closing brace. In the `while` loop, the behavior of
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the closing brace changes to be that of a branch back to just before the
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evaluation of the boolean condition (the "Decision").
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A new label is placed before evaluating the "Decision".
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A new unconditional branch is placed after the end of the "Code Block."
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For review, consider this C or C++ code:
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```c
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if (a >= b) {
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// CODE BLOCK
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}
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```
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here is the assembly language for this `if` statement:
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```asm
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// Assume value of a is in x0 // 1
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// Assume value of b is in x1 // 2
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cmp x0, x1 // 3
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ble 1f // 4
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// CODE BLOCK // 5
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1: // 6
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```
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Now, consider this `while` loop:
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```c
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while (a >= b) {
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// CODE BLOCK
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}
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```
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Here is the code for the `while` showing the addition of one new label
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and one new unconditional branch:
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```asm
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// Assume value of a is in x0 // 1
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// Assume value of b is in x1 // 2
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// 3
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1: cmp x0, x1 // 4
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bgt 2f // 5
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// CODE BLOCK // 6
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b 1b // 7
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// 8
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2: // 9
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```
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Temporary label `2` on `line 9` takes the place of the line after the
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closing brace in a `while` loop.
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Temporary label `1` on `line 4` is the end point of the red arrow in the
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right hand flow chart above.
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## Summary
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A `while` loop is an extension of the `if` statement. A simple `if`
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contains one conditional branch and one label.
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A `while` loop contains at least two labels, one conditional branch and
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one unconditional branch. We acknowledge the possibility that the
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unconditional branch could be made a conditional one, but this is rarely
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done in assembly language and impossible in higher level languages like
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C and C++ since the branch is simply the closing `}`.
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## Questions
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### 1
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(T | F) This code has a problem:
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```asm
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// Assume value of a is in x0 // 1
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// Assume value of b is in x1 // 2
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// 3
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1: cmp x0, x1 // 4
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b 2f // 5
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// CODE BLOCK // 6
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b 1b // 7
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// 8
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2: // 9
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```
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Answer: True - the unconditional branch on `line 5` doesn't reference the
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results of the `cmp` that comes before it. The `CODE BLOCK` will never be
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executed.
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### 2
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(T | F) This attempt at a `while` loop has a problem:
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```asm
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// Assume value of a is in x0 // 1
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// Assume value of b is in x1 // 2
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// 3
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1: cmp x0, x1 // 4
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ble 2f // 5
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// CODE BLOCK // 6
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// 7
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2: // 8
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```
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Answer: True - missing a branch back to label `1`, there is no loop.
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