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CS 115 Lecture 10 Structured programming; for loops Neil Moore Department of Computer Science University of Kentucky Lexington, Kentucky 40506 neil@cs.uky.edu 8 October 2015 13 October 2015 The bad old days: GOTO In the early days of

  1. Selection “Selection” means choosing which code to run based on some condition or question. In Python, an if - else statement. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 7 / 31

  2. Selection “Selection” means choosing which code to run based on some condition or question. In Python, an if - else statement. Two branches: true and false. ◮ Each branch is another control structure (most often a sequence). Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 7 / 31

  3. Selection “Selection” means choosing which code to run based on some condition or question. In Python, an if - else statement. Two branches: true and false. ◮ Each branch is another control structure (most often a sequence). Guarantees: ◮ Always starts with the question/condition. ◮ Runs one branch or the other, never both. ◮ . . . and never neither. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 7 / 31

  4. Selection “Selection” means choosing which code to run based on some condition or question. In Python, an if - else statement. Two branches: true and false. ◮ Each branch is another control structure (most often a sequence). Guarantees: ◮ Always starts with the question/condition. ◮ Runs one branch or the other, never both. ◮ . . . and never neither. Avoid dead code : code that is never executed. ◮ Often because the condition is always true or always false. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 7 / 31

  5. Selection “Selection” means choosing which code to run based on some condition or question. In Python, an if - else statement. Two branches: true and false. ◮ Each branch is another control structure (most often a sequence). Guarantees: ◮ Always starts with the question/condition. ◮ Runs one branch or the other, never both. ◮ . . . and never neither. Avoid dead code : code that is never executed. ◮ Often because the condition is always true or always false. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 7 / 31

  6. Iteration “Iteration” means running code multiple times (a loop). In structured programming, “repeat this body until a condition is false”. In Python, a while loop (in about a week). ◮ for loops are a special case of iteration. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 8 / 31

  7. Iteration “Iteration” means running code multiple times (a loop). In structured programming, “repeat this body until a condition is false”. In Python, a while loop (in about a week). ◮ for loops are a special case of iteration. Guarantees: ◮ Always starts with the question/condition. ◮ If the condition is true, executes the entire body, then comes back to the condition. ◮ Otherwise (the condition is false), leaves the loop. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 8 / 31

  8. Iteration “Iteration” means running code multiple times (a loop). In structured programming, “repeat this body until a condition is false”. In Python, a while loop (in about a week). ◮ for loops are a special case of iteration. Guarantees: ◮ Always starts with the question/condition. ◮ If the condition is true, executes the entire body, then comes back to the condition. ◮ Otherwise (the condition is false), leaves the loop. Be careful to avoid infinite loops , where the condition is always true. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 8 / 31

  9. Iteration “Iteration” means running code multiple times (a loop). In structured programming, “repeat this body until a condition is false”. In Python, a while loop (in about a week). ◮ for loops are a special case of iteration. Guarantees: ◮ Always starts with the question/condition. ◮ If the condition is true, executes the entire body, then comes back to the condition. ◮ Otherwise (the condition is false), leaves the loop. Be careful to avoid infinite loops , where the condition is always true. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 8 / 31

  10. Subprograms Sometimes we may need to repeat the same combination of control structures in several different places. It would be nice if we didn’t have to write the code multiple times. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 9 / 31

  11. Subprograms Sometimes we may need to repeat the same combination of control structures in several different places. It would be nice if we didn’t have to write the code multiple times. A subprogram is a chunk of the flowchart treated as a single unit. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 9 / 31

  12. Subprograms Sometimes we may need to repeat the same combination of control structures in several different places. It would be nice if we didn’t have to write the code multiple times. A subprogram is a chunk of the flowchart treated as a single unit. When we need to execute those steps, we call the subprogram. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 9 / 31

  13. Subprograms Sometimes we may need to repeat the same combination of control structures in several different places. It would be nice if we didn’t have to write the code multiple times. A subprogram is a chunk of the flowchart treated as a single unit. When we need to execute those steps, we call the subprogram. ◮ Run the subprogram, wait for it to finish. ◮ Keep going where you left off. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 9 / 31

  14. Subprograms Sometimes we may need to repeat the same combination of control structures in several different places. It would be nice if we didn’t have to write the code multiple times. A subprogram is a chunk of the flowchart treated as a single unit. When we need to execute those steps, we call the subprogram. ◮ Run the subprogram, wait for it to finish. ◮ Keep going where you left off. ◮ Sometimes we send values to the subprogram. ◮ And sometimes the subprogram sends a value back. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 9 / 31

  15. Subprograms Sometimes we may need to repeat the same combination of control structures in several different places. It would be nice if we didn’t have to write the code multiple times. A subprogram is a chunk of the flowchart treated as a single unit. When we need to execute those steps, we call the subprogram. ◮ Run the subprogram, wait for it to finish. ◮ Keep going where you left off. ◮ Sometimes we send values to the subprogram. ◮ And sometimes the subprogram sends a value back. In Python, subprograms are called functions . ◮ Arguments are the values we send to the subprogram. ◮ And the function can return a result. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 9 / 31

  16. Subprograms Sometimes we may need to repeat the same combination of control structures in several different places. It would be nice if we didn’t have to write the code multiple times. A subprogram is a chunk of the flowchart treated as a single unit. When we need to execute those steps, we call the subprogram. ◮ Run the subprogram, wait for it to finish. ◮ Keep going where you left off. ◮ Sometimes we send values to the subprogram. ◮ And sometimes the subprogram sends a value back. In Python, subprograms are called functions . ◮ Arguments are the values we send to the subprogram. ◮ And the function can return a result. ◮ Can you think of Python functions that: ⋆ Take one or more arguments? Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 9 / 31

  17. Subprograms Sometimes we may need to repeat the same combination of control structures in several different places. It would be nice if we didn’t have to write the code multiple times. A subprogram is a chunk of the flowchart treated as a single unit. When we need to execute those steps, we call the subprogram. ◮ Run the subprogram, wait for it to finish. ◮ Keep going where you left off. ◮ Sometimes we send values to the subprogram. ◮ And sometimes the subprogram sends a value back. In Python, subprograms are called functions . ◮ Arguments are the values we send to the subprogram. ◮ And the function can return a result. ◮ Can you think of Python functions that: ⋆ Take one or more arguments? ⋆ Take no arguments? Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 9 / 31

  18. Subprograms Sometimes we may need to repeat the same combination of control structures in several different places. It would be nice if we didn’t have to write the code multiple times. A subprogram is a chunk of the flowchart treated as a single unit. When we need to execute those steps, we call the subprogram. ◮ Run the subprogram, wait for it to finish. ◮ Keep going where you left off. ◮ Sometimes we send values to the subprogram. ◮ And sometimes the subprogram sends a value back. In Python, subprograms are called functions . ◮ Arguments are the values we send to the subprogram. ◮ And the function can return a result. ◮ Can you think of Python functions that: ⋆ Take one or more arguments? ⋆ Take no arguments? ⋆ Return a result? Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 9 / 31

  19. Subprograms Sometimes we may need to repeat the same combination of control structures in several different places. It would be nice if we didn’t have to write the code multiple times. A subprogram is a chunk of the flowchart treated as a single unit. When we need to execute those steps, we call the subprogram. ◮ Run the subprogram, wait for it to finish. ◮ Keep going where you left off. ◮ Sometimes we send values to the subprogram. ◮ And sometimes the subprogram sends a value back. In Python, subprograms are called functions . ◮ Arguments are the values we send to the subprogram. ◮ And the function can return a result. ◮ Can you think of Python functions that: ⋆ Take one or more arguments? ⋆ Take no arguments? ⋆ Return a result? ⋆ Don’t return a result? Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 9 / 31

  20. Subprograms Sometimes we may need to repeat the same combination of control structures in several different places. It would be nice if we didn’t have to write the code multiple times. A subprogram is a chunk of the flowchart treated as a single unit. When we need to execute those steps, we call the subprogram. ◮ Run the subprogram, wait for it to finish. ◮ Keep going where you left off. ◮ Sometimes we send values to the subprogram. ◮ And sometimes the subprogram sends a value back. In Python, subprograms are called functions . ◮ Arguments are the values we send to the subprogram. ◮ And the function can return a result. ◮ Can you think of Python functions that: ⋆ Take one or more arguments? ⋆ Take no arguments? ⋆ Return a result? ⋆ Don’t return a result? Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 9 / 31

  21. Control structures summary Sequence (one statement after the other: easy to forget) Selection (conditionals: if ) Iteration (loops: for and while ) Subprograms (functions: def ) Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 10 / 31

  22. Control structures summary Sequence (one statement after the other: easy to forget) � Selection (conditionals: if ) � Iteration (loops: for and while ) Subprograms (functions: def ) We’ve seen sequence and selection already, so now let’s look at iteration in more detail. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 10 / 31

  23. Control structures summary Sequence (one statement after the other: easy to forget) Selection (conditionals: if ) Iteration (loops: for and while ) Subprograms (functions: def ) We’ve seen sequence and selection already, so now let’s look at iteration in more detail. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 10 / 31

  24. Repeating yourself What if we wanted to draw a tic-tac-toe board with 4 × 4 lines? We could write code to draw a vertical line. . . . . . and code to draw a horizontal line. . . Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 11 / 31

  25. Repeating yourself What if we wanted to draw a tic-tac-toe board with 4 × 4 lines? We could write code to draw a vertical line. . . . . . and code to draw a horizontal line. . . We need to do that four times each. ◮ With different coordinates each time. Do we have to copy-and-paste each one 4 times? Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 11 / 31

  26. Repeating yourself What if we wanted to draw a tic-tac-toe board with 4 × 4 lines? We could write code to draw a vertical line. . . . . . and code to draw a horizontal line. . . We need to do that four times each. ◮ With different coordinates each time. Do we have to copy-and-paste each one 4 times? ◮ Of course not! ◮ Loops allow you to execute code multiple times. . . . with a variable that is different each time. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 11 / 31

  27. Repeating yourself What if we wanted to draw a tic-tac-toe board with 4 × 4 lines? We could write code to draw a vertical line. . . . . . and code to draw a horizontal line. . . We need to do that four times each. ◮ With different coordinates each time. Do we have to copy-and-paste each one 4 times? ◮ Of course not! ◮ Loops allow you to execute code multiple times. . . . with a variable that is different each time. Two kinds of loop: definite and indefinite. ◮ Definite loops know in advance how many times to run. ◮ Indefinite loops run until some condition is satisfied. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 11 / 31

  28. Repeating yourself What if we wanted to draw a tic-tac-toe board with 4 × 4 lines? We could write code to draw a vertical line. . . . . . and code to draw a horizontal line. . . We need to do that four times each. ◮ With different coordinates each time. Do we have to copy-and-paste each one 4 times? ◮ Of course not! ◮ Loops allow you to execute code multiple times. . . . with a variable that is different each time. Two kinds of loop: definite and indefinite. ◮ Definite loops know in advance how many times to run. ◮ Indefinite loops run until some condition is satisfied. ◮ Today we’ll see how to write definite loops in Python. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 11 / 31

  29. Repeating yourself What if we wanted to draw a tic-tac-toe board with 4 × 4 lines? We could write code to draw a vertical line. . . . . . and code to draw a horizontal line. . . We need to do that four times each. ◮ With different coordinates each time. Do we have to copy-and-paste each one 4 times? ◮ Of course not! ◮ Loops allow you to execute code multiple times. . . . with a variable that is different each time. Two kinds of loop: definite and indefinite. ◮ Definite loops know in advance how many times to run. ◮ Indefinite loops run until some condition is satisfied. ◮ Today we’ll see how to write definite loops in Python. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 11 / 31

  30. The for loop Syntax: for var in sequence : ◮ Followed by a block (collection of indented lines) called the body . ⋆ The body must be indented past the “for”! ◮ var is an identifier (variable name). Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 12 / 31

  31. The for loop Syntax: for var in sequence : ◮ Followed by a block (collection of indented lines) called the body . ⋆ The body must be indented past the “for”! ◮ var is an identifier (variable name). Semantics: Execute the body once for each item in the sequence. ◮ Each time, the variable var will have the value of that item. ◮ Each run of the body is called an iteration . Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 12 / 31

  32. The for loop Syntax: for var in sequence : ◮ Followed by a block (collection of indented lines) called the body . ⋆ The body must be indented past the “for”! ◮ var is an identifier (variable name). Semantics: Execute the body once for each item in the sequence. ◮ Each time, the variable var will have the value of that item. ◮ Each run of the body is called an iteration . A very simple for loop: for color in (’red’, ’green’, ’blue’): print(color, ’is a primary color.’) Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 12 / 31

  33. The for loop Syntax: for var in sequence : ◮ Followed by a block (collection of indented lines) called the body . ⋆ The body must be indented past the “for”! ◮ var is an identifier (variable name). Semantics: Execute the body once for each item in the sequence. ◮ Each time, the variable var will have the value of that item. ◮ Each run of the body is called an iteration . A very simple for loop: for color in (’red’, ’green’, ’blue’): print(color, ’is a primary color.’) We’re giving a tuple , but a list in square brackets would work too. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 12 / 31

  34. The for loop Syntax: for var in sequence : ◮ Followed by a block (collection of indented lines) called the body . ⋆ The body must be indented past the “for”! ◮ var is an identifier (variable name). Semantics: Execute the body once for each item in the sequence. ◮ Each time, the variable var will have the value of that item. ◮ Each run of the body is called an iteration . A very simple for loop: for color in (’red’, ’green’, ’blue’): print(color, ’is a primary color.’) We’re giving a tuple , but a list in square brackets would work too. When executed it does: Iteration 1: print(’red’, ’is a primary color.’) Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 12 / 31

  35. The for loop Syntax: for var in sequence : ◮ Followed by a block (collection of indented lines) called the body . ⋆ The body must be indented past the “for”! ◮ var is an identifier (variable name). Semantics: Execute the body once for each item in the sequence. ◮ Each time, the variable var will have the value of that item. ◮ Each run of the body is called an iteration . A very simple for loop: for color in (’red’, ’green’, ’blue’): print(color, ’is a primary color.’) We’re giving a tuple , but a list in square brackets would work too. When executed it does: Iteration 1: print(’red’, ’is a primary color.’) Iteration 2: print(’green’, ’is a primary color.’) Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 12 / 31

  36. The for loop Syntax: for var in sequence : ◮ Followed by a block (collection of indented lines) called the body . ⋆ The body must be indented past the “for”! ◮ var is an identifier (variable name). Semantics: Execute the body once for each item in the sequence. ◮ Each time, the variable var will have the value of that item. ◮ Each run of the body is called an iteration . A very simple for loop: for color in (’red’, ’green’, ’blue’): print(color, ’is a primary color.’) We’re giving a tuple , but a list in square brackets would work too. When executed it does: Iteration 1: print(’red’, ’is a primary color.’) Iteration 2: print(’green’, ’is a primary color.’) Iteration 3: print(’blue’, ’is a primary color.’) Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 12 / 31

  37. The for loop Syntax: for var in sequence : ◮ Followed by a block (collection of indented lines) called the body . ⋆ The body must be indented past the “for”! ◮ var is an identifier (variable name). Semantics: Execute the body once for each item in the sequence. ◮ Each time, the variable var will have the value of that item. ◮ Each run of the body is called an iteration . A very simple for loop: for color in (’red’, ’green’, ’blue’): print(color, ’is a primary color.’) We’re giving a tuple , but a list in square brackets would work too. When executed it does: Iteration 1: print(’red’, ’is a primary color.’) Iteration 2: print(’green’, ’is a primary color.’) Iteration 3: print(’blue’, ’is a primary color.’) Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 12 / 31

  38. Other kinds of sequences Strings can also be used as sequences. Each iteration of the loop operates on a single character: name = input("What is your name? ") for char in name: print(char) Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 13 / 31

  39. Other kinds of sequences Strings can also be used as sequences. Each iteration of the loop operates on a single character: name = input("What is your name? ") for char in name: print(char) Prints this: M o o r e Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 13 / 31

  40. Other kinds of sequences Strings can also be used as sequences. Each iteration of the loop operates on a single character: name = input("What is your name? ") for char in name: print(char) Prints this: M o o r e Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 13 / 31

  41. Numeric ranges One of the most common, and most useful, kinds of sequence for a for loop is a numeric range. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 14 / 31

  42. Numeric ranges One of the most common, and most useful, kinds of sequence for a for loop is a numeric range. In Python, you create numeric ranges with the range function. There are three ways to call range : Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 14 / 31

  43. Numeric ranges One of the most common, and most useful, kinds of sequence for a for loop is a numeric range. In Python, you create numeric ranges with the range function. There are three ways to call range : range(3) : counts from 0 up to 2. ◮ Computer scientists usually count from zero, not one. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 14 / 31

  44. Numeric ranges One of the most common, and most useful, kinds of sequence for a for loop is a numeric range. In Python, you create numeric ranges with the range function. There are three ways to call range : range(3) : counts from 0 up to 2. ◮ Computer scientists usually count from zero, not one. ◮ Goes up to but not including the number. (just like randrange !) Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 14 / 31

  45. Numeric ranges One of the most common, and most useful, kinds of sequence for a for loop is a numeric range. In Python, you create numeric ranges with the range function. There are three ways to call range : range(3) : counts from 0 up to 2. ◮ Computer scientists usually count from zero, not one. ◮ Goes up to but not including the number. (just like randrange !) for i in range(3): print(i, "squared is", i**2) Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 14 / 31

  46. Numeric ranges One of the most common, and most useful, kinds of sequence for a for loop is a numeric range. In Python, you create numeric ranges with the range function. There are three ways to call range : range(3) : counts from 0 up to 2. ◮ Computer scientists usually count from zero, not one. ◮ Goes up to but not including the number. (just like randrange !) for i in range(3): print(i, "squared is", i**2) Prints: 0 squared is 0 1 squared is 1 2 squared is 4 Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 14 / 31

  47. Numeric ranges One of the most common, and most useful, kinds of sequence for a for loop is a numeric range. In Python, you create numeric ranges with the range function. There are three ways to call range : range(3) : counts from 0 up to 2. ◮ Computer scientists usually count from zero, not one. ◮ Goes up to but not including the number. (just like randrange !) for i in range(3): print(i, "squared is", i**2) Prints: 0 squared is 0 1 squared is 1 2 squared is 4 ◮ Notice the loop ran 3 times (0, 1, 2). ⋆ Don’t make a fencepost error! Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 14 / 31

  48. Numeric ranges One of the most common, and most useful, kinds of sequence for a for loop is a numeric range. In Python, you create numeric ranges with the range function. There are three ways to call range : range(3) : counts from 0 up to 2. ◮ Computer scientists usually count from zero, not one. ◮ Goes up to but not including the number. (just like randrange !) for i in range(3): print(i, "squared is", i**2) Prints: 0 squared is 0 1 squared is 1 2 squared is 4 ◮ Notice the loop ran 3 times (0, 1, 2). ⋆ Don’t make a fencepost error! Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 14 / 31

  49. More ranges We can also tell range to start at a different number: Syntax: range(start, stop) ◮ Produces a sequence of integers from start to stop. ◮ Does include the start (inclusive), not the stop (exclusive). Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 15 / 31

  50. More ranges We can also tell range to start at a different number: Syntax: range(start, stop) ◮ Produces a sequence of integers from start to stop. ◮ Does include the start (inclusive), not the stop (exclusive). for i in range(3, 6): print(i) Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 15 / 31

  51. More ranges We can also tell range to start at a different number: Syntax: range(start, stop) ◮ Produces a sequence of integers from start to stop. ◮ Does include the start (inclusive), not the stop (exclusive). for i in range(3, 6): print(i) Prints: 3 4 5 Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 15 / 31

  52. More ranges We can also tell range to start at a different number: Syntax: range(start, stop) ◮ Produces a sequence of integers from start to stop. ◮ Does include the start (inclusive), not the stop (exclusive). for i in range(3, 6): print(i) Prints: 3 4 5 ◮ Runs for (stop - start) iterations. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 15 / 31

  53. More ranges We can also tell range to start at a different number: Syntax: range(start, stop) ◮ Produces a sequence of integers from start to stop. ◮ Does include the start (inclusive), not the stop (exclusive). for i in range(3, 6): print(i) Prints: 3 4 5 ◮ Runs for (stop - start) iterations. What if we wrote range(1, 1) ? Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 15 / 31

  54. More ranges We can also tell range to start at a different number: Syntax: range(start, stop) ◮ Produces a sequence of integers from start to stop. ◮ Does include the start (inclusive), not the stop (exclusive). for i in range(3, 6): print(i) Prints: 3 4 5 ◮ Runs for (stop - start) iterations. What if we wrote range(1, 1) ? ◮ Empty sequence: stops before getting to 1. ◮ The loop wouldn’t run at all! Loops can run for 0 iterations. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 15 / 31

  55. More ranges We can also tell range to start at a different number: Syntax: range(start, stop) ◮ Produces a sequence of integers from start to stop. ◮ Does include the start (inclusive), not the stop (exclusive). for i in range(3, 6): print(i) Prints: 3 4 5 ◮ Runs for (stop - start) iterations. What if we wrote range(1, 1) ? ◮ Empty sequence: stops before getting to 1. ◮ The loop wouldn’t run at all! Loops can run for 0 iterations. ◮ Similarly, range(5, 1) is an empty sequence. ⋆ So this loop will do nothing: for i in range(1, 5, -1): print(i) ⋆ The body never executes (is dead code ). Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 15 / 31

  56. More ranges We can also tell range to start at a different number: Syntax: range(start, stop) ◮ Produces a sequence of integers from start to stop. ◮ Does include the start (inclusive), not the stop (exclusive). for i in range(3, 6): print(i) Prints: 3 4 5 ◮ Runs for (stop - start) iterations. What if we wrote range(1, 1) ? ◮ Empty sequence: stops before getting to 1. ◮ The loop wouldn’t run at all! Loops can run for 0 iterations. ◮ Similarly, range(5, 1) is an empty sequence. ⋆ So this loop will do nothing: for i in range(1, 5, -1): print(i) ⋆ The body never executes (is dead code ). Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 15 / 31

  57. Counting with steps Finally, we can tell range to count by steps, only considering every n th number: Syntax: range(start, stop, step) ◮ Instead of adding 1 in each iteration, adds step . ◮ The first number is still start . ◮ The next number is start + step , then start + 2*step , . . . ◮ What will this do? for i in range(10, 25, 5): print(i) Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 16 / 31

  58. Counting with steps Finally, we can tell range to count by steps, only considering every n th number: Syntax: range(start, stop, step) ◮ Instead of adding 1 in each iteration, adds step . ◮ The first number is still start . ◮ The next number is start + step , then start + 2*step , . . . ◮ What will this do? for i in range(10, 25, 5): print(i) ◮ Prints: 10 Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 16 / 31

  59. Counting with steps Finally, we can tell range to count by steps, only considering every n th number: Syntax: range(start, stop, step) ◮ Instead of adding 1 in each iteration, adds step . ◮ The first number is still start . ◮ The next number is start + step , then start + 2*step , . . . ◮ What will this do? for i in range(10, 25, 5): print(i) ◮ Prints: 10 15 Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 16 / 31

  60. Counting with steps Finally, we can tell range to count by steps, only considering every n th number: Syntax: range(start, stop, step) ◮ Instead of adding 1 in each iteration, adds step . ◮ The first number is still start . ◮ The next number is start + step , then start + 2*step , . . . ◮ What will this do? for i in range(10, 25, 5): print(i) ◮ Prints: 10 15 20 Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 16 / 31

  61. Counting with steps Finally, we can tell range to count by steps, only considering every n th number: Syntax: range(start, stop, step) ◮ Instead of adding 1 in each iteration, adds step . ◮ The first number is still start . ◮ The next number is start + step , then start + 2*step , . . . ◮ What will this do? for i in range(10, 25, 5): print(i) ◮ Prints: 10 15 20 ◮ Does not include 25: stop is still exclusive. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 16 / 31

  62. Counting with steps Finally, we can tell range to count by steps, only considering every n th number: Syntax: range(start, stop, step) ◮ Instead of adding 1 in each iteration, adds step . ◮ The first number is still start . ◮ The next number is start + step , then start + 2*step , . . . ◮ What will this do? for i in range(10, 25, 5): print(i) ◮ Prints: 10 15 20 ◮ Does not include 25: stop is still exclusive. What about range(10, 2) ? Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 16 / 31

  63. Counting with steps Finally, we can tell range to count by steps, only considering every n th number: Syntax: range(start, stop, step) ◮ Instead of adding 1 in each iteration, adds step . ◮ The first number is still start . ◮ The next number is start + step , then start + 2*step , . . . ◮ What will this do? for i in range(10, 25, 5): print(i) ◮ Prints: 10 15 20 ◮ Does not include 25: stop is still exclusive. What about range(10, 2) ? ◮ Two arguments are start and stop, not step. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 16 / 31

  64. Counting with steps Finally, we can tell range to count by steps, only considering every n th number: Syntax: range(start, stop, step) ◮ Instead of adding 1 in each iteration, adds step . ◮ The first number is still start . ◮ The next number is start + step , then start + 2*step , . . . ◮ What will this do? for i in range(10, 25, 5): print(i) ◮ Prints: 10 15 20 ◮ Does not include 25: stop is still exclusive. What about range(10, 2) ? ◮ Two arguments are start and stop, not step. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 16 / 31

  65. Counting backwards You can count down by providing a negative step. for i in range(3, 0, -1): print("Counting down:", i) Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 17 / 31

  66. Counting backwards You can count down by providing a negative step. for i in range(3, 0, -1): print("Counting down:", i) print("Lift off!") Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 17 / 31

  67. Counting backwards You can count down by providing a negative step. for i in range(3, 0, -1): print("Counting down:", i) print("Lift off!") Prints: Counting down: 3 Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 17 / 31

  68. Counting backwards You can count down by providing a negative step. for i in range(3, 0, -1): print("Counting down:", i) print("Lift off!") Prints: Counting down: 3 Counting down: 2 Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 17 / 31

  69. Counting backwards You can count down by providing a negative step. for i in range(3, 0, -1): print("Counting down:", i) print("Lift off!") Prints: Counting down: 3 Counting down: 2 Counting down: 1 Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 17 / 31

  70. Counting backwards You can count down by providing a negative step. for i in range(3, 0, -1): print("Counting down:", i) print("Lift off!") Prints: Counting down: 3 Counting down: 2 Counting down: 1 Lift off! The stop is still exclusive. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 17 / 31

  71. Counting backwards You can count down by providing a negative step. for i in range(3, 0, -1): print("Counting down:", i) print("Lift off!") Prints: Counting down: 3 Counting down: 2 Counting down: 1 Lift off! The stop is still exclusive. range(1, 5, -1) is an empty sequence. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 17 / 31

  72. Counting backwards You can count down by providing a negative step. for i in range(3, 0, -1): print("Counting down:", i) print("Lift off!") Prints: Counting down: 3 Counting down: 2 Counting down: 1 Lift off! The stop is still exclusive. range(1, 5, -1) is an empty sequence. Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 17 / 31

  73. Tic-tac-toe grid Now we can make that tic-tac-toe grid. We’ll have one loop to draw the vertical lines. And another to draw the horizontal lines. grid.py Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 18 / 31

  74. Tic-tac-toe grid Now we can make that tic-tac-toe grid. We’ll have one loop to draw the vertical lines. And another to draw the horizontal lines. grid.py A neat “display hack” (simple code to make an intricate picture) using for loops and if: moire.py Neil Moore (UK CS) CS 115 Lecture 10 Fall 2015 18 / 31

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