PHY 107 – Programming For Science

Today’s Goal  How to use while & do / while loops in code  Know & explain when use of either loop preferable  Understand loop termination conditions  Be able to write bounded and unbounded loops  Explain how for, while & do / while loops differ  Convert between the 3 loops with same end result  Explain why we would never convert between loops

Code Structures in Programming  Sequence  Decision  Repetition true false true false

Loop Structure

Loop Termination Condition  Most important detail of a loop  This is what tells computer when loop should execute  Errors common in loop termination condition  Computers are dumb & happily execute infinite loops  Waste lot of time waiting for this to finish EXACTLY  Remember: computers do EXACTLY what they're told  Have infinite patience with the stupid tasks we ask

Loop Body  Actions loop performs each repetition  Loop body completed before checking condition  Even  Even if variable reassigned no rechecking within body  May contain any code including another loop  Nested loops common in science & engineering  Finding and then fixing errors can be hard  Error may be small, but executed many times…  Always test out on paper to verify idea works, first

for Loop Structure

for Loop

for Loop Strengths  for loops good at bounding times loop is run  (Optional) Declare & initialize variable doing counts  Loop statement also clearly shows loop termination  (Optional) Variable‘s update made quite clear  May not always have a variable controlling update  Might want to execute only while some expression true  Update within loop body and not just at the end  Not just a single variable, but multiple variables needed

while Loop while (expression) { statement;... }  Evaluates expression to find its value  If true, executes entire loop body  Re-check after each pass through loop, only  Continue with loop while expression checks true

Tracing while Loop int mPerMi = 1; float mph; while (mPerMi > 1) { mph=(1.0/mPerMi)*60.0; printf(“%d min/mile = ", mPerMi); mPerMi += 1; printf("%f mph\n", mph); } printf("Now I know.\n“);

Tracing while Loop int mPerMi = 1; float mph; while (mPerMi >= 1) { mph=(1.0/mPerMi)*60.0; printf(“%d min/mile = ", mPerMi); mPerMi += 1; printf("%f mph\n", mph); } printf("Now I know.\n“);

Tracing while Loop int mPerMi = 1; float mph; while ((mPerMi >= 1) && (mPerMi < 4)) { mph=(1.0/mPerMi)*60.0; printf(“%d min/mile = ", mPerMi); mPerMi += 1; printf("%f mph\n", mph); } printf("Now I know.\n“);

Sometimes you feel like a nut…  May want loop only if expression is true  while checks before entering loop to see if true  But may want to execute loop body at least once  do - while checks after executing of loop  Why use one and not the other?  Depends on when you want check to occur  Otherwise, there are not any real differences

do - while Loop

Sample do - while Loop int mPerMi = 1; float mph; do { mph=(1.0/mPerMi)*60.0; printf(“%d min/mile = ", mPerMi); mPerMi += 1; printf("%f mph\n", mph); } while (mPerMi > 1) ;

Your Turn  Get in groups & work on following activity

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