Lesson 6 · 6 of 20

if, then and else · Pascal

Make decisions in Pascal with if, then and else: weather warnings and exam grades, with begin…end blocks and else if chains.

Pascal · if, then and else in DMarlon Studio: the example's code and its output in the terminal

`if` condition `then` statement runs the statement only when the condition is true. With temp at 31, temp > 30 holds and the heat warning appears; temp < 10 does not, so the coat line is skipped.

After then (and after else) comes a single statement. If you need several, group them between `begin` and `end`, like the two lines for a pass.

`else` says what to do when the condition is false. One Pascal rule trips everyone up at first: there is never a semicolon right before `else`, because the ; would close the if and leave the else hanging. That is why the code has end, not end;, just before it.

To choose between several options, chain `else if`: the conditions are checked in order and only the first one that holds runs. With a 6, grade >= 9 and grade >= 7 fail, grade >= 5 succeeds and Pass is printed.

Code · main.pas

program main;var  temp, grade: Integer;begin  temp := 31;  // The line after then only runs if the condition is true.  if temp > 30 then    WriteLn(temp, ' C: it is hot, drink water.');  if temp < 10 then    WriteLn('Take a coat.');  // Several statements go between begin and end.  // There is never a semicolon just before else.  grade := 6;  if grade >= 5 then  begin    WriteLn('Grade ', grade, ': passed.');    WriteLn('See you next year.');  end  else    WriteLn('Grade ', grade, ': failed.');  // else if checks the conditions in order.  if grade >= 9 then    WriteLn('Outstanding')  else if grade >= 7 then    WriteLn('Very good')  else if grade >= 5 then    WriteLn('Pass')  else    WriteLn('Fail');end.

Output · fpc main.pas && ./main

31 C: it is hot, drink water.
Grade 6: passed.
See you next year.
Pass

Key points

if ... then runs a statement when the condition holds.
Several statements go between begin and end.
Never put a semicolon right before else.
else if checks several conditions in order.

Exercise

Merge the two temperature warnings into a single if ... else if ... else that prints «Hot» above 30 degrees, «Cold» below 10 and «Mild» otherwise, and try it with temp set to 5, 18 and 31.