When you're traveling, you'll sometimes come to a point at which you make a decision - to go one way or to go another. Sometimes the decision is a small one - for example, if I'm driving through Marlborough there's a choice of the town centre or the road around the back; whichever I take, they come back together again very quickly. And at other times the decision is much bigger - we decided to fly on our recent trip to Slovenia, and once that decision was made and we started to act on it, the whole course of our time from Sunday lunchtime through to the following Friday evening was mapped out as alternative "B" rather than "A".
All the programming languages that we teach have conditional statements -
if statements, or commands - and there's a requirement in every language to define the start
and end of the conditional code - where the two tracks merge again, as well as where they separate. There are a variety of ways of doing this - using insets (Python), using an
endif,
if or
do, done pair (Shell programming) but perhaps the most common is to use curly braces. That applies in PHP and in C (today's example) and - slightly varied - in Perl and Tcl.
#include <stdio.h>
int main() {
int dayslong;
printf("How long is the course ");
scanf("%d",&dayslong);
/* Condition applies only to first printf */
if (dayslong < 5)
printf ("Less than a week\n");
printf ("No weekend activities\n");
printf ("First Job done\n");
/* Condition applies to TWO printf-s */
if (dayslong < 5) {
printf ("Less than a week\n");
printf ("No weekend activities\n"); }
printf ("Second Job done\n");
}
In the first section, no curly braces are used and so the condition applies ONLY to the first printf, but in the second section with curly braces, the condition applies to the first and second printfs.
Let's run that ...
How long is the course 7
No weekend activities
First Job done
Second Job done
[trainee@daisy cd07]$ ./cb
How long is the course 4
Less than a week
No weekend activities
First Job done
Less than a week
No weekend activities
Second Job done
[trainee@daisy cd07]$
(written 2007-12-18, updated 2007-12-19)
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Y103 - Python - Conditionals and Loops [299] What - no switch or case statement? - (2005-05-03)
[353] Wimbledon Neck - (2005-06-20)
[657] The ternary operator in Python - (2006-03-25)
[668] Python - block insets help with documentation - (2006-04-04)
[788] New - Conditional expressions in Python 2.5 - (2006-07-01)
[835] Python - when to use the in operator - (2006-08-16)
[909] Python is like a narrowboat - (2006-10-30)
[1201] No switch in Python - (2007-05-23)
[1661] Equality, sameness and identity - Python - (2008-05-31)
[1696] Saying NOT in Perl, PHP, Python, Lua ... - (2008-07-04)
[2778] Learning to program in Python 2 ... and / or in Python 3 - (2010-05-24)
[2899] Groupsave tickets - 3 or 4 train tickets for the price of 2 - (2010-08-02)
[3083] Python - fresh examples from recent courses - (2010-12-11)
[3200] How a for loop works Java, Perl and other languages - (2011-03-12)
[3397] Does a for loop evaluate its end condition once, or on every iteration? - (2011-08-18)
[3439] Python for loops - applying a temporary second name to the same object - (2011-09-14)
[3558] Python or Lua - which should I use / learn? - (2011-12-21)
[3620] Finding the total, average, minimum and maximum in a program - (2012-02-22)
[3762] Learning to program - the if statement. Python. - (2012-06-12)
[3895] Flowchart to program - learning to program with Well House - (2012-10-14)
[4092] Identity in Python - (2013-05-17)
[4210] If elif elif elif - multiway selection in Python - (2013-11-16)
[4322] Learning to Program - the conditional statement (if) - (2014-11-21)
[4323] Learning to program - Loop statements such as while - (2014-11-22)
[4402] Finding sum, minimum, maximum and average in Python (and Ruby) - (2015-01-19)
[4541] Setting up and tearing down with the Python with keyword - (2015-10-16)
[4545] Method, Class, Module, Package - how to they relate in Python? - (2015-10-17)
[4713] Equality (in Python) - (2016-10-30)
[4723] Conditional operators in Python - (2016-11-05)
T203 - Tcl/Tk - Conditionals and Loops [210] Joining lists in Tcl. Indirect variables in Tcl. - (2005-02-12)
[1401] Tcl - using [] or {} for conditions in an if (and while) - (2007-10-23)
[2261] Tcl - nice and nasty - (2009-06-29)
[2471] A short form of if ... then ... else - (2009-10-23)
[2681] Tcl - a great engineering language - (2010-03-17)
[3189] Tcl - the danger of square brackets in a while command - (2011-03-02)
[3570] Trapping errors in Tcl - the safety net that catch provides - (2012-01-06)
[3571] Comparing loop commands in Tcl - (2012-01-06)
[4455] Working out distance between places, using OS grid references and a program in Tcl - (2015-03-11)
P204 - Perl - Conditionals and Loops [930] -> , >= and => in Perl - (2006-11-18)
[1191] Smart English Output - via PHP and Perl ? : operator - (2007-05-18)
[1468] Lexical v Arithemetic testing, Bash and Perl - (2007-12-11)
[1607] Learning to program in Perl - (2008-04-11)
[1727] Equality and looks like tests - Perl - (2008-07-29)
[2351] Ternary operators alternatives - Perl and Lua lazy operators - (2009-08-12)
[2550] Do not copy and paste code - there are much better ways - (2009-12-26)
[2711] For loop - checked once, or evety time? Ruby v Perl comparison and contrast - (2010-04-07)
[2832] Are you learning Perl? Some more examples for you! - (2010-06-27)
[3004] Increment operators for counting - Perl, PHP, C and others - (2010-10-18)
[4031] Showing what programming errors look like - web site pitfall - (2013-03-06)
H104 - PHP - Control Statements [340] Code and code maintainance efficiency - (2005-06-08)
[406] Assignment, equality and identity in PHP - (2005-08-08)
[421] Don't repeat code - use loops or functions - (2005-08-21)
[863] Double and Triple equals operator in PHP - (2006-09-12)
[962] Breaking a loop - Ruby and other languages - (2006-12-03)
[1199] Testing for one of a list of values. - (2007-05-22)
[1220] for loop - how it works (Perl, PHP, Java, C, etc) - (2007-06-06)
[1825] Question Mark - Colon operator (Perl and PHP) - (2008-10-08)
[2304] Extracting real data from an exported file in PHP or Perl - (2009-07-25)
[2912] Predictions for the seagull population - (2010-08-09)
[3914] While, for, foreach or something else to loop. - (2012-11-06)
Some other Articles
The Christmas LetterNext course - 7th January 2008, Regular ExpressionsFSB leaves its members feeling like mushroomsSome new C programming examples - files, structs, unions etcDecisions - small ones, or big ones?Shopping for Christmas and looking forwardTcl/Tk - updating your display while tasks are runningUsing Tcl/Tk resource files for flexible applicationsMaking a variable dynamically visible in a Tcl/Tk GUIThe Horse goes on and on