FORTRAN PROGRAMMING BASICS

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The basic layout of all programs is as follows (p.33)

PROGRAM name = heading (i.e., title)Specifications (e.g., define constants)Executions (the main part)Subprograms (see much later in the semester)END PROGRAM statement

Let’s look at each…

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Program name…

You get to choose a name!Make it meaningful!Name MUST start with a letter.Can be followed by letters, numbers, and the

underscore.

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Examples…

Areaofcircle - OKTemp_conversion (for deg F to deg C) - OK

2Version_speed – NOT OK (“2”)Mybro’scode – NOT OK (“ ’ ”)

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Specifications…

Any quantity in your program MUST be given a name – by you.

Includes known things e.g., “g” – acceleration of gravity (9.8 m/s2) things like data that you might input.

Also includes unknowns – the things you are trying to calculate.

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Example …

Speed of sound (c) is given by: c = √(RdT).

To compute “c” using Fortran, we need to “name” the quantities:

c … suppose we call it SPEEDRd … suppose we call it RDT … suppose we call it TEMP

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Memory …

Once we do this, the compiler allocates a memory location to each variable.

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Specifications…

Always choose meaningful names.Gravity → GRAVValue of → PICoriolis parameter → CORIOLIS Kinetic energy → KE (but this starts with a “K” which can be a

problem because…)

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Data types…

Fortran allows 5 data types:

INTEGERREALCOMPLEXCHARACTERLOGICAL

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IN OTHER WORDS, A Fortran program will assume a quantity is one of these (and nothing else).

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Rule:When you choose a name for a quantity starting with the letters I,J,K,L,M,N, Fortran will make that quantity an INTEGER.

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Example:

If we decide to name “gravity” as “KGRAV”, it will be treated as an integer!

Its value (entered as 9.8) would be recorded in memory as either “9” if truncated – or “10” if rounded up.

Will give an error in calculations

If we name kinetic energy as “KE”, it will be treated as an integer!

We must be careful to avoid these problems…see below

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INTEGER data…

A whole numberPositive or negative

Must include minus sign if negative

Cannot contain a periodSee examples on p.16 @ top

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REAL data…

Not a whole number!Positive or negative

MUST include minus sign if negative

MUST include a decimal (e.g., 9.81 or 17.0)

Often write in exponent form:

6371x103 6.371E61.48x10-17 1.48E-17

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COMPLEX data…

Let’s deal with this later

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CHARACTER data…

Things like letters!Often used to label some data (e.g., on printed

tables)Character data in a program is enclosed between

apostrophes.

Example:“ALISON” is character data of length 6 … use “-”‘Allison is gone’ is character data of length 15 …

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LOGICAL data…

Let’s deal with this later (or never)

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To repeat…when you choose a name for a quantity starting with the letters I,J,K,L,M,N, Fortran will make that quantity an INTEGER.

Otherwise it is a REAL data type.

We often choose names to make use of this…but we also make mistakes.

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Having REAL and INTEGER quantities together can lead to problems (examples below).

Thus it is VITAL to explicitly TELL THE COMPILER which is which.

This is called type declaration.Type declaration statements go at the start of

the program.

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Examples of format:

REAL : : grav INTEGER : : timeofday

Says the following:1) “grav” will be a REAL number2) “timeofday” will be an INTEGER number

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Statements like these MUST go @ start of code (“specifications”).

As a safeguard, your first statement should be this:

IMPLICIT NONE

Which says to the compiler … “do not assume anything … I will declare everything”!

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So your program structure so far is:

PROGRAM myfirstcodeIMPLICIT NONEREAL : : RD, GAMMA, TEMP, SPEEDExecution statements (not done yet)Subprograms (not done yet)END PROGRAM

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More stuff:

REAL : : grav = 9.81

tells the computer the value of “grav”!

This gets put into the memory location for “grav”.

ALL VARIABLES MUST HAVE VALUES DEFINED before the main computations happen!

Otherwise – they may be assumed to be zero

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For CHARACTER data, we type:

CHARACTER (5) : : month

This says :

“month” will be CHARACTER data with 5 characters

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Other ways to write:

CHARACTER (5) : : month, day

This says :

“month” and “day” will both be CHARACTER data with 5 characters

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Assigning values

In Fortran, the statement:

A = B

means: the value stored in memory for “B” will be assigned to (transferred into) the memory space for “A”

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So – the computer gets the value of a variable on the LHS of the “=” by first computing the value on the RHS.

A = B

So it’s OK to write: c = √(RdT)

But not: √(RdT) = c

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Arithmetic operations…

There are five allowed:

addition A+B subtraction A-B multiplication A*B not AB division A/B exponentiation A**3 means A3

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Putting it all together…

To evaluate: x = b2 – 4acwith a=1, b=7, and c=-2, we might have lines of code like:

IMPLICIT NONE REAL : : X, B, A, C A=1.0 ! A IS REAL, SO A=1.0 – NOT A=1 B=7.0 C=-2.0 X = B**2 -4.0*A*C

Everything after the “!” is ignored!!!

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Another example…

To evaluate: c = √(RdT)

with =1.4, b=7, and c=-2:

IMPLICIT NONE REAL : : GAMMA, RD, TEMP, SPEED GAMMA = 1.0 RD = 287.0 TEMP = 273.0 SPEED = SQRT(GAMMA*RD*TEMP)

Here, we have used the intrinsic function: SQRT = √

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An intrinsic function is something built into Fortran.

COS(X) SIN(X) EXP(X) SQRT(X) MAX(X) ABS(X)

And more – see Table 2-2 and Appendix A.9/1/2011

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We/you now know enough to:

a) Try writing tiny bits of codeb) Debug bad code – anything that even

slightly violates one of the rules above!

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