Loops

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B.6.2 Loops

The for loop

for iter in seq:
<statements>

iterates over elements of a sequence seq, assigning each in turn to iter. The sequence is usually a list of values.

For example,

splitms = ’polcal_20080224.cband.all.split.ms’
srclist = [’0137+331’,’2136+006’,’2202+422’,’2253+161’,’0319+415’,’0359+509’]
spwlist = [’0’,’1’]

for src in srclist:

    for spwid in spwlist:

        imname = splitms + ’.’ + src + ’.’ + spwid + ’.clean’
        clean(vis=splitms,field=src,spw=spwid,imagename=imname,
            stokes=’IQUV’,psfmode=’hogbom’,imagermode=’csclean’,
            imsize=[288,288],cell=[0.4,0.4],niter=1000,
            threshold=1.3,mask=[134,134,154,154])

    # Done with  spw

# Done with sources

As usual, blocks are closed by blank lines of the previous indentation level.

You can use the range (§ B.4) Python function to generate a numerical loop:

vis = ’polcal_20080224.cband.all.ms’
for i in range(0,6):
fld = str(i)
plotxy(vis,field=fld,xaxis=’uvdist’,yaxis=’amp’)

# Done with fields [0, 1, 2, 3, 4, 5]

There is also a while loop construct

while <expression>:
<statements>

which executes the statement block while the <expression> is True. The while loop can also take an else block.

For example,

# Do an explicit set of clean iterations down to a limit
prevrms = 1.e10
while rms > 0.001 :
    clean(vis=splitms,field=src,spw=spwid,imagename=imname,
          stokes=’IQUV’,psfmode=’hogbom’,imagermode=’csclean’,
          imsize=[288,288],cell=[0.4,0.4],niter=200,
          threshold=1.3,mask=[134,134,154,154])

    offstat=imstat(imname+’.residual’,box=’224,224,284,284’)
    rms=offstat[’sigma’][0]
    if rms > prevrms:
         break                # the rms has increased, stop

    prevrms = rms

# Clean until the off-source rms residual, reaches 0.001 Jy

Note that you can exit a loop using the break statement, as we have here when the rms increases.

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